NEC CONDUCTOR CALCULATIONS
PDH Enterprises, LLC
PO Box 942
Morrisville, NC 27560
(919)208-5296
NEC Conductor Calculations
Course #201
This course provides a review of the basic calculations associated with
Article 310 of the National Electrical Code. Students successfully
completing this course will be able to correctly size circuit conductors and
apply necessary temperature correction and derating factors. Students will
also be shown the difference between continuous and non-continuous loads
and the considerations that must be adhered to when working with them.
You may need to reference the National Electrical Code book for tables that
will be used in this course.
To receive credit for this course, each student must pass an online multiple
choice exam of ten (10) questions. A passing score is 70% or better.
Completion of this course and successfully passing the exam will qualify the
student for one (1) hour of continuing education credit. All information
necessary to complete the examination will be presented within this course
document and within the 2008 version of the National Electrical Code or
NEC.
Course Author:
Chris Barrow, PE
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NEC CONDUCTOR CALCULATIONS
General
In this chapter we will be learning how to size general branch circuit conductors and
overcurrent protection devices. The following definitions and NEC Articles should be
understood or looked up for you to get a basic understanding of the subject matter
covered in this chapter. Code definitions are presented in Article 100 of the NEC.
Ampacity. The current, in amperes, that a conductor can carry continuously under the
conditions of use without exceeding its temperature rating.
Ampacity is the amount of current a conductor can carry as listed in the NEC Tables or
after the conditions of use have been applied such as (1) ambient temperature derating
factor; (2) the number of current carrying conductors in a raceway derating factors.
Ambient Temperature is the temperature of the area surrounding a conductor and/or the
raceway. The ambient temperature selected for ampacity correction factors will be based
upon the highest temperature a conductor is subjected to from and including its source to
the load being served.
Branch Circuit Rating - Article 210.3 states that the rating of the overcurrent device
determines the circuit rating.
Article 210.3 also states that the overcurrent protection device (OCPD) determines the
maximum rating of a branch circuit even if the ampacity of the conductor is higher.
Example: We have a 20 amp fuse connected to a 30 amp wire.
This would be a 20 amp circuit according to Article 210.23 Permissible Loads.
The total loads shall not exceed the rating of the branch circuit.
For selecting the over-current protection device for a circuit, Article 240.4(B) states that
where the ampacity of the conductor does not correspond with the standard ampere rating
of a fuse or circuit breaker, the next higher rated device may be used if the following
conditions are met
(1) The conductors being protected do not supply cord and plug connected loads
(2) The ampacity does not correspond to a standard rating
(3) The next higher standard rating is less than 800 amps.
The standard size ratings of Fuses and Fixed-trip circuit breakers are listed in Article
240.6. They are as follows: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125,
150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 100, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes.
Conductor Lettering. When looking at wiring properties, you will notice that the wires
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NEC CONDUCTOR CALCULATIONS
contain some type of letter identifier such as THHN or THW. These letters serve to
identify specific properties of the conductor and/or its insulation. Listed below are some
of the letters commonly used (See 310.13, Table 310.13, and Table 402.3)
T - Thermoplastic
F - Fixture Wires
W - Wet or Damp
R - Rubber
H - Heat
FF - Fixture wire, flexible stranding
The combination of letters will tell you most of what you need to know about the
conductor. The first letter indicates what the insulation is made of. One H indicates a
conductor insulation rating of 75° C. HH would indicate a conductor insulation rating of
90° C. Zero H’s usually indicate an insulation rating of 60° C.
Examples:
TW - Thermoplastic (T) insulation, (W) suitable for wet and dry locations (When suitable
for wet generally means it can be used for dry also), and is rated 60° C (no H in group).
RHW - Rubber (R) insulation, (H) rated 75° C and (W) suitable for wet and dry
locations.
NOTE: This is a general rule and there are exceptions such as THW (see Table 310.13).
The one H indicates 75° C rating but it can be used at a 90° C conductor with ballasts in
dry locations.
TF - Thermoplastic (T) insulation, (F) fixture wire
RFH – Rubber (R) insulation, (F) fixture wire, (H) rated 75° C
Using Wire Tables
Tables 310.16 through 310.19 are the ampacities of one to three conductors rated 0-2000
volts. These tables all work basically the same and you will probably find several of them
useful. Table 310.16 is the most commonly used table for the type of conductors and
installations generally used and it is based on temperatures and conditions most likely to
be found in normal field conditions.
For each Table 310.16 through 310.19 the conditions under which the ampacities given in
the table are based on is always stated at the top of the table.
Table 310.16 is based on an ambient temperature of 30°C (86° F), and not more than
three conductors in a raceway. This means the conductors listed have been tested and
found suitable for use under these conditions.
Table 310.16 is basically two tables, the left half is temperature rating of the conductor,
size and insulation type for copper and the right side is for aluminum.
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The main body of this table gives us the ampacity of a conductor. Example: a #4 copper
THW has an ampacity of 85 amps.
The lower part of the Table 310.16 gives us the correction factors for temperatures other
than 30° C or 86° F. These correction factors when used will establish a new ampacity for
any conductor. The absence of a multiplying factor in a column indicates that this
conductor with this insulation shall not be used in these ambient temperatures.
The asterisk (*) next to wire sizes 10, 12, and 14 AWG refers to article 240.4(D) and is
letting you know that conductor sizes #14, #12, and #10 copper and aluminum are limited
to certain size overcurrent protection devices (OCPD) even if the ampacity of the #14,
#12, and #10 conductor is higher. The limitations are:
•
#14 copper limited to 15 amp OCPD
#12 copper limited to 20 amp OCPD
#10 copper limited to 30 amp OCPD
•
#14 aluminum not allowed
#12 aluminum limited to 15 amp OCPD
#10 aluminum limited to 25 amp OCPD
Table 310.15(B)(2)(a): Adjustment Factors
Due to the closeness of conductors in the same raceways or trench when more than three
current carrying conductors are together, the ampacity of the conductors must be lowered
because the heat generated cannot be displaced as quickly due to the bunching of
conductors. This is called derating.
When there are more than three current carrying conductors in a raceway, we must use
310.15(B)(2)(a) to reduce conductor ampacities.
The following are not considered current carrying conductors:
(1) Control wiring conductors, T310.15(B)(2)(a), Exception No. 1
(2) Conductors in cable trays, 310.15(B)(2)(a), Exception No. 2
(3) Conductors in nipples (24” or less), Exception No. 3
(4) Neutral conductors of balanced circuits of three or more conductors,
310.15(B)(4)(a)
(5) A grounding or bonding conductor, 310.15(B)(5)
The following are current carrying conductors:
(1) All phase conductors or Ungrounded conductors (hot wires)
(2) Neutral conductors in a circuit consisting of two phase conductors and the
neutral conductor of a 4-wire 3-phase wye-connected system. 310.15(B)(4)(b)
(3) Neutral conductor of a 3-phase, 4-wire, wye circuit where the major portion of
the load consist of nonlinear loads, 310.15(B)(4)(c)
Example: Fluorescent lighting, computer equipment.
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(4) Any 2-wire circuit (series circuit)
Practice Problems
I. Wire sizing. Using Table 310.16, find the conductor ampacity and overcurrent
protection.
Wire Size/Type
#12 THW
#12 THHN
#4 TW
3/0 XHHW-2 al
500 kcmil THHN
Ampacity
OCPD Required
II. Ambient Temperature Correction Factors. Using Table 310.16, what is the
correction factor for the conductor and ambient temperatures?
Ambient
Temperature
55° C
72° C
35° C
Conductor
Correction Factor
THHN
THWN
TW
III. To find new ampacity of a conductor using the correction factors you multiply the
ampacity of the conductor by the correction factor.
New Ampacity = (conductor ampacity * correction factor)
Find the new ampacity for the following conductor operating in the ambient temperature
listed.
Conductors
#6 TW cu.
250 kcmil THHN
#1 RH al.
#3 TW
#12 THW
Ambient
Temperature
47° C
78° C
89° F
44° C
50° C
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New Ampacity
OCPD
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IV. Table 310.15(B)(2)(a) - Derating - No Load Diversity
Derated Ampacity = Conductor ampacity * factors in Table 310.15(B)(2)(a)
Find the correction factor and new ampacity for the following conductors in a conduit.
NOTE: To work the problems below first find the ampacity of the conductor in Table
310.16.
Conductor in a Conduit
8 - no. 3 THW wires
6 - no. 1/0 ZW wires
40 - no. 10 THHN wires
12 - no. 2/0 XHHW dry
Correction Factor
New Ampacity
V. New ampacity of conductors when you have more that 3 conductors in a raceway
and a temperature correction factor is determined as follows.
New ampacity = (conductor ampacity * temperature correction factor * adjustment factor
in Table 310.15(B)(2)(a))
Find the maximum load for the following:
Conductors
6 - no. 6 TW
25 - no. 3/0 THW al.
43 - no. 12 SIS
20 - no. 10 RHH al.
5 - no. 3/0 RHH
Ambient Temperature
105° F
40° C
112° F
47° C
150° F
New Ampacity
Continuous vs. Non-continuous loads
The ampacities listed on the Tables in Article 310 are tested and approved for continuous
use. The table ampacities can be used as continuous or non-continuous loads after all
temperature and derating factors have been applied.
A continuous load is a load where the maximum current is expected to continue for 3
hours or more. It is for this reason that the conductors and their overcurrent protection
devices face special rules to account for the increased heat that it will encounter during
operation.
Typical continuous loads are those such as lighting in an office, bank, etc. Outdoor signs
are also assumed to be continuous when sizing conductors and over current protection.
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NEC CONDUCTOR CALCULATIONS
Non-continuous: Overcurrent protection devices (fuses and circuit breakers) are rated
100% for non-continuous loads. In other words, no special consideration or increased
loading factors are given to non-continuous loads outside of what we looked at in the
previous section.
Conductor Calculations – Non-continuous Loading
We have learned how to find new ampacity of conductors using temperature correction
factors and derating factors for more than three conductors in a raceway. Most noncontinuous loads are sized by going to the tables and selecting a conductor then using
240.6 and 240.4(B) to select the overcurrent device.
Example: What size circuit breaker and THW conductor would you need for noncontinuous load of 73 amps?
Protective device 80 amps, Article 240.4(B), 240.6
Conductor size - #4THW copper, Table 310.16
If ambient temperature and/or more than three conductors are involved, the conductor
ampacity needed must be calculated to find the conductor size. The overcurrent
protection device is still selected out of the NEC by using 240.4(B) and 240.6
Formula:
Wire ampacity needed from table =
Load at 100% (Non - Continuous)
Temp Correction Factors x T310.15(B)(2)(a)
Protection device = 100% of non-continuous
Example: There is a 34 amp load and the raceway is installed over a roof where the
ambient temperature is 110° F, 2 wire circuit. Assume a non-continuous load.
(A) What size protection device is required?
(B) What size RHW conductor is required?
(A) Protection device = 100% of non continuous load
Protection device = 35 amps, 240.4(B), 240.6
(B) Wire ampacity needed from table =
Load at 100% (Non - Continuous)
Temp Correction Factors x T310.15(B)(2)(a)
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ampacity = 34 amps
Ambient Temperature 110°F = 82 TCF for RHW
T310.15(B)(2)(a) does not apply because there are only 2 wires
Wire ampacity needed from table =
34 amps
= 41.46 amps
.82
Wire Ampacity needed = 41.5 amps
Using Table 310.16, we find the wire size needed to be #8 RHW
Example: There are ten current carrying conductors installed in an ambient temperature
of 57°C. A 48 amp load 2-wire circuit must be added.
(A) What size protection device is required for the 48 amp load?
(B) What size THHN copper conductor is required for the 48 amp load?
(A) Protection device = 100% of non-continuous load.
240.4(B), 240.6, Protection device = 50 amps
(B) Wire ampacity needed from table =
Load at 100% (Non - Continuous)
Temp Correction Factors x T310.15(B)(2)(a)
Temperature correction factor for the THHN is .71 (from Table 310.16)
10 existing current carrying conductors plus 2 new current carrying conductors = 12
current carrying conductors. Using T310-15(B)(2)(a) gives us a derating factor of 50% or
.5
Wire ampacity needed =
48ampsat100%
= 135.3 amps
.71CF * .5 DF
Wire ampacity needed = 135.3 amps
Using table 310.16 gives us a wire size of #1 THHN copper
Sizing overcurrent protection for non-continuous loads
In order to select OCP for a non-continuous load, you would select the same size as the
conductor amperage rating, or next size up.
Example:
We have four current carrying conductors in a raceway and two are #6TW. The ambient
temperature is 40° C.
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(A) What is the new ampacity?
(B) What size is the protection device?
(C) What is the maximum non-continuous load?
A. New Ampacity = Conductor Ampacity * TCF * T310.15(B)(2)(a) Derate
Factor
Table 310.16 #6 TW = 55 amps
Table 310.16 400°C = .82 TCF
Note from table 310.15(B)(2)(a), 4 current carrying conductors = 80%
New Ampacity = 55 amps x .82 TCF * .8 derating factor
New Ampacity = 36.1 amps
B. Size OCP device: using Article 240.4(B) and 240.6 we get:
36.1 amps = 40 amp device
C. The maximum non-continuous load on the 40 amp OCP device is 40 amps but
the ampacity of the conductor is 36.1 amps. Article 310.10 states that a
conductor cannot be used in such a manner that its operating temperature
rating is exceeded.
To determine the maximum non-continuous load, compare the OCP at 100%
against the ampacity of the conductor.
40 amp OCP device = 40 amps, non-continuous
#6 TW new ampacity = 36.1 amps
Conductor Calculations – Continuous Loading
The first step in continuous load problems is to always find the overcurrent protection
device.
Article 210.20(A) Continuous Loads. The total load on any overcurrent device located in
a panel board shall not exceed 80% of its rating where in normal operation the load will
continue for three hours or more. You could also look at it and say that the over-current
protection of a continuous load shall be sized at 125% of the full load current. According
to Article 210.19(A)(1), continuous loads shall be protected at 125% of the branch circuit
rating.
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Example: What is the maximum continuous load placed on a 20 amp fuse or breaker?
20 x .8 = 16 amps is the maximum continuous load that can be applied to a 20
amp breaker
Sizing Overcurrent Protection - Continuous loads
(1) To find the maximum continuous load of a protection device, use the following
formula.
Maximum continuous load = OPD x .8 (or 80%)
Example: What is the maximum continuous load of a 100 amp over current protection
device?
Maximum continuous load = 100 x .8
Maximum continuous load = 80 amps
(2) To find the overcurrent protection device for continuous load, use the following
formula.
OPD (continuous) = continuous load x 125%
Example: What is the overcurrent protection device for an 80 amp continuous load?
Using Articles 240.4(B), 240.6 and the formula for a continuous load:
OPD (continuous) = 80 amps x 1.25
OPD (continuous) = 100 amps
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