Engineering Encyclopedia
Saudi Aramco DeskTop Standards
Designing Low Voltage
Non-Motor Industrial Feeder Circuits
Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s
employees. Any material contained in this document which is not
already in the public domain may not be copied, reproduced, sold, given,
or disclosed to third parties, or otherwise used in whole, or in part,
without the written permission of the Vice President, Engineering
Services, Saudi Aramco.
Chapter : Electrical
File Reference: EEX10301
For additional information on this subject, contact
Engineering Encyclopedia
Electrical
Designing Low Voltage
Non-Motor Industrial Feeder Circuits
CONTENTS
PAGES
CALCULATING AMPACITY RATINGS OF FEEDERS CONDUCTORS................ 1
Factors Affecting Ampacity Ratings ........................................................................ 1
Factors Affecting Continuous Current Ratings ........................................................ 2
Busway................................................................................................................... 14
CALCULATING SHORT CIRCUIT RATINGS OF FEEDER
CONDUCTORS........................................................................................................... 16
Introduction ............................................................................................................ 16
Factors Affecting Conductor Short Circuit Ratings ............................................... 16
Conductor Temperature Rise.................................................................................. 20
Protective Device Clearing Times.......................................................................... 24
CALCULATING VOLTAGE DROPS OF A FEEDER CONDUCTOR
CIRCUIT...................................................................................................................... 27
Introduction ............................................................................................................ 27
Factors Affecting Voltage Drop Calculations ........................................................ 28
Effects of Resistance Variables on Voltage Drop .................................................. 31
Effects of Reactance Variables on Voltage Drop ................................................... 33
Voltage Drop Limits............................................................................................... 34
Approximation Formula......................................................................................... 35
SELECTING FEEDER PROTECTIVE DEVICES...................................................... 38
Types of Protective Devices................................................................................... 38
Low Voltage Power Circuit Breaker (LVPCB) Ratings......................................... 43
Protective Device Time/Current (T/C) Characteristics........................................... 45
Factors Affecting Selection .................................................................................... 60
SELECTING FEEDER CONDUCTOR RACEWAY SIZES ...................................... 63
Types of Raceways................................................................................................. 63
Factors Affecting Raceway Size ............................................................................ 65
WORK AID 1: RESOURCES USED TO DESIGN A LOW VOLTAGE
NON-MOTOR FEEDER CIRCUIT............................................................................. 68
Work Aid 1A: 1993 National Electric Code Handbook......................................... 68
Work Aid 1B: ANSI/IEEE Standard 141-1986 (Red Book) .................................. 68
Work Aid 1C: SAES-P-114 ................................................................................... 68
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Work Aid 1D: SAES-P-100 ................................................................................... 70
Work Aid 1E: SAES-P-104.................................................................................... 71
Work Aid 1F: Applicable Procedures for Calculating Ampacity Ratings
of Feeder Conductors ............................................................................................. 73
Work Aid 1G: Applicable Procedures for Calculating the Short Circuit
Rating of a Feeder Conductor ................................................................................ 75
Work Aid 1H: Applicable Procedures for Calculating the Voltage Drop
of a Feeder Conductor............................................................................................ 78
Work Aid 1I: Applicable Procedures for Selecting a Feeder Conductor
Protective Device ................................................................................................... 79
Work Aid 1J: Applicable Procedures for Selecting a Feeder Conductor
Raceway Size ......................................................................................................... 80
Work Aid 1K: Feeder Circuit Design Flow Chart.................................................. 81
GLOSSARY................................................................................................................. 82
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CALCULATING AMPACITY RATINGS OF FEEDERS CONDUCTORS
Note: Work Aid 1F has been developed to teach the Participant procedures to calculate
feeder conductor ampacity ratings.
Factors Affecting Ampacity Ratings
Although there are many factors that affect the ampacity ratings of conductors, this Module
will restrict the discussion to the following three factors:
•
•
•
Continuous Current
Short Circuit Current
Voltage Drop
Continuous Current
The National Electric Code (NEC) defines the continuous current rating of a conductor as
ampacity, which means “the current in amperes that a conductor can carry continuously under
the conditions of use without exceeding its temperature rating”.
The NEC recognizes that the maximum continuous current carrying capability of a conductor
varies with the different conditions of use and the insulation temperature rating of the
conductor itself. For example, ambient temperature is a condition of use. A 75°C conductor
installed outdoors in Saudi Arabia would have very little ampacity capability compared to the
same temperature-rated conductor installed indoors in an air-conditioned space. Another
example of condition of use is the number of conductors that are installed in a raceway
(conduit).
Short Circuit Current
The short circuit current rating of a conductor is the maximum current that a conductor can
carry, for a specific and very short time interval, without reaching temperatures that will
permanently damage the conductor insulation. Note: The next Information Sheet will discuss
in detail the calculation of the short circuit ratings of conductors.
Voltage Drop
Because most electrical equipment is voltage-sensitive, it is very important to have within
equipment and design standard tolerances, the proper voltage at the terminals that are serving
the equipment. Any excessive voltage drop could possibly damage the equipment and impair
its proper operation. Note: Calculating the voltage drop of feeder conductors will be
discussed later in this Module.
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Factors Affecting Continuous Current Ratings
Conductor Material Types
The resistivity of a conductor, represented by the Greek letter rho (ρ), is defined as the
resistance, in ohms (Ω), of the material for a specific sample of given cross-sectional area (A)
and with given length (L). The most commonly used dimension for ρ is circular-mil-foot.
Resistance (R) is directly proportional to ρ, and it is represented by the following equation:
•
•
R = ρL/A
where:
R
ρ
L
A
=
=
=
=
dc resistance of the conductor in ohms (Ω)
resistivity of the conductor in Ω-cmil/ft
length of the conductor in feet (ft)
cross-sectional area of the conductor
(cmil)
in
circular
mils
The relationship should be very obvious that, if the resistivity (ρ) of the material is greater
(Figure 1), the resistance (R) of the material also increases.
Type of
Material
Resisitivity at 200C
Ω-cmil/ft
silver
9.8
copper
10.4
aluminum
17.0
tungsten
33.0
nickel
50.0
iron
60.0
nichrome
650.0
Figure 1. Resistivity Table
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Aluminum is widely used as a transmission-line conductor, especially on high-voltage lines.
Compared to a copper conductor of the same physical size, the aluminum wire only has
60 percent of the conductivity, only 45 percent of the tensile strength, and only 33 percent of
the weight. An aluminum wire, to have the same conductivity as copper, would have to be
1.64 (100/61) times larger than the copper conductor. For the same conductor current
carrying capacity, an aluminum wire would have approximately 75 percent of the tensile
strength and 55 percent of the weight of a copper wire. This lighter weight makes aluminum
conductors the preferred choice for longer spans; fewer towers, crossarms, insulators, etc., are
required for the spans. Note: Although permitted by the NEC, SAES-P-104 does not permit
the use of aluminum conductors for wires and cables in buildings, direct burial cables,
overhead cable feeders, etc. ACSR is permitted for overhead distribution lines only. This
section on aluminum is presented for general information purposes only.
When an aluminum conductor is stranded, the center strand is most often made of steel, which
provides greater strength. ACSR is especially suited for long spans. Figure 2 shows the
comparison (cross-sectional area) between a copper, an aluminum, and an ACSR conductor,
each of which has equal current-carrying capacity.
Figure 2. Conductor Type Comparison
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Copper (Annealed) is the most generally used building conductor in insulated wires and
cables. When covered with rubber insulation, the copper is coated with tin or lead alloy for
its own protection, as well as for protection of the insulation, because both copper and
insulation contain substances that attack each other.
Copper is also quite often used in overhead distribution line conductors. Three kinds of
copper are used: hard-drawn copper, medium hard-drawn copper, and annealed copper,
which is often called “soft-drawn” copper. Copper wire is hard-drawn as it comes from the
drawing die. To obtain soft or annealed copper wire, the hard-drawn wire is heated to a very
high temperature (red heat) to soften it. Most utilities use medium hard-drawn copper for
distribution lines, especially in wire sizes smaller than No. 2 AWG.
Stranded Versus Solid Conductors - Conductors are classified as either solid or stranded
and by size, ranging from very small (AWG No. 18) to very large (2000 kcmil) sizes.
As the name implies, a solid conductor is a single strand conductor with a solid circular cross
section. Size AWG No. 1 is approximately the largest solid conductor that is manufactured.
Anything larger would not be flexible enough to run in ducts, conduits, etc. Large solid
conductors also are easily damaged by bending.
Stranded conductors are a group of solid wires (single strands) made into a single conductor.
The strands of the conductor are arranged in concentric layers around a single core. Size
AWG No. 1/0 and larger are the usual sizes for stranded wire, although, because of its
flexibility, it is available and readily used in smaller sizes as well. The smallest number of
wires in a stranded conductor is three, and this number increases to a very large number for
special applications.
Note: SAES-P-104 requires the use of Class B or Class C stranding for all power conductors
for normal power requirements.
Classification of stranded cables for industrial applications are as follows:
•
Class B - This class of stranding is used for industrial cables for 600, 5000, and
15,000 volt power systems. The number of strands vary from 7 to 127,
depending on the wire size.
•
Class C - This class of stranding is used when there is a need for additional
flexibility. The number of strands varies from 19 to 169. Also, the diameter of
each strand is smaller than it is in wires of the same size that have Class B
stranding. The combination of more strands and smaller diameter strands
allows for more flexibility.
•
Classes G and H - These classes of stranding allow for extremely flexible
cables. Classes G and H also are called rope or bunch stranding. Class G uses
133 strands, and Class H uses 259 strands. Welding cable is an example of
Class G and Class H cables.
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Figure 3 shows various strand configuration for cables.
Figure 3. Stranded Cable Configuration
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Conductor Sizes
American Wire Gage (AWG) System - Wire sizes are expressed in numbers. Because there
are different numbering methods, it is imperative to always state which numbering method is
being used when stating wire sizes. In America, the most often used wire gage is Browne and
Sharpe, which is also called the American Wire Gage (AWG). Wire sizes range from No. 40
AWG (very small) to 0000 AWG or 4/0 AWG (pronounced four-aught). Larger wires greater
than No. 4/0 AWG are sized in circular mils (cmil) or kilo-circular mils (kcmil), where 1
kcmil equals 1000 cmils. The previous term for kcmil was MCM, which is now obsolete.
One cmil equals the diameter of the wire in mils (1/1000 of an inch) squared. As the wire size
diameter and cross-sectional area (A) increases, its resistance decreases, which is once again
expressed by the following formula:
•
R = ρL/A
International System (SI) - Saudi Aramco specifies the nearest metric wire size to the
standard U.S. ICEA NEMA standard sizes (AWG or kcmil). Note: See Work Aid 1, Figure
36. The following formula is presented for informational purposes only:
•
1 cmil = 5.067 x 10-4 mm2 = 7.854 x 10-7 in2
Temperatures
Operating Temperature - The maximum continuous current carrying capability of a
conductor is determined by the temperature at which it is allowed to operate over its lifetime.
The type of insulation surrounding the material ultimately determines the operating
temperature. The NEC temperature rating classifications are 60°C, 75°C, and 90°C. Note:
SAES-P-104 does not permit use of 60°C insulation. If the operating temperature is exceeded
for any long periods of time, the insulation ages prematurely, becomes hard and brittle, and
eventually leads to early failure.
Ambient Temperature - The ambient temperature is defined as the temperature of the
medium (air or earth) surrounding the conductor. When the ambient temperature increases,
there is less of a temperature differential surrounding the conductor, which means that the
heat dissipating rate of the conductor is less. As the ambient temperature increases, the
current carrying capability of the conductor must be decreased to prevent the conductor’s
operating temperature, which is based on the insulation material, from being exceeded. The
NEC ampacity ratings for building wire are based on an ambient temperature of 30°C. SAESP-100 specifies an ambient temperature of 35°C in indoor air-conditioned spaces, whereas
SAES-P-104 specifies 30°C in indoor air-conditioned spaces. See Work Aid 1 for other
SAES-P-100 and SAES-P-104 listed ambient temperatures.
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NEC Derating Factors - If the ambient temperature exceeds 30°C, the NEC requires derating
of the conductor in accordance with the correction factors listed in NEC Table 310-16 (see
Work Aid 1A, Handout 1).
Example A:
NEC Table 310-16 lists the ampacity rating of a 500 kcmil (240 mm2) 75°C
copper conductor at 380 A. What is the ampacity rating of the conductor at
35°C?
Answer A:
The correction factor listed in the NEC is 0.94. The corrected ampacity is 357
A (.94 x 380).
Conductor Insulations
Types - The five most common types of insulation are listed as follows:
•
•
•
•
•
Thermosetting compounds, solid dielectric
Thermoplastic compounds, solid dielectric
Paper-laminated tapes
Varnished cloth, laminated tapes
Mineral insulation, solid dielectric - granular
Current industrial applications use synthetic materials for cable insulation. Their resistance to
moisture and ease of handling make synthetic materials popular. These synthetic materials
also have excellent electrical and mechanical properties. Synthetic insulations are grouped
into either thermoplastic or thermosetting. Thermosetting type cables have little tendency to
soften upon reheating, whereas thermoplastic will soften when reheated. Saudi Aramcoapproved insulations for power conductors are the following:
•
Polyvinyl chloride (PVC) - Thermoplastic Compound
This cable is flexible and has excellent electrical properties. PVC cables are
used for 600 volt systems. The maximum operating temperature is 60°C. The
maximum short-circuit temperature rating is 150°C. PVC cable is available in
many different colors, and it usually carries a designation of T or TW. The
maximum voltage of PVC cable is 600 volts. Note: Polyvinyl chloride cable is
not used on Saudi Aramco installations.
•
Cross-Linked Synthetic Polymer
This cable insulation is also for 600 volt systems, and it is moisture and heat
resistant. The maximum operating temperature is 90°C, and the insulation is
designated as XHHW.
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•
Ethylene Propylene Rubber (EPR)
This type of insulation offers excellent resistance to ozone, heat, and weather.
The maximum operating temperature is 90°C. The voltage range of EPR cable
is 600 to 69,000 volts.
•
Silicone Rubber - Thermosetting Compound
This insulation system is highly resistant to flame, corona, and ozone, but it has
very poor mechanical strength. Silicone rubber insulation has a maximum
operating temperature of 125°C with a short circuit temperature rating of 250°C.
Silicone rubber cable is used in high temperature areas or fire protection
applications only. The voltage range of silicone rubber cable is from 600 to
5000 volts.
•
Crossed-Linked Polyethylene (XLPE) - Thermosetting Compound
This cable has excellent electrical, moisture-resistant, and environmentalresistant properties. Corona affects XLPE type insulation. The maximum
operating temperature is 90°C. The maximum short circuit temperature is
250°C. XLPE cable voltages range from 600 to 69,000 volts.
Applicable Provisions - NEC Table 310-13 lists the applicable provisions for insulated
conductors. For example, certain types of conductors may be used only in dry locations,
whereas another type may be used only in wet or damp locations. Some of the conductors
may be used in both locations. The applicable provisions ultimately determine the conductor
ampacity ratings. For example, a type XHHW conductor is rated for 90°C in dry and damp
locations, and for 75°C in wet locations.
Example B:
What is the ampacity rating of a No. 4/0 AWG, type XHHW copper conductor
that is being used in a wet location?
Answer B:
Because the conductor is being used in a wet location, NEC Table 310-13
(applicable provisions) lists the conductor operating temperature as 75°C. Per
NEC Table 310-16, the ampacity rating of a No. 4/0 AWG, type XHHW
copper conductor, is 230 A.
Number of Conductors
The ampacity ratings of conductors, as listed in NEC Table 310-16, are based on no more
than three conductors installed in a raceway (conduit) at a given ambient temperature of 30°C.
Exceeding the number of conductors (3) requires derating.
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Current Carrying Conductors - Only conductors that normally carry current are considered
as current carrying conductors for determining the number of conductors in a raceway. For
example, the equipment grounding conductor (green wire) is not considered a current
carrying conductor. On the other hand, in a 3-phase, 4-wire, wye-connected system, the
common conductor (neutral) carries approximately the same current as the phase conductors,
and is therefore considered to be a current carrying conductor. Note: See Note 10 to NEC
Table 310-16 for other examples.
Fill Rate and Derating Factors - When the number of current carrying conductors, as
explained above, exceeds 3 in a raceway, Note 8 to Article 310-16 requires derating of the
conductors. For example, 4 to 6 current carrying conductors in a raceway require that each
conductor be derated by 20 percent (.80 reduction factor). Seven to 9 conductors require that
each conductor be derated by 30 percent (.70 reduction factor). Note: For a complete listing
of the reduction factors, see Note 8 to NEC Table 310-16 (Work Aid 1J, Handout 2).
Example C:
Determine the ampacity of a 3-phase, 4-wire, feeder conductor that is using 500
kcmil copper conductors, type THHN insulation (90°C), installed in steel
conduit, 40°C ambient temperature, and feeding a mercury vapor lighting load.
Answer C:
Because the ambient temperature exceeds 30°C, the conductors must be
derated using the correction factor (0.91) from NEC Table 310-16. Note 10 to
NEC Table 310-16 also requires that the neutral conductor be considered as a
current carrying conductor for arc discharge (i.e., mercury vapor) lighting.
Therefore, the conductors must be derated using the adjustment factor (0.80)
for 4 to 6 current carrying conductors in a raceway (Note 8A to NEC Table
310-16). The conductor ampacity is listed in NEC Table 310-16 is 430 A.
Therefore, the ampacity rating is 315 A (430 x 0.91 x 0.80).
Example D:
What is the ampacity rating of 6, type THHW, 75°C, size No. 4/0 AWG copper
conductors in a raceway? Assume that the ambient temperature is 30°C.
Answer D:
Per NEC Table 310-16, the ampacity is 230 A for no more than three
conductors in a raceway. Per Note 8 to NEC Table 310-16, the ampacity
should be reduced to 80 percent for 6 conductors. Therefore, the ampacity
rating
is
184
A
(.80 x 230).
Parallel Conductors - NEC Article 310-4 permits parallel conductors as long as all of the
following conductor conditions are met: same length, same conductor material, same
insulation type, and the same terminations.
Because of the skin effect in ac circuits, the current carrying capability per cmil of conductor
area decreases with the size of the conductor. Additionally, it is harder to dissipate the heat
within large conductors. It is also more difficult to “pull” a large conductor through raceway.
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For these reasons, it is often preferable to parallel small conductors rather than to use large
conductors.
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Conductor Terminations
Although the NEC specifies the ampacity ratings of conductors, the Underwriters
Laboratories, Inc. (UL) approves the use and conditions of the electrical equipment. The
termination provisions of the UL-approved equipment are based on 60°C and 75°C
terminations (terminals, lugs, etc.).
100 A or Less - NEC Article 110-14 (c) (1) specifies that termination provisions of equipment
for circuits rated 100 A or less or marked for sizes No. 14 through No. 1 AWG conductors
shall be used with conductors rated for an operating temperature of 60°C. The NEC permits
the following two exceptions:
•
Higher-temperature-rated conductors may be used, provided the ampacity of
the conductors is based on the 60°C operating temperature ampacity ratings.
•
The equipment is listed for the higher-temperature-rated conductors.
Greater Than 100 A - NEC Article 110-14 (c) (2) specifies that termination provisions of
equipment for circuits rated over 100 A or marked for conductors larger than No. 1 AWG
shall be used with conductors rated for an operating temperature of 75°C. The same two
exceptions apply for circuits rated more than 100 A, except that the ampacity of the
conductors is based on an operating temperature of 75°C.
Example E:
What is the ampacity rating of a 75°C, type THHW, size No. 4 AWG
conductor that is being terminated at a 70 A molded case circuit breaker
(MCCB)?
Answer E:
Because the MCCB is rated at less than 100 A, the ampacity rating of the
conductor must be limited to its 60°C rating of 70 A versus its 75°C rating of
85 A.
Types of Feeders and Sizing Guidelines
Bus Feeders - SAES-P-104 specifies that bus feeders be sized in accordance with the NEC,
but that a 20 percent growth factor must be added. However, the size, including the 20
percent growth factor, must not exceed the maximum rating of the bus. SAES-P-104 allows
deletion of the growth factor where 2 or more feeders serve the same load bus.
Transformer Feeders - SAES-P-104 specifies that feeders shall have an ampacity rating of
not less than the full load ratings (fan-cooled ratings if fans are installed) of all connected
transformers and all other connected loads.
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Other Feeders - Although not explicitly specified in SAES-P-104, other feeders should be
sized to carry the load in accordance with the NEC.
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Busway
In large commercial and industrial complexes, where high density (ampacity) loads are
connected, it is usually not economical to use insulated conductors as feeders. An alternative
method is to use a busway, which is a metal enclosure containing factory assembled
conductors of copper bars. It is standard industry practice, as well as Saudi Aramco practice,
to consider the use of a copper busway whenever the load is greater than or equal to 1000 A.
Ampacity Ratings
Busways are available in the following three design types: low impedance feeder busway,
high impedance feeder busway, and simple plug-in busway (Figure 4).
Plug-in busways are available in ampacity ratings ranging from 100 to 3000 A and in short
circuit current withstand ratings ranging from 10 to 85 kA. Feeder type busways are available
in ampacity ratings ranging from 600 to 5000 A and in short circuit current withstand ratings
ranging from 42 to 200 kA.
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Figure 4. Busway Types
Industrial Feeder Uses
In industrial applications, feeder busways are almost exclusively used for horizontal runs from
the main switchboards to the major power centers located throughout the plant. Busway risers
are also often used to distribute power to all floors of multistory buildings.
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CALCULATING SHORT CIRCUIT RATINGS OF FEEDER CONDUCTORS
Note: Work Aid 1G has been developed to teach the Participant procedures to calculate
short circuit ratings of feeder conductors.
Introduction
A cable must be protected from overheating due to excessive short circuit (fault) currents.
The fault point may be on a section of the protected cable or on any other part of the electric
system. During a phase fault, the I2R losses in the phase conductor elevate first the
temperature of the conductor, followed by the insulation materials, protective jacket, raceway,
and surroundings. Since the short circuit current should be interrupted either instantaneously
or in a very short time by the protective device, the amount of heat transferred from the
metallic conductors outward to the insulation and to other materials is very small; therefore,
the heat from I2R losses is almost entirely in the conductors. For practical purposes, it is
assumed that 100% of the I2R losses is consumed to elevate the conductor temperature.
During the period that the short circuit current is flowing, the conductor temperature should
not be permitted to rise to the point where it may damage the insulation. Providing cable
protection during a short-circuit condition involves, as a minimum, the following factors:
•
•
•
•
Maximum available short circuit currents.
Maximum conductor temperature that will not damage the insulation.
Cable conductor size that affects the I2R value and its capacity to contain the
heat.
Longest time that the fault will exist and the fault current will flow.
Factors Affecting Conductor Short Circuit Ratings
Operating Temperature Versus Short Circuit Temperature
The operating temperature is the initial temperature of the conductor prior to the occurrence
of the short circuit. This operating temperature is assumed to be the temperature of the
conductor in a 30°C ambient temperature environment and at rated ampacity, which is 60°C,
75°C, or 90°C, as discussed in the previous section of this Information Sheet.
The short circuit temperature is the final temperature of the conductor after the short circuit
occurs. This short circuit temperature is assumed to be the maximum temperature rise that the
conductor can sustain for a specified period of time without damaging the insulation.
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Figure 5 lists the initial and final temperature ratings for various types of insulation commonly
used in industrial applications. The temperature rise is related by the following formula,
which will be discussed in detail later in this Information Sheet.
•
(I/A)2t = 0.0297 log10 [(T2 + 234)/(T1 + 234)]
Type of
Insulation
Continuous (Initial)
Temperature Rating
T10C
Short Circuit (Final)
Temperature Rating
T20C
Rubber
75
200
Cross-Linked Polymer
90
250
125
250
60, 75, 90
150
Paper
85
200
Varnished Cloth
85
200
Silicone Rubber
Thermoplastic
Figure 5. Conductor Temperature Ratings
Operating Speed of Protective Devices
The extraordinary temperatures generated under short circuit conditions may damage cables
over their entire length if the fault current is not interrupted quickly enough. The upstream
protective devices must clear the fault before the damaging temperatures occur. Many
engineers select cables and/or protective devices such that the backup protective device could
also clear the fault before thermal damage occurs.
Cable Impedance
Although the cable impedance decreases as the cable size increases, the increased crosssectional area of the cable increases the withstand capability of the cable. For example, a No.
2/0 AWG copper conductor having an impedance of 0.1150 Ω/1000 ft can withstand 30 kA of
short circuit current for approximately 4 cycles (≈ .067 sec). Conversely, a 500 kcmil copper
conductor having roughly half the impedance (0.0551 Ω/1000 ft) can withstand the same 30
kA of short circuit current for almost 48 cycles (≈ 0.8 sec). The increased diameter of the
cable allows the cable to absorb more heat without damaging the insulation.
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Conductor Insulation
Figure 5 listed the maximum conductor temperature ratings. Any prolonged load/ambient
temperature combinations that increase the conductor temperature above the continuous
(initial) temperature ratings, for example 75°C for a type THWN conductor, will damage the
insulation. Short circuits that elevate the conductor temperature above the final temperature,
for example 150°C for the type THWN conductor, will also damage the insulation.
Short Circuit Current Available (ISCA)
It is general industry practice to use the subtransient reactance (X”d) to calculate the
RMS symmetrical short circuit current available where instantaneous overcurrent relays
(ANSI Device 50) are used to protect cables. And where cables are protected by fuses, cable
limiters, or low voltage breakers, it is industry practice to use X”d to calculate the
asymmetrical current available. Note: Short circuit currents were discussed in detail in EEX
102.
Type of Fault
Three-phase faults and line-to-ground faults in a solidly grounded system are typically of such
a large magnitude that the protective devices quickly detect and clear the faults
instantaneously, and therefore, usually provide adequate protection for the cables.
On the other hand, high impedance arcing faults are often persistent and escalate into more
serious, high magnitude phase-to-phase faults, resulting in extensive cable damage.
Type of System Grounding
For solidly grounded systems, although the fault current magnitudes are high, extensive cable
damage is usually avoided because of the very quick detection and clearing of faults.
Cable damage in low resistance grounded systems is also typically not extensive because the
ground fault current magnitudes are small (100 - 1200 A) and are quickly detected and
cleared.
Because of the very low current magnitudes in high-resistance-grounded systems, cable
damage is limited unless the fault is not cleared and another ground fault escalates to a phaseto-phase fault resulting in extensive damage.
Line-to-ground faults in ungrounded systems can result in extensive cable insulation damage
by the sustained overvoltages rather than by the fault current magnitudes (typically 1 - 10 A).
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In ungrounded systems, cables should have, as a minimum, 133% or 173% insulation level
ratings. Note: Saudi Aramco design standards do not permit use of ungrounded systems.
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Conductor Temperature Rise
Insulation, Material Type, Size
On the assumptions that all heat is absorbed by the conductor metal (copper or aluminum) and
that there is no heat transmitted from the conductor metal to the insulation, the temperature
rise is a function of the conductor size (area in cmils), the magnitude of the short circuit
current (ISCA in amperes), and the fault duration (time in sec). These variable are related by
the following formula:
•
(I/A)2t = (k1) log10 [(T2 + k2)/T1 + k2)]
•
where:
I
A
t
T1
=
=
=
=
short circuit current (asymmetrical) in amperes (A)
conductor cross-sectional area in circular mils (cmils)
time of short circuit in seconds (sec)
initial conductor temperature in degrees Celsius (°C), as listed by
the manufacturer
T2 = final conductor temperature in degrees Celsius (°C), after the
short circuit, as listed by the manufacturer
k1 = 0.0297 for copper = 0.0125 for aluminum
k2 = 234 for copper = 228 for aluminum
Variables in Formula for Allowable Short Circuit Interrupting Time
The initial (T1) and final (T2) conductor temperatures are predetermined on the basis of the
continuous current rating and the insulation material. For low voltage thermoplastic
conductors specified in Saudi Aramco design standards, T1 will equal 75°C or 90°C and T2
will always equal 150°C (see Figure 5). Because Saudi Aramco design standards do not
permit the use of low voltage aluminum conductors, R1 equals 0.0297 and k2 equals 234.
The short circuit current (I) used in the formula to plot the cable damage curves is not
necessarily the short circuit current available (ISCA) in the event of a fault. The operating time
of the protective device must, however, allow the device to clear the fault for all values of
fault current in less time than the time (t) calculated in the above equation.
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Short Circuit Cable Damage Charts
Cable damage charts (curves) are often used to ensure that the protective device will clear the
fault prior to damaging the conductor. Figure 6 shows an Insulated Cable Engineers
Association (ICEA) format for representing cable damage curves, and Figure 7 shows cable
damage representative curves used by many coordination engineers.
Example F:
How quickly must a protective device clear a 30 kA (asymmetrical current)
fault to protect a size No. 4/0 AWG copper conductor?
Answer F:
A No. 4/0 AWG copper conductor can withstand a 30 kA fault for
approximately 16 cycles (≈ 0.2667 sec).
Example G:
A low voltage power circuit breaker (LVPCB), with an instantaneous operating
time of .05 sec (≈ 3 cycles), is used to protect a 90°C rated thermoplastic
conductor in a circuit where 35 kA of asymmetrical fault current is available.
What is the minimum size conductor (cross-sectional area in cmil) that will
safely carry the 35 kA for 0.05 sec?
Answer G:
Rewriting the formula described previously in this Information Sheet yields the
following:
•
A =[(I2t)/(.0297 log10 ((T2 + 234)/(T1 + 234)))]1/2
=[(35000)2 (.05)/(.0297 log10 ((150 +234)/(90+234)))]1/2
=167, 165 cmil
•
Per NEC Table 8, Chapter 9, a size 3/0 AWG conductor has a crosssectional area of 167,800 cmil.
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Figure 6. ICEA Cable Damage Curves
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Figure 7. Cable Damage Curves For Thermoplastic Insulation
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Protective Device Clearing Times
The total clearing time of various types of protective devices depends on the types of circuit
breakers (molded case or low voltage power) or fuses (current or non-current limiting) that
are being used.
Circuit Breakers
Note: All of the following times are approximate; the time/current characteristics of the
specific device being used should be checked for exact times.
Molded Case Circuit Breakers (MCCBs) typically operate in 1.1 (0.018 sec) cycles for less
than or equal to 100 ampere frame (AF) sizes and 2.25 cycles (0.038 sec) for 225 AF and
larger sizes in their instantaneous ranges (5 times ampere trip), and over 100 seconds for low
magnitude overloads.
Low Voltage Power Circuit Breakers (LVPCBs) typically operate in 3 cycles (0.05 sec) in
their instantaneous ranges, 6 - 30 cycles (0.10 - 0.50 sec) in their short time ranges, over 6000
cycles (100 sec) in their long time ranges, and 6 - 30 cycles (0.10 - 0.50 sec) in their ground
fault ranges.
Low Voltage Fuses
Low voltage fuses clear faults in approximately 60,000 cycles (1000 sec) at 1.35 to 1.5 times
their continuous current ratings and in 0.25-0.50 cycles (0.004-0.008 sec) in their currentlimiting ranges.
Typical Time/Current (T/C) Plot
Circuit Breakers - Figure 8 is a time/current characteristic plot of a LVPCB (DS-206)
protecting a 500 kcmil copper conductor.
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Figure 8. LVPCB Protection of a Cable
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Fuses - Figure 9 is a time/current characteristic plot of a 20 A fuse protecting a No. 12 AWG
copper wire and a 400 A fuse protecting a 500 kcmil copper wire.
Figure 9. Fuse Protection of Cables
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CALCULATING VOLTAGE DROPS OF A FEEDER CONDUCTOR CIRCUIT
Note: Work Aid 1H has been developed to teach the Participant procedures to calculate
voltage drops of a feeder conductor circuit.
Introduction
Designers of power systems must have practical knowledge of voltage drop calculations, not
only to meet required codes and standards, but to ensure that the required voltage of a
particular piece of equipment, for example a motor, is kept within manufacturer’s specified
tolerances in order to prevent damage to the equipment. Note: The effects of voltage drops
on different types of electrical equipment was discussed in Modules EEX 102.02 and EEX
102.04.
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Factors Affecting Voltage Drop Calculations
The voltage drop on a feeder depends on the following factors (Figures 10 and 11).
•
Load Data
•
Feeder Lengths
•
Feeder Impedance
Figure 10. One-Line Diagram
Figure 11. Feeder Diagrams
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Load Data
Voltage - The voltage drop (VD) is the difference between the voltage at the source end (VS)
of the feeder, which is assumed to be a fixed value, and the voltage at the load end (VL) of the
feeder, which varies as a function of the load or feeder current (IL). The voltage drop VD
calculated is a line-to-neutral drop (one-way), the line-to-line voltage drop calculated for a
single-phase system is 2VD, and the line-to-line voltage drop calculated for a three-phase
system is 3VD.
Current - The current (IL) flowing in the circuit is assumed to be the load or feeder current.
The voltage drop is the load current times both the resistance (ILR) and reactance (ILX) of the
circuit.
Power Factor - The power factor (p.f.) of the load directly affects the voltage drop. As the
power factor decreases, the voltage drop will increase. For purposes of this Module, assume
that all power factors are lagging power factors.
Feeder Lengths
The voltage drop (ILZ) is directly proportional to the feeder lengths because the line
impedance Z depends on the length. As the length of the feeder increases, the impedance of
the line increases.
Feeder Impedance
Resistance - The resistance (R) per unit of length depends on the following factors.
•
Conductor Material
•
Conductor Size/Length
•
Conductor Operating Temperature
Reactance - The inductive reactance (X) per unit of length depends on the following factors:
•
Conductor Diameter
•
Conductor Spacing
•
Power Supply Frequency
•
Raceway Type
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Effects of Resistance Variables on Voltage Drop
Conductor Material
The resistance (R) of a conductive material is directly proportional to the resistivity (ρ) of the
material as expressed by the following formula:
•
R = ρL/A
Because the resistivity of copper is 10.4 Ω-cmil/ft, versus 17.0 Ω-cmil for aluminum, the
resistance of a copper wire, of a given length and cross-sectional area, is less than the
resistance of an aluminum wire of the same length and cross-sectional area.
Conductor Size/Length
Resistance is inversely proportional to the size (cross-sectional area) and directly proportional
to the length, which is again expressed by the following formula:
•
R = ρL/A
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Operating Temperature
Resistance values of conductors are usually provided at a given temperature, for example,
30°C (86°F). As the temperature of the conductor increases, either because of an increase in
load or because of ambient temperature, resistance will also increase. For a copper conductor,
the change in resistance as a function of temperature can be represented by the following
formula
(Figure 12):
•
R2 = R1 [(234.5 + T2)/(234.5 + T1)]
•
where R1 and R2 are the resistances of the conductor in ohms (Ω) at
temperatures T1 and T2 respectively, in degrees Celsius (°C).
Figure 12. Resistance Versus Temperature Relationships
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Example H:
The ac resistance of a 500 kcmil copper conductor in steel conduit in a 30°C
ambient environment is .029 Ω/1000 ft. What is the ac resistance of 2500 ft of
a 500 kcmil copper conductor in a 50°C ambient environment?
Answer H:
•
T1 = 30°C, T2 = 50°C
•
R1 = (.029 Ω/1000 ft)(2500 ft) = .0725 Ω
•
R2 = R1 [(T2 + 234.5)/(T1 + 234.5)]
= (.0725)[(50 + 234.5)/(30 + 234.5)]
≈ .078 Ω
Effects of Reactance Variables on Voltage Drop
Conductor Diameter
As the conductor diameter increases (larger wire sizes), the reactance of the conductor
decreases because of mutual self-inductance.
Conductor Spacing
The total reactance of a line (X) actually consists of two terms, Xa and Xd, where Xa is a
function of the conductor material and Xa is a function of conductor spacing. As the spacing
increases, the reactance (Xd) increases because of mutual inductance. This increase in
reactance is typically not evident for low voltage conductors because the reactance values in
tables already account for spacing.
Frequency
As indicated by the following formula, conductor reactance is directly proportional to
frequency:
•
X = ωL = 2πfL
•
where:
Example I:
ω = angular frequency in radians/sec
f = frequency in cycles/sec or Hertz (Hz)
The reactance of a 2000 ft run of a 500 kcmil copper conductor in steel is .096
Ω at 60 Hz. What is the conductor’s reactance at 50 Hz? at 400 Hz?
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Answer:
(1)
X60/60 = X50/50
X50 = (50/60) X60 = (5/6)(.096) = .08 Ω
(2)
X60/60 = X400/400
X400 = (400/60) X60 = (20/3)(.096) = .64 Ω
Raceway Type
If the material (raceway) surrounding the conductor is magnetic, such as steel conduit, the
reactance is greater than if the raceway is non-magnetic, such as aluminum or plastic conduit.
Voltage Drop Limits
National Electric Code (NEC)
NEC Article 215-2(b), FPN No. 2, states that “conductors for feeders as defined in Article
100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power,
heating, and lighting loads, or combinations of such loads, and where the maximum total
voltage drop on both feeder and branch circuits to the farthest outlet does not exceed 5
percent, will provide reasonable efficiency of operation”.
Saudi Aramco Standard SAES-P-100
SAES-P-100 specifies the following criteria pertaining to feeder voltage drops for non-motor
circuits under 600 volts:
•
Maximum steady state voltage drop for a main, feeder, and branch circuit shall
not exceed five (5) percent.
•
Summation of voltage drops in circuit from main to distribution center, and
from feeder breaker to panelboard shall be two (2) percent at full load.
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Approximation Formula
Phasor Relationships
Due to the phasor relationships shown in Figure 13, exact methods of calculating line voltage
drops require extensive knowledge of complex phasor algebra as was previously discussed in
Module EEX 102.03. However, in most industrial electrical distribution systems, such as
found on Saudi Aramco installations, the approximate method formula, as presented below, is
adequate for most line voltage drop calculations.
•
Approximate Method Formula:
VD = IR cos θ + IX sin θ = I(R cos θ + X sin θ) = IZ
•
where:
VD
= line-to-neutral voltage drop, volts (V), (one way)
I
= line current, amperes (A)
R
= circuit line resistance, ohms (Ω)
X
= circuit line reactance, ohms (Ω)
Z
= circuit line impedance, ohms (Ω)
Z
= R cos θ + X sin θ
θ
= load power factor angle, degrees
cos θ = load power factor, decimals
sin θ = load reactance factor, decimals
VS
= source voltage, line-to-neutral, volts (V)
VL
= load voltage, line-to-neutral, volts (V)
VD(line-to-line, 1φ system) = 2VD
VD(line-to-line, 3φ system) = 3VD
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Figure 13. Voltage Drop Phasor Diagrams
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Variable Assumptions
The error caused by variation of load current and power factor because of the voltage applied
to the load is not considered in the above formula for calculating voltage drops. In other
words, the load current (IL) is assumed to be constant regardless of the voltage. However, if
the error is significant, an iterative method may be used. Once the voltage drop is calculated,
subtract the voltage drop (VD) from the source voltage (VS) and use this value to calculate a
new load current, and then, using the new load current calculated, repeat the voltage drop
calculation. Generally, if the total drop calculated is less than ten percent, the iterative
method is not used.
By convention, in the above formula, sin θ is considered to be positive for lagging power
factor loads. As the angle of the load voltage approaches the angle of the line voltage, the
error in the calculation approaches zero.
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SELECTING FEEDER PROTECTIVE DEVICES
Note: Work Aid 1I has been developed to teach the Participant procedures to select feeder
protective devices.
Types of Protective Devices
Note: SAES-P-114 restricts low voltage line protection to circuit breakers.
Circuit Breakers
Molded-Case Circuit Breakers (MCCB) are a class of breaker rated at 600 volts and below,
and they consist of a switching device and an automatic protective device assembled in an
integral housing of insulated material. MCCBs are capable of clearing a fault more rapidly
than a low voltage power circuit breaker (LVPCB). Solid-state trip units incorporated into
some styles of MCCBs provide for their coordination with LVPCBs. MCCBs are generally
sealed to prevent tampering, and this sealing, in turn, precludes any inspection of the MCCB
contacts. MCCBs are generally not designed to be maintained in the field, and manufacturers
recommend total replacement if a defect appears.
MCCBs are available in several different types. The thermal magnetic type, which is the most
widely used, employs thermal tripping for overloads and magnetic tripping for short-circuits.
The magnetic type MCCB employs only instantaneous magnetic tripping for cases where only
short circuit interruption is required. The integrally-fused type MCCB combines regular
thermal magnetic protection, together with current limiting fuses, to respond to applications
where higher short circuit currents are available. In addition, the current limiting type MCCB
offers high interrupting capacity protection, while at the same time limiting the let-through
current to a significantly lower value than is usual for conventional MCCBs.
Low-Voltage Power Circuit Breakers (LVPCB), like MCCBs, are rated 600 volts and
below. They differ from MCCBs, however, because they are typically open-construction
assemblies on metal frames and all parts are designed for accessible maintenance, repair, and
ease of replacement. LVPCBs are intended for service in switchgear compartments or in
other enclosures of dead- front construction. Tripping units are field-adjustable and include
electromagnetic, direct-acting, and solid-state types. LVPCBs can be used with integral
current-limiting fuses to meet interrupting requirements up to 200 kA RMS symmetrical.
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Saudi Aramco Applications of Fuses
Low voltage fuses (600 volts or less) come in two basic shapes: the plug fuse and the
cartridge fuse. Fuses may be current limiting or non-current limiting. Cartridge fuses are
either renewable or nonrenewable. Nonrenewable cartridge fuses are assembled at the
factory, and they are replaced after they open (melt) in service. Renewable cartridge fuses
can be disassembled, and the fusible element can be replaced. A special type of low voltage
fuse that is sometimes used is the dual-element or time-delay fuse. The dual-element fuse has
one element that is fast-acting, and it responds to overcurrents in the short circuit range, while
its other element permits short-duration overloads, but melts if the overload is sustained.
Note: Saudi Aramco has very limited application of low voltage fuses.
Molded Case Circuit Breaker (MCCB) Ratings
The MCCB gets its name from the material (plastic) and manufacturing process (molded)
used to make the frame (case) of the breaker. Figure 14 describes a MCCB. The MCCB
serves as the disconnect, overload, and fault protection for the feeder circuit.
Figure 14. Molded Case Circuit Breaker (MCCB)
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Frame Size
The frame size of the MCCB describes the physical size and continuous current ratings of the
MCCB. Each different type and size of MCCB is assigned a frame designation. MCCBs
have many different trip and interrupting ratings for the numerous frame sizes. Figure 15 lists
the different frame sizes for typical MCCBs.
Trip Ratings
The function of the trip unit in a MCCB is to trip the operating mechanism in the event of a
prolonged overload or short circuit current. The traditional MCCB uses electromechanical
(thermal-magnetic) trip units. Protection is provided by combining of a temperature sensing
device (bimetallic strip) with a current sensing electromagnetic device, both of which act
mechanically on the trip mechanism. More modern breakers use sophisticated electronic trip
units, which can be more precisely modeled than the electromechanical trip units. NEC
Article 240-6 lists the standard trip ratings of MCCBs. Figure 15 also lists the continuous
current or trip ratings of typical MCCBs.
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Interrupting Ratings
The interrupting rating or short circuit rating at a 40°C ambient temperature is commonly
expressed in root mean square (RMS) symmetrical amperes. The interrupting capability of
the breaker may vary with the applied voltage. For example, a breaker applied at 480 volts
could have an interrupting rating of 25,000 A at 480 volts, but the same breaker applied at
240 volts may have an increased interrupting rating of 65,000 A.
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All MCCBs operate instantaneously at currents well below their interrupting rating. Nonadjustable MCCBs will usually operate instantaneously at current values approximately five
times (5x) their trip rating. Low voltage breaker contacts separate and interrupt the fault
current during the first cycle of short circuit current. Because of this fast operation, the
momentary and interrupting duties are considered to be the same. Therefore, all fault
contribution from generators, motors, and the dc components of the fault waveform must be
considered. Figure 15 also lists the symmetrical and asymmetrical interrupting ratings of
typical MCCBs.
Line
No.
Frame
Size
(amps)
(AF)
Rated
Continuous
Current
(amps)
(AT)
Interrupting Current Rating (AIC)
(amps)
240 Volts
480 Volts
600 Volts
Sym
Asym
Sym
Asym
Sym
Asym
1
100
10-100
18,000
20,000
14,000
15,000
14,000
15,000
2
100
10-100
65,000
75,000
25,000
30,000
18,000
20,000
3
100
10-100
100,000
4
225
125-200
22,000
25,000
18,000
20,000
14,000
15,000
5
225
70-225
25,000
30,000
22,000
25,000
22,000
25,000
6
225
70-225
65,000
75,000
35,000
40,000
25,000
30,000
7
225
70-225
100,000
--
8
225
70-225
35,000
40,000
9
400
200-400
65,000
10
400
200-400
100,000
11
400
200-400
42,000
12
600
300-600
100,000
13
800
300-800
42,000
50,000
30,000
35,000
22,000
25,000
14
800
300-800
65,000
75,000
35,000
40,000
25,000
30,000
15
800
600-800
100,000
16
1000
600-1000
42,000
50,000
30,000
35,000
22,000
25,000
17
1200
700-1200
42,000
50,000
30,000
35,000
22,000
25,000
--
75,000
-50,000
--
--
100,000
--
100,000
25,000
35,000
--
100,000
--
--
30,000
22,000
25,000
40,000
25,000
30,000
100,000
30,000
100,000
-35,000
100,000
--
100,000
--
100,000
22,000
100,000
100,000
-25,000
--
--
Figure 15. Typical MCCB Ratings
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Low Voltage Power Circuit Breaker (LVPCB) Ratings
Frame Size
The rated continuous current of a LVPCB is the designated limit of RMS current, at rated
frequency, that it is required to carry continuously without exceeding the temperature
limitations based on a 40°C ambient temperature. The temperature limit on which the rating
of circuit breakers are based are determined by the characteristics of the insulating materials
used and the metals that are used in the current carrying components and springs. Standard
frame size ratings for low voltage power circuit breakers are 800, 1600, 2000, 3200, and 4000
amperes. Some manufacturers may have additional frame sizes. LVPCPs have either an
electromechanical trip or a solid-state trip that is adjustable or interchangeable from a
minimum rating up to the ampere rating of the frame. Figure 16 lists the frame sizes of
typical LVPCBs.
Frame Size (amperes)
800
1600
2000
3200
4000
Available Sensor Ratings (amperes)
50, 100, 150, 200, 300, 400, 600, 800
100, 150, 200, 300, 400, 600, 800, 1200, 1600
100, 150, 200, 300, 400, 600, 800, 1200, 1600, 2000
2400, 3200
4000
Figure 16. LVPCB Frame and Sensor Ratings
Sensor Ratings
A sensor on an LVPCB is simply a current transformer that reduces (transforms) the high
magnitude line currents to a magnitude that the trip units (typically electronic) can safely
carry for short periods of time. Figure 16 also lists the sensor ratings for an LVPCB.
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Short Time Ratings
LVPCBs are designed and marked with the maximum voltage at which they can be applied.
LVPCBs can be used on any system where the voltage is lower than the breaker rating. The
applied voltage will affect the interrupting rating of the breaker. Standard maximum voltage
ratings for LVPCBs are 635 volts, 508 volts, and 254 volts. LVPCBs are usually suitable for
both 50 and 60 Hz.
The short-time current rating of a LVPCB specifies the maximum capability of the circuit
breaker to withstand the effects of short circuit current flow for a stated period, typically 30
cycles or less. The short-time delay on the breaker’s trip units corresponds to the short-time
current rating. This time delay provides time for downstream protective devices closer to the
fault to operate and to isolate the circuit. The short-time current rating of a modern day
LVPCB without an instantaneous trip characteristic is usually equal to the breaker’s short
circuit interrupting rating. By comparison, MCCBs usually do not have a short-time rating.
Figure 17 lists the short-time interrupting rating of typical LVPCBs.
Frame
Size
(amperes)
800
1600
2000
3200
4000
Interrupting Ratings (RMS Symmetrical Amperes)
With Instantaneous Trip
Short-Time Ratings (30cycles)
(With Short-Delay)
208-240V
42,000
65,000
65,000
85,000
130,000
480 V
30,000
50,000
65,000
65,000
85,000
600 V
30,000
42,000
50,000
65,000
85,000
208-240V
30,000
50,000
65,000
65,000
85,000
480 V
30,000
50,000
65,000
65,000
85,000
600V
30,000
42,000
50,000
65,000
85,000
Figure 17. LVPCB Short-Time and Interrupting Ratings
Interrupting Ratings
The interrupting rating of a LVPCB is the RMS symmetrical current rating of the circuit
breaker. The asymmetrical interrupting rating is implied, and it is based on an X/R ratio of
6.6 for unfused breakers and on an X/R ratio of 4.9 for fused breakers. An X/R ratio of 6.6
corresponds to an asymmetrical factor (Ma) of 1.17. Under short circuit conditions, most low
voltage systems have an X/R ratio of less than 6.6. Therefore, if an asymmetrical interrupting
rating is not listed by the manufacturer, assume that the asymmetrical rating is 1.17 times the
symmetrical rating. Figure 17 also lists the symmetrical interrupting ratings of typical
LVPCBs.
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Protective Device Time/Current (T/C) Characteristics
The response curves of all protective devices are plotted on common graphs so that they may
be compared at all current and time points. The standard means used to plot device T/C
characteristics is to plot the devices on log-log graph paper (Figure 18).
Standard log-log graphs are 4.5 cycles on the horizontal scale that represents current. The
current axis ranges from 0.5 to 10,000 amperes. The vertical axis, representing time, ranges
from 0.01 to 1000 seconds and/or 0.6 to 60,000 cycles. Because current limiting fuses and
molded case circuit breakers may operate in less than 0.5 cycles (.00835 seconds),
manufacturers of these devices may reproduce T/C characteristic curves with 6 cycle vertical
scales that represent times ranging from 0.001 to 10,000 seconds (.06 to 600,000 cycles). The
horizontal current scale is also often “shifted” for a particular plot by multiplying the current
scale by a factor of 10, 100, or 1000 (x10, x100, x1000).
Thermal-Magnetic MCCB
SAES-P-114 permits thermal-magnetic (inverse-time) MCCBs for the protection of feeders.
The MCCB is sized to protect the low voltage feeder conductors in accordance with their
ampacities after all derating factors have been applied. The NEC permits the next standard
size MCCB that is available to protect the conductor if the MCCB rating does not exceed 800
A. If the breaker rating being selected exceeds 800 A, the NEC requires the next smaller size
breaker to be selected. Figure 19 shows a 600 A MCCB that protects a parallel set of 500
kcmil, copper, type THWN feeder conductors.
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Figure 18. Typical Log-Log Paper
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Figure 19. MCCB Protecting A Feeder Conductor
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An MCCB is tested in open air (not in an enclosure) to verify its nameplate ampere rating.
The nameplate specifies a value of current that the circuit breaker is rated to carry
continuously, without tripping, within specific operating temperature guidelines. In most
cases, however, an MCCB is used in an enclosure, but not in open air. Therefore, the
performance of a breaker inside an enclosure, as with any other overcurrent device, could be
adversely affected by heat dissipation and temperature rise. These factors must be considered
regarding the ability of the breaker to comply with its nameplate ampere rating.
Section 220-10(b) of the NEC, which covers continuous and noncontinuous loads, states that:
“Where a feeder supplies continuous loads or any combination of continuous and
noncontinuous loads, the rating of the overcurrent device shall be less than the noncontinuous
load plus 125% of the continuous load”. The same NEC section permits the following
exception: “Where the assembly including the overcurrent devices protecting the feeder(s)
are listed for operation at 100% of their rating, neither the ampere rating of the overcurrent
device nor the ampacity of the feeder conductors shall be less than the sum of the continuous
load plus the noncontinuous load”. Figure 20a shows a typical application using a standard
design approach and Figure 20b shows the same application using 100 percent rated MCCBs.
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Figure 20. Typical MCCB Applications
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Low Voltage Power Circuit Breaker (LVPCB)
Long Time Functions include the long time pickup and the long time delay functions. Long
time pickup (LTPU) functions or long delay pickup (LDPU) settings are adjustable from
0.5 - 1.0 times the plug rating (In). The plug rating is also a function of the sensor rating
(current transformer), which establishes the continuous current rating of the breaker. Typical
tolerances for a modern day solid-state trip (SST) (e.g., Westinghouse Digitrip RMS) are 0%, + 10% (Figure 21).
Figure 21. Long Time Pickup (LTPU) T/C Characteristics
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Long time delay (LTD) or long delay time (LDT) settings are adjustable from 2 - 24 seconds
at
6 times the plug rating (6In). Typical tolerances are + 0%, - 33% (Figure 22).
Figure 22. Long Time Delay (LTD) T/C Characteristics
Short Time Functions include the short time pickup and short time delay functions. Short
time pickup (STPU) or short delay pickup (SDPU) settings are adjustable from 2 to 6 times
the plug rating (2 - 6In) plus two variable settings of S1 (8In) and S2 (10In). Typical tolerances
are + 10%, - 10% (Figure 23).
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Figure 23. Short Time Pickup (STPU) T/C Characteristics
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Short time delay (STD) or short delay time (SDT) settings are available with five flat
responses of 0.1, 0.2, 0.3, 0.4 and 0.5 seconds. Typical tolerances are variable depending on
the setting (Figure 24).
I2t Function settings are available in three responses of 0.1*, 0.3*, and 0.5* seconds, and the
settings revert back to a flat response at 8In (Figure 24). The asterisk (*) refers to I2t settings.
Figure 24. Short Time Delay (STD) With I2t T/C Characteristics
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Instantaneous Trip (IT) settings are adjustable from 2 to 6 times the plug rating (2 - 6In)
plug two variable settings of M1 (8In) and M2 (12In). Typical tolerances are + 10%, - 10%
(Figure 25).
Figure 25. Instantaneous Trip (IT) T/C Characteristics
Ground Fault Functions - SAES-P-114 requires ground fault protection, where required by
the NEC, that uses window-type current transformers (CTs) similar to the BYZ CT, as shown
in Figure 26. The tripping function, unlike its MCCB counterpart, is part of the same SST
unit.
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Figure 26. GFP With Window-Type CT
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Ground fault pickup (GFPU) settings have eight discrete adjustments (A, B, C, D, E, F, H, K),
which are a function of the plug ratings (In). Figure 27 shows a sample listing of the settings
for plug ratings of 100, 200, 250, and 300 amperes. Typical tolerances are + 10%, - 10%
(Figure 28).
In
100
200
250
300
A
25
50
63
75
B
30
60
75
90
C
35
70
88
105
D
40
80
100
120
E
50
100
125
150
F
60
120
150
180
H
75
150
188
225
K
100
200
250
300
Figure 27. Sample GFPU Code Letters and Settings
Figure 28. Ground Fault Pickup (GFPU) T/C Characteristics
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Ground fault time (GFT) or ground fault delay time (GFDT) settings, like the SDT settings,
are available with 5 flat responses of 0.1, 0.2, 0.3, 0.4, and 0.5 seconds. Typical tolerances
are variable depending on the setting (Figure 29).
I2t functions settings are also available for ground fault functions in 3 responses of 0.1*, 0.2*,
and 0.3* seconds, and the settings revert back to a flat response at 0.625 In (Figure 29).
Figure 29. Ground Fault Time (GFT) With I2t T/C Characteristics
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Coordination Principles
Molded Case Circuit Breakers (MCCBs) are difficult to coordinate, especially in the
smaller frame sizes of 100 and 225 amperes. Even the larger frame breakers are difficult to
coordinate unless sufficient impedance (very long cable runs) exists between the feeder
breakers and the other downstream breakers. Modern-day larger frame breakers with solidstate (electronic) trips are much easier to coordinate.
Two MCCBs in series will coordinate if their plotted T/C characteristic curves do not overlap
or cross one another.
Low Voltage Power Circuit Breakers (LVPCBs) are fairly simple to coordinate because of
the numerous settings and adjustments available for their long time, short-time, instantaneous,
and ground fault functions.
As with MCCBs, two LVPCBs in series will coordinate if their plotted T/C characteristic
curves do not overlap or touch one another.
Figure 30 shows a main breaker (DS-632) coordinating with a feeder breaker (DS-206) that is
protecting a 500 kcmil copper conductor.
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Figure 30. LVPCB Coordination
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Factors Affecting Selection
The properly rated MCCB for a specific application can be selected by determining the
following listed parameters. There are other factors, for example, unusual operating
conditions, altitude, frequency, etc; however, for purposes of this Module, the factors
affecting MCCB selection are limited to the following:
•
•
•
•
•
Temperature
Continuous Loads
Voltage
Round-up Rule
Round-down Rule
Temperature
Thermal magnetic circuit breakers are temperature sensitive. At ambient temperatures below
40°C, circuit breakers carry more current than they are rated to carry in continuous current.
Nuisance tripping is not a problem under these lower temperature conditions, although
consideration should be given to closer protection coordination to compensate for the
additional current carrying capability. In addition, the actual mechanical operation of the
breaker could be affected if the ambient temperature is significantly below the 40°C standard.
Note: The 40°C standards for MCCBs is based on the fact that 40°C is the average
temperature of an enclosure, for example, a lighting panelboard.
For ambient temperatures above 40°C, breakers will carry less current than they are rated to
carry in continuous current. This condition promotes nuisance tripping, and it can create
unacceptable temperature conditions at the terminals. Under this condition, the circuit
breaker should be recalibrated for the higher ambient temperature. Recalibrating MCCBs is
not a “user” option; recalibration must be accomplished by the manufacturer. The best option
when ordering new breakers is to order ambient-compensated breakers.
Continuous Loads
MCCBs are rated in amperes at a specific ambient temperature. The ampere rating is the
continuous load current that the breaker will carry in the ambient temperature for which it is
calibrated. Most manufacturers calibrate their standard breakers for a 40°C (104°F) ambient.
The continuous current ampere rating typically signifies the amount of current that the MCCB
is designed to carry continuously in open air. In accordance with Article 220-10(b) of the
NEC, all overcurrent protection devices may be loaded to a maximum of 80% of their
continuous ampere rating, unless specifically listed for 100% application. A number of
manufacturers offer circuit breakers that can be applied at 100% of their continuous rating.
These 100% rated breakers specifically outline on their nameplates the minimum size
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enclosure, the minimum ventilation (if needed), and the minimum conductor size for
application at 100% of their ratings.
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Voltage
The voltage rating of an MCCB is determined by the maximum voltage that can be applied
across its terminals, the type of distribution system, and how the breaker is applied in the
system. For example, the most prevalent secondary distribution voltage in commercial and
institutional buildings today is 480Y/277 volts, with a solidly grounded neutral. It is also a
very common secondary voltage in industrial plants and even in some high rise commercial
buildings that are centrally air-conditioned. In panelboards, it is important that the MCCB
have the lowest possible voltage rating that will do the job and meet the specifications. An
overly conservative selection can make a considerable difference in the cost.
It is critical that the proper application, testing, and governing standards be understood before
any attempt is made to apply dual voltage-rated breakers. Article 240-83(e) of the NEC
defines the allowed application of circuit breakers with straight or slash voltage markings.
International applications, where 415Y/240, 380Y/220, or 220Y/127 volt systems are used,
are all grounded “Y” systems similar to the U.S. 480Y/277 volt system. Only IEC 947-2
breakers tested per Appendix C and marked accordingly (C 480V) are suitable for application
on delta systems. Single pole testing at maximum voltage is not a requirement of IEC 947-2
except for this optional test under Appendix C. This test is a standard test under the UL 489
requirements for all voltage ranges.
Round-up and Round-down Rules
Per NEC Article 240-3, conductors should be protected against overload in accordance with
their ampacities as specified by NEC Article 310-15 and Tables 310-16 through 310-19 for
conductors rated 0-2000 V. The next higher standard device is permitted if the protective
device does not exceed 800 A (round-up rule). See NEC Article 240-3(b). The conductor
ampacity must be greater than the protective device setting/rating if the protective device
exceeds 800 A (round-down rule). See NEC Article 240-3(c).
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SELECTING FEEDER CONDUCTOR RACEWAY SIZES
Note: Work Aid 1J has been developed to teach the Participant procedures to select feeder
conductor raceway sizes.
Types of Raceways
Note: For purposes of this Module, only wireways and conduits will be considered in
selecting of feeder conductor raceway sizes.
Wireways
The NEC (Article 362-1) defines wireways as sheet metal troughs with hinged or removable
covers for housing and protecting electric wires and cable and in which conductors are laid in
place after the wireway has been installed. Figure 31 describes a typical length of wireway
with a hinged cover. Note: The NEC does not permit concealment of wireway; it must
remain exposed.
Figure 31. Length of Wireway
Conduits
SAES-P-114 lists the following specifications pertaining to conduits:
•
Conduit, above ground in outdoor, industrial facilities shall be hot-dip
galvanized rigid steel.
•
Direct buried conduit shall be (a) threaded, hot-dip galvanized rigid steel and
polyvinyl chloride (PVC) coated or (b) type direct burial (DB) PVC conduit per
NEMA TC 8.
•
Other types of conduits shall be permitted where used and installed in
accordance with the NEC.
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Rigid Metal Conduit (RMC) has almost unlimited restrictions for use in industrial
installations. RMC is most often used where moisture is present and where cables are
subjected to mechanical damage. RMC uses threaded fittings.
Intermediate Metal Conduit (IMC) is very similar to RMC, and it also has very limited
restrictions for use. The major difference between the two conduits is that RMC provides
better mechanical damage for conductors than does IMC. Note: SAES-P-104 prohibits the
use of IMC in classified areas.
Electric Metallic Tubing (EMT) is a non-threaded metallic conduit used primarily for indoor
protection of conductors. EMT is not permitted in permanent moisture locations, although it
is permitted for both exposed and concealed wet locations. EMT uses compression type
couplings and fittings.
Rigid Nonmetallic Conduit (PVC) is permitted for both concealed and exposed locations as
long as it is not subject to physical damage. PVC is not permitted in hazardous locations.
Flexible Metal Conduit is most often used for motor circuits or other types of circuits
subjected to vibrations. Flexible metal conduit is not permitted in underground installations,
in most hazardous locations, in wet locations, or in any areas subject to physical damage.
Factors Affecting Raceway Size
The number of conductors permitted in a raceway is restricted by the NEC. The total crosssectional area of the conductors, which includes the insulation, must not exceed a specified
percentage of the wireway or conduit cross-sectional area. The NEC refers to this restriction
as “percentage fill”. Exceeding the percentage fill can cause physical damage to the
conductors as they are being installed (pulled) through the raceway. Additionally, the heat
buildup in the raceway could be excessive, resulting in damage to the insulation.
Conductor Insulation
Because each type of conductor has different insulation thicknesses, the percentage fill for
each type of conductor (insulation) is different. Chapter 9, Table 5, of the NEC lists the
dimensions (diameter and area) for each size and type of rubber-covered and thermoplasticcovered conductors.
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Number of Conductors
The number of the same type of conductors (insulation) in conduit can be determined from
Chapter 9, Tables 3A and 3B of the NEC. The number of conductors in wireway, or mixed
sizes and types of conductors (insulations), must be calculated based on the fill rates permitted
by the NEC.
Tables - NEC Tables 3A and 3B are based on a 40 percent fill rate of conductors in a given
trade size of conduit (Figure 32). If conductor insulation types are mixed, the tables cannot
be used and the fill rate for a particular mix of conductors in a given trade size of conduit
must be calculated. The number of conductors, for fill rate purposes, includes all conductors,
regardless of whether they are considered as current-carrying conductors. For example, the
“green” equipment grounding conductor is considered for percentage fill restrictions even
though it does not carry current except under line-to-ground fault conditions.
Figure 32. Conduit Fill Rate Example
Example J:
Referring to Table 3B of the NEC, what is the maximum number of 500 kcmil,
type THHN conductors in 3-inch conduit?
Answer J:
Per Table 3B (page 914 of Work Aid 1A, Handout 1), the maximum number of
500 kcmil type THHN conductors in 3-inch conduit is 4.
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Fill Rate Calculations - NEC Article 362-5 restricts the fill rate of wireways to the following:
no more than 30 current-carrying conductors, and/or the sum of all conductor areas shall not
exceed a fill rate of 20 percent of the interior cross-sectional area of the wireway. Note: The
derating factors specified in Article 310 for more than 3 conductors in a raceway are not
applicable to the 30 current-carrying conductors and 20 percent fill rate specified above.
Table 1, Chapter 9, of the NEC specifies a fill rate of 40 percent for more than three
conductors in a raceway. Table 4 lists the diameter, total area, and 40 percent fill rate area of
conduit. Tables 5, 5A, and 5B lists the dimensions (diameter and area) of bare, rubbercovered, and thermoplastic-covered conductors. All of the tables must be used to calculate
the fill rate of a conduit, when the insulation types and wire sizes installed in conduit are
mixed.
Example K:
Given the following list (Figure 33) of conductors, what is the minimum size
conduit permissible to enclose the conductors?
Circuit
Identification
Insulation
Type
Feeder A
Feeder B
Feeder C
THHW
THHN
THHN
Size of Copper
Conductors
3-No.4/0, 1-No.2/0
4-No.2/0, 1-No.1/0
4-No.1/0, 1-No.2
Figure 33. Example K Conductor Listing
Answer K:
Feeder A: 3 x 0.3904 + 1 x 0.2781
Feeder B: 4 x 0.2265 + 1 x 0.1893
Feeder C: 4 x 0.1893 + 1 x 0.1182
Total
=
=
=
=
1.4493 in2 (Table 5)
1.0953 in2 (Table 5)
0.8754 in2 (Table 5)
3.4200 in2
Per Table 4, a 3.5-inch conduit (3.96 in2) is acceptable at a 40 percent fill rate.
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WORK AID 1: RESOURCES USED TO DESIGN A LOW VOLTAGE NON-MOTOR
FEEDER CIRCUIT
Work Aid 1A:
1993 National Electric Code Handbook
For the content of Work Aid 1A, refer to Handout 1.
Work Aid 1B:
ANSI/IEEE Standard 141-1986 (Red Book)
For the content of Work Aid 1B, refer to Handout 2.
Work Aid 1C:
SAES-P-114
1.
Section 4.2.1 specifies that all system and equipment protection shall conform to
NFPA 70 (National Electric Code - NEC) as supplemented by this Standard.
2.
Section 4.7.1 specifies that low voltage feeder circuits shall be protected by circuit
breakers and that all interrupting devices shall be fully rated.
3.
Section 4.7.3 specifies that overcurrent protection for wires and cables connected to
full-size molded case circuit breakers shall be based on NEC Article 110-14.
4.
Section 4.8.3 specifies that all low voltage power circuit breakers shall include either
an integral protective device with adjustable ground unit or shall be tripped by a
ground overcurrent relay.
5.
Section 4.8.4 specifies that low voltage 480 volt molded case circuit breakers rated 70
amperes and greater shall be provided with one of the following:
6.
a)
An electronic trip device with adjustable ground unit, where available from the
manufacturer for the required frame size, or
b)
A thermal-magnetic trip unit with separate adjustable 50G ground sensor and
shunt trip device.
Section 10.2.3 specifies that protection of low voltage radial feeders may be provided
by integral solid-state breaker trip devices, which shall be supplied with long-time,
short-time phase overcurrent trips, and time-delayed ground fault trips. Instantaneous
trips shall only be provided for dedicated feeders where there is no downstream
protective device.
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Work Aid 1D:
SAES-P-100
1.
Section 5.6 specifies that maximum steady state total voltage drop for a main, feeder,
and branch circuit shall not exceed five (5) percent.
2.
Section 5.6.1 (E) specifies that summation of voltage drops in circuit from main to
distribution center and from feeder breaker to panelboard shall be two (2) percent at
full load.
3.
Section 5.6.1 (F) specifies that other voltage drops shall average two (2) percent with a
maximum of four (4) percent to the most distant outlet at full load.
4.
Section 6.2 specifies that the following criteria shall be used to establish equipment
derating when specific requirements are not covered in an SAES or SAMSS.
Location
Ambient Temperature
Average Monthly
Maximum
Normal Maximum
Daily Peak
0
0
C
C
Outdoors
45
50
Indoors Well Ventilated Buildings
40
50
Indoors Air-Conditioned Buildings
Unmanned Areas
35
35
Indoors Air-Conditioned Buildings
Manned Areas
30
30
Figure 35. SAES-P-100 Ambient Temperatures
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Work Aid 1E:
SAES-P-104
1.
Section 4.1 specifies that design and installation of wiring and cable systems shall be
in accordance with NFPA 70 (National Electric Code - NEC), as supplemented by this
Standard.
2.
Section 4.2.1 specifies that wire and cable shall have copper conductors.
3.
Section 4.2.2 specifies that low voltage wire and cable (600 V or 600/1000 V and
below) shall have a minimum rating of 75°C.
4.
Section 4.2.5 specifies that power conductors shall be of stranded copper, except that
2
solid copper conductors 6 mm (No. 10 AWG) and smaller may be used in nonindustrial locations and for specialty applications.
5.
Section 4.2.10 specifies that for 600 V and below power conductors, the minimum size
permitted is 2.5 mm2 (No. 14 AWG).
6.
Section 4.3.1 specifies that direct buried conduit shall be threaded, rigid steel, hot-dip
galvanized, and PVC coated, or type DB PVC conduit.
7.
Section 4.3.2 specifies that conduit above ground in outdoor industrial facilities shall
be threaded, rigid steel, and be hot-dip galvanized.
8.
Section 4.3.4 specifies that the minimum conduit size shall be 3/4 inch, except on
instrument panels, inside buildings, and on prefabricated skids, where the minimum
size conduit shall be 1/2 inch.
9.
Section 4.3.6 specifies that electric metallic tubing (EMT) is acceptable only in
nonhazardous indoor locations.
10.
Section 5.1 specifies that the minimum burial depths for 600 V and below
underground installations shall be the following:
•
Direct Buried cables - 600 mm (24 in.)
•
Direct buried PVC conduit - 460 mm (18 in.)
•
Duct bank and direct buried rigid steel conduit - 460 mm (18 in.)
•
PVC conduit, rigid steel conduit, or duct bank under roads, parking lots, and
other areas that are subject to vehicular traffic - 600 mm (24 in.)
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11.
Section 8.1 specifies that the sizing of power cables in the Saudi Aramco system shall
be the following:
•
load factor - 100 percent of maximum steady state load
•
direct buried cable - 40°C ambient temperature
•
cable in conduit (in air) - 60°C exposed to sun and 50°C shaded or indoor.
12.
Section 8.2.1 specifies that feeders supplying transformers shall have an ampacity of
not less than the sum of the full-load ratings (fan-cooled ratings, if fans are installed)
of all connected transformers and all other connected loads.
13.
Section 8.2.2 specifies that a feeder cable serving a load bus shall be sized in
accordance with the NEC plus a 20 percent growth factor, but the size is not to exceed
the maximum rating of the bus. Note: Section 8.2.3 permits deletion of the growth
factor where two or more feeders serve the same bus.
14.
Section 8.2.4 specifies that the ampacity of a feeder directly connecting the secondary
of a transformer to a load bus shall not be less than the full-load rating (forced-air
rating, if fans are installed) of the transformer.
15.
Section 8.2.5 specifies that a derating factor of 15 percent shall be applied to power
cable that requires fireproofing.
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Work Aid 1F:
Step 1.
Applicable Procedures for Calculating Ampacity Ratings of Feeder
Conductors
Determine the feeder load current in accordance with the following formulas: Note:
1.2 factor is the 20 percent growth factor required by SAES-P-104.
a.
b.
Three-phase non-motor loads:
•
IL = 1.2 x kVA/( 3 x kV) or
•
IL = 1.2 x kW/( 3 x kV x p.f.)
Three-phase transformer feeder loads based on the self-cooled (OA) rating
or forced-air (FA) rating if fans are installed:
•
IL = kVA/( 3 x kV)
Step 2.
Initially select a 75°C or 90°C conductor form NEC Table 310-16 (Handout 2,
page 248). Note: Consider use of parallel conductors for required conductor sizes
500 kcmil and larger.
Step 3.
Apply derating (correction) factors (if applicable) as follows:
Step 4.
a)
Fireproofing (where required): 15 percent
b)
Ambient temperature: Select the correction factor from NEC Table 310-16
(Handout 2, page 248).
c)
More than three current-carrying conductors in a raceway: Select the
correction factor from Note 8 to NEC Table 310-16 (Handout 2, page 253).
Specify the selected conductor (or parallel conductors) as follows:
•
Size - AWG or kcmil Note: See Figure 36 for the nearest metric equivalent
size conductor.
•
Material - Copper (Cu)
•
Insulation Type - i.e., THWN, THHN, etc.
•
Number of Conductors - i. e., 4/C, 8/C
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CONDUCTOR SIZES
AWG
or
kcmil*
mm2
14
12
10
8
6
4
2
1/0
2.5
4
6
10
16
25
35
50
AWG
or
kcmil*
mm2
2/0
4/0
250*
350*
500*
750*
1000*
70
120
120
185
240
400
500
Figure 36. Standard Saudi Aramco Wire Sizes
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Work Aid 1G:
Step 1.
Step 2.
Applicable Procedures for Calculating the Short
Feeder Conductor
Circuit Rating of a
Estimate the protective device clearing times as follows:
a)
Molded Case Circuit Breakers (MCCBs): 1.1 cycles (18 msec) for less than
100 ampere frame (AF) and 1.5 cycles (25 msec) for 225 AF and larger in
the MCCB’s instantaneous range (approximately five times the ampere
trip), and over 100 sec at low magnitude overloads.
b)
Low Voltage Power Circuit Breakers (LVPCBs): 3 cycles (0.05 sec) for
instantaneous ranges, 6 to 30 cycles (0.10 - 0.50 sec) in the short time
ranges, over 100 sec in the long time ranges, and 6 to 30 cycles (0.10 - 0.50
sec) for their ground fault ranges.
Calculate the magnitude of the fault current (total asymmetrical) in accordance with
the following formula:
•
Iasy = Isym x ko
•
where:
Isym = rms symmetrical fault current
Note: Isym is assumed as a “given quantity” for purposes of this Work Aid.
ko= correction factor accounting for the dc component of current.
= 1.6 for MCCBs
= 1.3 for LVPCBs
Step 3.
Calculate the duration (t) of of the short circuit in accordance with the following
formula:
•
t = .0297 log10 [(T2 + 234)/T1 + 234)] x (A/Iasy)2
•
where: t = short circuit duration in seconds
T1 = initial conductor temperature in degrees Celcius (°C)
= 75°C or 90°C for NEC thermoplastic (PVC) conductors
T2 = final conductor temperature in degrees Celsius (°C)
= 150°C for NEC thermoplastic (PVC) conductors
A = conductor cross-sectional area in circular mils (cmils)
Note: See Table 8 of the NEC Handbook (Handout 1,
page 919).
Iasy= maximum asymmetrical short circuit current in amperes
calculated in Step 2.
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Step 4.
Compare the short circuit duration (t) calculated in Step 3 to the estimated
protective device clearing times from Step 1. If t is less than the protective
device’s clearing time, increase the conductor size to the next standard available
size and repeat Steps 3 and 4 until t is greater than the protective device’s clearing
time.
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Work Aid 1H:
Applicable Procedures for Calculating the Voltage
Feeder Conductor
Step 1.
Obtain the specified one-line diagram.
Step 2.
Calculate load current (IL).
Drop
of
a
IL = (kVA)/[( 3)(kV)] = (kW)/[( 3)(kV)(p.f.)]
Step 3.
Calculate the load power factor angle θ = cos-1 p.f.
Step 4.
Calculate the load reactive factor (sin θ). sin θ = sin (cos-1 p.f.)
Step 5.
Determine feeder impedance (ZΩ) per 1000 feet from NEC Table 9 (Handout 1,
page 920).
Step 6.
Calculate the feeder impedance.
ZΩ = ((R + jX) Ω per 1000 ft) (number of feet)
Step 7. Calculate VD line-to-neutral.
VD = I(R cos θ + X sin θ)
Step 8.
Calculate VD line-to-line.
VD = 3 VD (3f system)
VD = 2VD (1φ system)
Step 9.
Calculate the load voltage (VL).
VL = VS - VD
Step 10.
Calculate VD as a percentage (VD%).
VD% = 100[(VS - VL)/VS]
Step 11.
If VD% exceeds 2 percent, increase the conductor to the next standard size and
repeat Steps 6 through 11.
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Work Aid 1I:
Applicable Procedures for Selecting a Feeder Conductor Protective
Device
Step 1.
Select the ampacity rating of an MCCB or an LVPCB based on the ampacity of the
conductors as calculated in Work Aid 1F. Note: NEC Article 240-6 (Handout 1,
pages 142 and 143) lists the standard ampere ratings of circuit breakers. Note:
NEC Article 240-3 (Handout 1, page 141) requires low voltage conductors to be
protected in accordance with their ampacities as listed in NEC Tables 310-16
through 310-19 (Handout 1, pages 248 through 251) and their accompanying notes
(Handout 1, pages 252 through 255).
Step 2.
If the conductor ampacity does not correspond with a standard ampere rating of a
circuit breaker, select the next standard ampere-rated device. (Note: See NEC
Article 240-3(b) (Handout 1, page 141). If the next standard ampere-rated
protective device exceeds 800 A, the conductor ampacity must be increased to
equal or exceed the device rating or the protective device must be decreased to the
next standard lower ampere-rated protective device. Note: See NEC Article 2403(c)
(Handout
1,
page 141).
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Work Aid 1J:
Applicable Procedures for Selecting a Feeder Conductor Raceway Size
Step 1.
Select the size of the equipment grounding conductor from NEC Table 250-95
(Handout 1, page 199) based on the size of the selected protective device from
Work Aid 1I.
Step 2.
For type of conductor and number of conductors all of the same size, select the
conduit (raceway) size from Tables 3A, 3B, or 3C of the NEC (Handout 1, pages
913, 914, and 915). Note: The term “conductors” includes the phase conductors,
the neutral conductor, and the equipment grounding conductor.
Step 3.
For type of conductor and number of conductors of different sizes:
a)
Using Table 5 of the NEC (Handout 1, page 916), compute the total crosssectional area of the conductors.
b)
Select a conduit from Table 4 of the NEC (Handout 1, page 915),where the
40% fill rate area of the standard conduit size is greaterthan the conductor
cross-sectional area computed in Step 3a.
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Work Aid 1K: Feeder Circuit Design Flow Chart
Figure 37. Feeder Circuit Design Flow Chart
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GLOSSARY
AWG American Wire Gauge
branch circuit
The circuit conductors that are between the final overcurrent protective
device and the load.
bus feeder A type of feeder that supplies power to the main bus of switchgear, a
switchboard, or a panelboard.
busway
Solid copper bars that are insulated and covered in a metal enclosure and that
are used to carry large currents for short distances.
cable A stranded conductor or a combination of conductors that are insulated from each
other.
circuit conductor The conductor that is the current carrying element of a branch or feeder
circuit. This circuit conductor is usually cable or busway.
circular mils (cmil) A measure of the diameter of a wire that equals the diameter of the wire
in mils (1/1000 of an inch) squared.
conduit
A metallic or non-metallic tube that is used to mechanically protect electrical
wires and cables.
correction factor
Factors that are used to derate equipment due to high ambient
temperatures, type of installation, spacing, etc.
derating factor
Factors that are used to derate equipment due to high ambient
temperatures, type of installation, spacing, etc. See correction factor.
direct burial A method of installing cable underground, where the cable is placed in a trench
or ditch and covered with soil.
duct bank
A method of underground installation of cable, where the duct bank consists of
metallic or non-metallic conduit encased in concrete. The complete assembly is buried in a
trench or ditch.
feeder circuit All circuit conductors that are between a source and the final branch circuit
overcurrent protective device.
forced-cooled rating
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A kVA rating on a transformer that is the transformer (FA)
rating with the fans operating.
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growth factor
expansion.
A factor that is applied to equipment to allow for future
ICEA
Insulated Cable Engineers Association
IEC
International Electrotecnical Commission
kcmil
(cmil).
A unit of measure of wire sizes that equals 1000 circular mils
NEC
National Electric Code
NEMA
National Electrical Manufacturer’s Association
power cable
equipment.
A conductor or a group of conductors that supplies current to
raceway
Any channel that holds wires, cables, or bus bars and it may be
metallic or non-metallic. Examples of raceway are conduit, cable duct, and cable tray.
self-cooled rating (OA)
A rating on a transformer that is the kVA rating of a transformer
without the use of any additional cooling methods, such as fans.
transformer feeder
A type of feeder that supplies power to a transformer.
utilization voltage
The operating voltage required by a specific piece of equipment.
Utilization voltage is the equipment’s rated operating voltage.
voltage drop
The amount of voltage that is measured (lost) on a conductor
between the source and the load. The amount of voltage drop depends on the cable
impedance, amount of current, cable size, and length of cable.
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