CHAPTER THREE
BUILDING ELECTRICAL DESIGN PRICIPLES
3.1 ELECTRICAL CODES, LICENSES AND PERMITS
The Electrical Code
 Design of an electrical system involves applying the standards written into the building
code.
 An electrical code specifies the minimum provisions necessary for protecting people and
property from the improper use of electricity and electrical equipment.
 It applies to both the manufacture and installation of electrical equipment .
National Electrical Code (NEC)
 The National Electrical Code (NEC) is a set of specifications and standards in the form
of a model code that can be adopted into local law by the local governmental entity.
Philippine Electrical Code (PEC)
 The primary objective of the code is to establish basic materials quality and electrical
works standards for the safe use of electricity for light, heat, power, communications,
signalling and for other purposes.
 Practical safeguarding of persons and property from hazards arising from the use of
electricity.
 Compliance to the PEC will ensure safety and prevent electrical fires.
Manufacturing Standard
 Today, all electrical equipment, appliances, and devices should meet specific safety
standards based on regular product testing.
 An approved product meets minimum safety standards as determined by extensive testing
by an independent testing company or organization.
 The National Electrical Manufacturing Association (NEMA) is the leading trade
association in the United States representing the interests of electrical manufacturers.
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Licensing
 Most municipalities have ordinances (local laws) that require that any person who wishes
to engage in the business of installing electrical systems must be licensed (usually by the
state or province).
 This means that the person must have a minimum number of years of experience working
with a licensed electrician and must pass a written test that deals with the electrical code
being used and with methods of installation.
 By requiring a license, it is assured that the electrician knows, at a minimum, the code
requirements and the installation procedures.
 There are areas where no laws require that only licensed electricians may install electrical
systems and, in effect, there is no protection for the consumer against an unskilled
electrician. It is good practice to insist on licensed electricians for all installations.
Permits
 Most municipalities require that a permit be issued before any electrical installations may
be made on a project.
 A complete electrical construction drawing may also be required for review and approval
by a plans examiner before installation begins.
 Municipalities that require a permit will have electrical inspectors who check the project
during regularly scheduled visits.
 Typically, they will inspect the installation after the rough wiring is in but before it is
concealed behind construction Materials.
 When all of the fixtures and devices are installed and wired back to the panel and the
service and meter installed, the final inspection is completed.
 On large projects, it may be necessary for several rough-in and final inspections as
electrical work may be done in phases. For example, conduit encased in concrete may
have to be inspected before the concrete is poured, and conduit to be built into masonry
walls will have to be inspected before the walls are begun.
 The installer and designer should become aware of when inspections are required and
what will be inspected.
 Also, it is important that close coordination and cooperation be maintained with the
inspector, because the inspector could slow down the progress of the work if the
inspection is not made promptly.
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 Whenever possible, the electrical inspector will need to know as early as possible when
inspections will be scheduled.
3.2 ELECTRICAL CONSTRUCTION DRAWINGS
 Electrical construction drawings show the layout and design of an electrical installation.
 A complete set of construction drawings and specifications of the building electrical
system is needed to convey design information to the contractor.
The following construction drawings and details are generally required:
1. Complete plans and specifications of all electrical work.
2. Labeling criteria of all electrical equipment.
3. Lighting floor plan(s) including electrical circuits indicating conduit and wiring sizes.
4. Power floor plans including electrical circuits indicating conduit and wiring sizes,
equipment, and disconnect switches
5. Exit sign/means of egress lighting location and power supply
6. Lighting fixture schedule
7. Symbol schedule and diagrams
Specifications and drawings should include requirements for:
1. Raceway and conduit with fittings
2. Wire and cable
3. Electrical boxes, fittings, and installation
4. Electrical connections
5. Electrical wiring devices
6. Circuit and motor disconnects
7. Hangers and supporting devices
8. Electrical identification
9. Service entrance and details
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10. Overcurrent protection
11. Switchboards
12. Grounding
13. Transformers
14. Panelboards
15. Motor control centers
16. Lighting fixtures
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3.3 BUILDING SYSTEM VOLTAGES
Supply Voltages
 Power is delivered by the utility company to the user at supply voltages. Supply voltage is
expressed as a nominal voltage because it varies slightly.
 During normal conditions, supply voltages can vary from about 90 to 105% of nominal
voltage.
 Variations from nominal voltages are caused by a number of reasons, including load
variation and changes in conditions at the utility power system.
 Additionally, transient voltages, caused by phenomena such as lightning strikes, some
types of faults, and the switching of some types of user loads, cause variations in voltage
available to the user.
System Voltages
 It is the target voltage entering the service panel.
 The principle voltages available in a building are called the system voltages.
 Medium and high voltage systems carry voltages above 600 V may be used in special
cases such as the 2400/4160 V three-phase system found in industrial and commercial
installations such as for large signage, sports lighting in stadiums, and services for large
manufacturing plants and skyscrapers.
 There are, however, drawbacks to voltages higher than 600 V because significant and
costly special precautions such as heavy insulation and conductor shielding are needed.
As a result, low voltage systems that that carry voltages less than 600 V are typically used
in buildings.
 System voltage is expressed as a nominal voltage because it varies slightly for the reasons
mentioned earlier.
 Design of a building’s electrical system begins with establishing the desired building
system voltage. A higher voltage means that a circuit can carry more current.
 A 208 V circuit can carry 1.73 times the current of a 120 V circuit (208 V/120 V 1.73); a
240 V circuit can carry twice the current of a 120 V circuit; a 277 V circuit can carry 2.31
times the current of a 120 V circuit; and so on. Thus, higher voltage means smaller
conductor sizes.
 The savings for larger conductors (feeders) of moderate length can be quite significant.
Higher voltage, however, is more dangerous.
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 There are a numerous system voltage levels and combinations used in buildings
throughout the world.
 Availability of a particular system voltage is dependent on utility lines and equipment at
or near the building site.
Circuit Wiring
 Before entering into a detailed discussion on system voltages, it is necessary to introduce
types of wiring found in a circuit.
 A minimum of two types of conductors is required to deliver alternating current in a
building electrical system: the ungrounded conductor and the neutral conductor. A third
conductor, called a grounding conductor, is added to most circuits. See Figure below.
 Each conductor plays an important role in how electricity is distributed to and circuited in
a building.
 The different function of each conductor is introduced here as a prelude to understanding
building system voltages.
Ungrounded Conductor
 It is the initial current-carrying conductor in an AC system.
 Frequently known as the hot or live conductor because it feeds current to the circuit.
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 When an ungrounded conductor is grounded (connected to ground), a closed circuit in
single phase results.
 This is the type of circuit used to power small appliances (e.g., toaster, portable
microwave oven, and so forth), small pieces of equipment (e.g., computer, electric drill,
and so on) and lighting (e.g., desk lamps, office lighting, and so forth).
 When two associated ungrounded conductors are connected in a single circuit, a higher
voltage is delivered.
Grounded/Neutral Conductor
 It is required to complete a single phase circuit by connecting the ungrounded (hot)
conductor to ground.
 The neutral conductor is a grounded conductor that serves more than one circuit.
 It carries the unbalanced load between two ungrounded (hot) conductors.
 Both conductors complete the circuit(s) by connecting it to ground and, as a result, are
treated as current carrying conductors.
Grounding Conductor
 It provides supplementary but important grounding protection.
 The grounding conductor is not normally a current-carrying conductor, but is energized
only on a temporary, emergency basis when there is a fault between an ungrounded (hot)
conductor and any metal associated to the electrical equipment.
 Confusion often exists between the definitions of the grounded conductor and the
grounding conductor because the grounding conductor is commonly referred to as a
“ground.” It is more correctly called a “grounding” conductor.
 In a simple single-phase circuit, the ungrounded conductor provides power to the load
and the grounded conductor provides a path from the load back to the power source,
which completes the circuit.
 Voltage in the circuit is equal to the voltage on the ungrounded conductor (e.g., 120 V on
a 120 V circuit). When two ungrounded (hot) conductors in a single-phase circuit are
connected in a circuit, voltage in the circuit is double the voltage available on each
ungrounded conductor (e.g., It is 240 V between two ungrounded conductors that are 120
V each).
 A three-phase circuit requires three ungrounded conductors that are one-third out of
phase of each other. In the case where two ungrounded conductors from a three-phase
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wye connected transformer are connected, voltage in the circuit is 1.732 times the voltage
available on each ungrounded conductor (e.g., 208 V between two ungrounded
conductors that are 120 V each but one-third out of phase). The factor 1.732 is the square
root of 3.
120 Volt, Alternating Current,
Single-Phase, Two-Wire System
(120 V AC, 1phase-2W)
 This system is used to serve outbuildings and farm buildings because its use is limited to
buildings with loads up to 6000 VA (50 A).
 The service entrance provided to the service equipment (switchboard or panelboard) is by
two conductors: one ungrounded (hot) conductor carrying 120 V and one neutral
conductor.
 Voltage measured between the ungrounded (hot) and a neutral conductor is 120 V.
120/240 Volt, Alternating Current,
Single-Phase, Three-Wire System
(120/240 V AC, 1phase-3W)
 It is the most common residential electrical service in use today. It is also used on a limited
basis in light commercial buildings such as small office buildings, churches, and retail shops
and stores.
 The availability of 120 V or 240 V leads to a number of circuit or feeder arrangements that
can supply the following loads:
 120 V, single-phase, two-wire branch circuit
 240 V single-phase, two-wire branch circuit
 120/240 V single-phase, three-wire feeder or branch circuit
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 On a 120/240 V system, a 120 V branch circuit provides electrical energy to convenience
(receptacle) outlets, small appliances, and light fixtures.
 A 240 V branch circuit serves large appliances and equipment such as electric-resistance
baseboard heaters, water heaters, and air conditioning equipment.
 A 120/240 V branch circuit provides both 120 V and 240 V to an appliance such as a range
and clothes dryer; controls, and light fixtures.
208 Y/120 Volt, Alternating Current,
Three-Phase, Four-Wire System
(208 Y/120 V AC, 3phase-4W)
 The 208 Y/120 V AC, 3_-4W is an older electrical service found in small commercial
buildings (e.g., office buildings and schools) and high-rise buildings where three-phase
motors (motors above about 1⁄2 horsepower) and equipment such as large air conditioners
are used.
 The availability of 120 V or 208 V in single- or three-phase leads to a number of circuit or
feeder arrangements that can supply the following loads:
 120 V, single-phase, two-wire branch circuit
 208 V single-phase, two-wire branch circuit
 208 V three phase, three-wire branch circuit
 120/208 V three phase, three wire feeder or branch circuit
480 Y/277 Volt, Alternating Current,
Three-Phase, Four-Wire System
(480 Y/277 V AC, 3phase-4W)
 It is a common electrical service in most modern medium to large commercial buildings. The
480 V three-phase power is used to power specially designed heavy machinery (e.g., at
machine shops and manufacturing plants).
 The availability of 277 V or 480 V in single or three -phase offers the following circuit or
feeder configurations:
 277 V, single-phase, two-wire branch circuit
 480 V single-phase, two-wire branch circuit
 480 V three-phase, three-wire branch circuit
 277/480 V three-phase, three-wire feeder or branch circuit
 When compared with the 208 Y/120 V system, the 480 Y/277 V system has economic
advantages from the standpoint of equipment and conductors. Because a given conductor can
carry more than twice the VA load at 480 V than at 208 V, the savings in wire size for
feeders can be quite significant with the 480 Y/277 V system.
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600 Y/346 Volt, Alternating Current,
Three-Phase, Four-Wire System
(600 Y/346 V AC, 3phase-4W)

This system is designed like the 480 Y/277 V AC, 3phase-4W described earlier, except that
600 V and 346 V are available in the circuit or feeder configurations.
 The 600 Y/346 V system has additional economic advantages from the standpoint of
equipment and conductor sizing in comparison to the 208 Y/120 V and 480 Y/277 V
systems.
 However, the 550 V or 575 V equipment used on the 600Y/346V system is not as readily
available as the 460 V equipment used on the 480 Y/277 V system.
240 _/120 Volt, Alternating Current,
Three-Phase, Four-Wire System
(240 Delta/120 V AC, 3phase-4W)
 It is another fairly common electrical service found in commercial and industrial buildings
where three-phase motors (motors above about 1⁄2 horsepower) and equipment such as large
air conditioners are used.
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System, Utilization, and Maximum Voltages
 System voltage is the target voltage entering the service panel.
 The utilization voltage accounts for anticipated voltage drops on branch circuit
conductors.
 Measured voltage at an outlet or connection is called the line voltage.
 The highest voltage to which a wiring device can be exposed is known as the maximum
voltage.
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3.4 GROUNDED AND UNGROUNDED CONDUCTORS
Grounded Conductor
 It serves as the grounded leg of the circuit.
 It completes the circuit by connecting the ungrounded (hot) conductor to ground.
 In circuit design, the grounded conductor is considered to be a current carrying conductor
because it serves as a return path back to the circuit’s power source.
Neutral Conductor
 Performs the function of a grounded conductor for at least two ungrounded (hot)
conductors that have sources from different voltage phases, such as on a multiwire branch
circuit, multiwire feeder, and the electrical service.
 Thus, a neutral conductor is frequently called a shared neutral or common neutral.
 Grounded and neutral conductors should be wired so a circuit breaker, fuse, or switch
does not interrupt them.
Load Balancing
 It is the practice of dividing loads as evenly as possible between the ungrounded
conductors on a multiwire circuit, feeder, or service.
 Unbalanced load can be excessive and unsafe, thus balancing the load can protect us from
the danger of electricity.
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3.5 SYSTEM AND CIRCUIT GROUNDING
Grounding
 In an electrical system, grounding is required to protect building occupants and electrical
equipment.
 The grounding conductor is a continuous conductor that connects the ground to the
neutral bus bar and the grounding conductor bus bar in the service equipment/main
panelboard.
 Grounding of an electrical branch circuit enables current to take an alternate path back to
the overcurrent protection device if an electrical device or appliance short-circuits.
Grounding Requirements
1. System grounding
2. Equipment grounding
System Grounding
 It is that part of a building electrical system that provides protection against electrical
shock, lightning, and fires.
 A lightning strike near the building or a high-voltage transmission line contacting the
service entrance conductors can introduce high voltage to a building electrical system.
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 A grounding system must be connected to some or all of the following elements if available
on the building premises:
 An underground metal water (not gas) pipe in direct contact with the earth for no less
than 10 ft (3 m); the metal building frame where it is effectively grounded.
 An electrode made of at least 20 ft (6 m) of electrically conductive steel reinforcing
bars (No. 4 AWG or greater) or bare copper wire no smaller than No. 2 AWG that is
encased in at least 2 in of concrete that is part of a foundation or footing in direct
contact with the earth.
 An electrode made of a steel or iron plate that is at least 1⁄4 in thick or copper plate
that is at least 0.06 in thick with at least 2 ft2 (0.2 m2) of the plate surface in contact
with exterior soil.
 An electrode made of a grounding ring of bare copper wire no smaller than No. 2
AWG that encircles the building at a depth no less than 2.5 ft (0.75 m) below grade.
 The structural metal frame of the building where the frame is effectively grounded.
Equipment Grounding
 Refers to a grounding conductor or grounding path that connects the noncurrent-carrying
metal components of equipment.
 The equipment-grounding conductor is a bare conductor or a green-colored, insulated
conductor that connects (bonds) the outlet boxes, metallic raceways, other enclosures and
frames on motors, appliances, and other electrical equipment.
Double Insulation

Double insulation of an appliance or power tool protects the user from electric shock by
creating a nonconducting barrier between the user and the electric components inside the
appliance or tool.
 Many of newer household electrical appliances and tools are double insulated. Examples
include coffee makers, blow dryers, electric drills, and other similar small power tools
and appliances.
3.6 THE BUILDING ELECTRICAL SYSTEM
Residential Systems
 Normally, a 120/240 V, three-wire, single-phase service entrance serves a residence.
 From a single panelboard rated from 100 to 200 A or more, power is distributed
throughout the residence through branch circuits.
 In large residences, a set of feeders may extend from the main panelboard and bring
power to one or more sub panelboards located at a remote area of the building.
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Multifamily Dwellings
 In multifamily dwelling units (e.g., apartments, condominiums), power is brought from a
utility transformer to the building service equipment.
 It is then divided at a main distribution panel, passes through individual meters, and is
distributed to the individual dwelling units through feeders. Each dwelling unit is served
by a separate panelboard located in the dwelling unit.
 Branch circuits extend from a panelboard to feed outlets within the unit.
Commercial/Industrial Systems
 In a typical large building, electrical power is provided to a transformer located outside
the building or it enters a transformer vault located at the service level in the building.
 A transformer vault is a basement- or ground-level structure or room in which power
transformers, network protectors, voltage regulators, circuit breakers, meters, and so on
are housed.
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3.7 CONDUCTOR REQUIREMENTS
Conductor Materials
 Electric conductors are substances or materials used to convey or allow the flow of
electric current.
 Materials Considered as Good Electric Conductors
1. Silver
6. Zinc
2. Copper
7. Platinum
3. Aluminum
8. Iron
4. Nickel
9. lead
5. Brass
10. Tin
 Good Conductors those substances with extremely low resistance to current flow.
Conductor Insulation
 Insulators are substances or materials that resist the flow of electric current.
 The conductor insulator serves as a physical shield of the wire against heat, water and
other elements of nature.
 Insulation is rated by voltage from 300 to 15000 volts.
 Various Kind of Insulators
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1. Rubber
7. Latex
2. Porcelain
8. Asbestos
3. Varnish
9. Paper
4. Slate
10. Oil
5. Glass
11. Wax
6. Mica
12. Thermoplastic
Conductor Ampacity Requirements
 Ampacity is defined as the ability of the wire or conductor to carry current without
overheating.
 Conductor Ampacity is determined by the maximum operating temperature that its
insulation can withstand continuously without heating.
 Conductor size and rating shall have sufficient Ampacity to carry load. They shall have
adequate mechanical strength and shall not be less than the rating of the branch circuit
and not less than the maximum load to be served
 Tables 2.14 through 2.16 and 2.18 through 2.24 contain ampacities for various
conductors, conductor insulations and sheathings.
Temperature Correction Factor
 Ambient temperature is the temperature of a surrounding medium (e.g., air, soil).
 Ambient temperature can affect allowable current-carrying capacity of a conductor.
 The ambient temperature rating of a conductor refers to the normal temperature range in
the environment in which that conductor is to be used (e.g., the temperature of the
surrounding air, water, or earth).
 A temperature correction factor (Ft) for conductors is applied based on the ambient
temperature of the conductor.
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Bundling Correction Factor
 A bundling correction factor (FN) must be applied for four or more conductors in a
raceway or cable installed in the same raceway or conduit or any bundled cables that are
more than 24 in (0.63 m) long.
 For raceways and cables with 4 through 6 current-carrying conductors, the bundling
correction factor is 0.80; for 7 through 9 conductors, it is 0.70; and for 10 through 20, it is
0.50.
 Thus, ampacity is
Where: Ft is ambient temperature factor
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Conductor Voltage Drop Requirements
 In addition to ampacity requirements, branch circuits and feeders should be analyzed for
voltage drop because of the adverse effect it can have on performance and operating life
of appliances and equipment.
 The basic formula for determining voltage drop (Edrop) in a two-wire AC circuit or
three-wire AC single-phase circuit with a balanced load at 100% power factor (neglecting
reactance) is based on the one-way circuit length (L), in feet or meters; conductor
resistance (R), in Ω/1000 ft or Ω/1000 m; and the circuit load (I) in amperes:
 The percentage of voltage drop is determined by the ratio of voltage drop and system
voltage.
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Insulation Color Coding and Identification Markings
 The commonly used but not mandatory color sequence of conductors serving single
phase circuiting.
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13.8 CABLE, RACEWAY, AND ENCLOSURE REQUIREMENTS
Cable and Raceway Requirements
 All building wiring must be enclosed in a cable, conduit, wireway, or raceway.
 During installation, conductors are snaked through conduit or tubing, are laid in a
wireway, or are contained in cables and secured to structural framing.
 Conductors that are run through a raceway must have sufficient open air space to prevent
overheating.
 The number of current-carrying conductors that can run through a raceway is limited by
code.
 Table 7 indicates the recommended maximum number of conductors allowed in conduit
or tubing for conductors with THWN or THHN insulation. Similar tables exist in the
electrical code for this and other insulation types.
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Box/Enclosure Requirements
 All electrical connections must be made in a protective enclosure such as a panelboard,
junction, or device box, fixture, or appliance.
 Every switch, outlet, and connection must be contained in an electrical box and every
lighting fixture must be mounted on a box.
 All electrical boxes must be adequately secured to the building structure.
 Conductors in an electrical box must have sufficient open air space to prevent
overheating.
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3.9 BRANCH CIRCUIT REQUIREMENTS
Branch Circuiting
 A branch circuit is that portion of a building wiring system that extends beyond the final
overcurrent protection device that is protecting a circuit.
 It provides power from a circuit breaker or fuse in the panelboard to single or multiple
points of use called outlets.
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 An outlet is a point in a wiring system where current is taken to supply an appliance,
piece of equipment, or lighting installation.
 A branch circuit is composed of an overcurrent protection device (fuse or circuit breaker),
wiring, and one or more outlets.
Types of branch circuit
1. Individual Branch Circuit



This type of branch circuit serves only one receptacle or piece of equipment such
as for a range, clothes dryer, large copy machine, or other piece of machinery.
These circuits usually lead directly from the distribution panel to the appliance
and do not serve any other electrical devices.
The individual branch circuit is sometimes known in the trade as a dedicated or
special purpose circuit.
2. General Purpose Branch Circuit


A branch circuit supplies two or more outlets for lighting and appliances.
This type of circuit may be referred to as a lighting circuit; this is a carryover
from the days when electricity was first used in buildings and its predominant
purpose was lighting.
3. Appliance Branch Circuit

This is the type of branch circuit that supplies energy to one or more outlets to
which appliances are to be connected.

They supply fixed electric equipment such as refrigerators, washers, and other
large appliances and electrical devices.

Appliance branch circuits do not supply lighting fixtures.

Appliance branch circuits cannot exceed 20 A.
4. Multiwire Branch Circuit


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A branch circuit consisting of two or more ungrounded (hot) conductors having a
voltage between them and a common neutral (grounded) conductor that is shared
between the ungrounded conductors such as in a 120/240 V three-wire circuit.
These branches are required in health care facilities such as hospitals, nursing
homes, and dental facilities.
5. Life Safety Branch Circuit
 An emergency system of feeders and branch circuits that provides adequate power to
patients and personnel.
 It must automatically connect to an alternate power source such as a generator when
the normal power source is interrupted.
6. Critical Branch Circuit
 An emergency system of feeders and branch circuits intended to provide power to
task illumination, special power circuits, and selected receptacles serving areas
and functions related to patient care.
 It must automatically connect to an alternate power source such as a generator
when the normal power source is interrupted.
7. Split Wiring Receptacles
 Split wired duplex receptacles are fed with a 120/240 V circuit having two
ungrounded (hot) conductors, a grounded (neutral) conductor, and a grounding
conductor.
 One ungrounded (hot) conductor feeds power to the upper outlet and the other
ungrounded (hot) conductor feeds the lower outlet.
 The grounded (neutral) conductor is shared between both circuits.
 Split wiring allows power to be drawn from two separate circuits on one duplex
receptacle.
8. Branch Circuit Rating and Loads
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
The branch circuit rating is determined by the rating of the overcurrent protection
device (fuse or circuit breaker) used to protect the wiring in the circuit from
excessive current flow.

The rating of the overcurrent protection device is related to the connected load or
loads being fed by the branch circuit.

The connected load on a branch circuit is the sum of all loads connected in a
circuit.
9. General Purpose Circuits
 General purpose circuits feed more than one outlet for lighting or other
purpose.
 According to requirements, the rating of general purpose branch circuits must
be 15 A, 20 A, 30 A, 40 A, or 50 A.
 General purpose circuits are typically limited according to what will be connected
to them:
1. When a general purpose circuit feeds fixed appliances and luminaires or
portable appliances, the total of the fixed appliances should be no more than
50% of the branch circuit rating.
2. A 20 A, 120 V branch circuit would have a theoretical maximum of 2400 VA
(20 A *120 V = 2400 VA), but it is common practice to limit the connected
load to 80% of the circuit rating (e.g., 20 A * 120 V * 80% = 1920 VA).
Refer to Table 11.
3. When the load on the circuit will be a continuous operating load (e.g., for
store lights), the total load should not exceed 80% of the circuit rating. The
lighting load must include any ballasts, transformers, or autotransformers,
which are part of the lighting system. Because a 15 A branch has a full rating
of 1800 VA, the limit would be 80%, or 12 A and 1440 VA (e.g., 15 A * 120
V * 80% = 1440 VA). A 20 A, 2400 VA branch would be limited to 16 A and
1920 VA of connected load. Again, refer to Table 11.
4. When portable appliances will be used on a general purpose circuit, the limit
for any one portable appliance is 80% of the branch circuit rating.
5. In commercial applications, convenience receptacles are computed at a load of
1.5 A (180 VA) per receptacle and are limited to 80% of the rating.
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10. Individual Circuits
 These circuits provide power to a single outlet such as a receptacle serving a
range, clothes dryer, or copy machine.
 Generally, individual circuits are required for the following appliances and
equipment:




Kitchen range (both stand-alone and counter-mounted units)
Oven
Microwave (built-in)
Waste disposal








Dishwasher
Clothes washer
Clothes dryer
Electric water heater
Furnace
Boiler circulating pump motor (large commercial and industrial)
HVAC air-handling unit
Large machinery (e.g., table saw, lathe, milling machine ,machining
center, elevator)
Large equipment (e.g., large copy machines, compressors, HVAC
blowers)

3. 45
11. Appliance Circuits
 These circuits serve two or more outlets to which only appliances are
connected.
 In dwelling units, two or more 20 A small appliance circuits for convenience
receptacle outlets in the kitchen, dining room, pantry, and breakfast room are
required.
Continuous Loads
 A continuous load is a connected load that operates for 3 hr or more at any time.
 Many electrical loads fit within this category such as circuits serving office and
classroom lighting installations.
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Branch Circuit Conductor Size
 On a branch circuit, conductor size is tied to circuit rating.
 Generally, ungrounded (hot) and grounded (neutral) conductors in the circuit must be
sized so that conductor ampacity is at least the branch circuit rating.
 The ampacity of a conductor can be larger than the circuit rating but not smaller.
Residential Branch Circuit Wiring
 Type NMB cable is the most widely used wiring method in residential dwellings.
 NM cable must have 194°F (90°C) conductor insulation rating, which is designated by a
“B” on the cable sheath.
 Type SER or other four-wire cable is used for electrical ranges, cooktops, wall ovens, and
clothes dryers.
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3.10 DEVICE AND EQUIPMENT REQUIREMENTS
Requirements for Switches and Receptacles
 Switches must be selected to match the load they control.
 Large lighting installations that require many switches may have the switches contained
within a panelboard-like enclosure called lighting control panel. Receptacles must be
selected to match the appliance or equipment they serve.
 There are no specific mounting height requirements for wall switches and receptacles.
Switches are normally mounted approximately 48 in (1.2 m) above finished floor (AFF),
unless otherwise specified. Convenience receptacles are normally mounted
approximately 16 in AFF (400 mm), unless otherwise specified.
 Convenience receptacles in bathrooms and restrooms are normally mounted
approximately 44 in AFF (1.1 m).
Overcurrent Protection (Circuit Breakers and Fuses) Requirements
 An overcurrent protection (OCP) device, a fuse, or circuit breaker serves to limit current
levels in a conductor by interrupting power when current limitations are exceeded.
 It prevents excessive heat from damaging conductors and related equipment.
 Therefore, the overcurrent device must be matched to the conductor and equipment so
that the current-carrying capacity of the conductor and equipment are not exceeded.
 The OCP device must protect the ungrounded conductors in a circuit to ensure that power
to the circuit is interrupted by the OCP device where the circuit originates (generally the
panelboard).
Feeder Requirements
 A feeder is a set of conductors that carry a comparatively large amount of power from the
service equipment to a second panelboard, called a subpanelboard, where branch circuits
further distribute the power.
 Feeders must be designed to provide sufficient power to the branch circuits they supply
so feeder conductor size is based on the maximum load to be supplied by the feeder.
 Feeders should be capable of carrying the amount of current required by the load, plus
any current that may be required in the future.
Switchboard and Panelboard Requirements
 Switchboards and panelboards can be used as distribution equipment, at a point
downstream from the service entrance equipment.
 Switchboards and panelboards used as service equipment should have a rating not less
than the minimum allowable service capacity of the computed load.
 Panelboards used as subpanelboards should have a rating not less than the minimum
feeder capacity of the computed load.
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Service Entrance Equipment Requirements
 Service equipment must be large enough to supply the computed load of the building or
area of the building being served.
 It is calculated using code requirements and utility regulations.
 The most common sizes of residential service equipment are 100, 125, 150, 175, and 200
A. RHW, THWN, THHN, XHHW, and USE aluminum conductors are commonly used.
 In single-family residences and multifamily dwellings, the main service panelboard can
be mounted either outside or inside the dwelling as near as possible to the point of
entrance of the service conductors to the building.
 All service equipment and electrical panels shall have a clear area 30 in (0.75 m) wide
and 36 in (0.9 m) deep in front.
 Typically, minimum vertical clearances of 18 ft (5.5 m) above roadways, 12 ft above
driveways, and 10 ft above sidewalks are the required minimum.
 An 8 ft (2.4 m) clearance is required above low-sloped (less than 4 in 12 slope) rooflines.
 A 3 ft (0.9 m) clearance is required for steep-sloped roofs.
 Minimum earth cover varies from 6 to 24 in (150 to 600 mm), depending on whether the
conductors are in a cable or protected by a conduit, or covered with soil or below a
concrete walk or street.
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 The maximum single-span distance that utilities will run overhead service drop
conductors to the point of service entrance varies, but typically it is 100 to 125 ft (30.5 to
38.1 m).
Transformer Requirements
 Transformers may be located in a building to step up or step down the building system
voltage.
 Transformer combinations, such as wye-wye (Y-Y), delta-delta (∆-∆ ), delta-wye (∆-Y),
and wye-delta (Y-∆ ) are available for use in buildings.
 A 480 V primary, 208 VY/120 V secondary, three-phase transformer is a popular unit
used in large commercial buildings and industrial facilities.
 Equipment designed to operate from delta-connected power, such as air conditioners or
motors, can also operate from wye-connected power, because the phase-to-phase voltages
are available in both systems.
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3.11 OCCUPANT PROTECTION REQUIREMENTS
Tamper-Resistant Receptacle Requirements
 The NEC has required tamper-resistant receptacles be installed in all 125-V, 15- and 20A electrical receptacles in hospital pediatric areas for nearly three decades. Recently, the
NEC introduced requirements for all 125-V, 15- and 20-A electrical receptacles in new
residential construction to be tamper-resistant receptacles.
 The move comes in an effort to better protect small children from suffering electrical
burns when they accidentally insert items into conventional outlets.
 Tamper-resistant receptacles should be considered in these areas. All tamper resistant
receptacles must have either the words “tamper resistant” or the letters “TR” (minimum
3⁄16 in or 5 mm high) on the device as a clear indication that this is a tamper resistant
receptacle.
Ground Fault Circuit Protection Requirements
 The NEC introduced requirements for use of GFCI in residences in 1973 when it required
GFCI protection of outdoor convenience receptacles within 61⁄2 ft (2 m) of grade level.
 The NEC introduced requirements for use of GFCI in residences in 1973 when it required
GFCI protection of outdoor convenience receptacles within 61⁄2 ft (2 m) of grade level.
 Bathroom convenience outlets (1975 edition)
 Readily accessible convenience outlets in garages (1978 edition), except where not
readily accessible such as outlets dedicated to an overhead door opener or freezer
 Convenience outlets within 6 ft (1.8 m) of kitchen sink (1987 edition) and revised to
include all kitchen convenience outlets that serve countertops, including islands (1996
edition) but not those serving fixed kitchen appliances (e.g. range and oven) and the
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refrigerator or freezer Convenience outlets in unfinished basements and crawl spaces, except
laundry (1990 edition)
 Convenience outlets within 6 ft (1.8 m) of laundry,
 utility room, or wet bar sink (1993 edition)
 Other locations requiring GFCI protection on 120 V, singlephase
 15 A and 20 A outlets include:
 Commercial kitchens
 Restrooms in commercial, industrial, and in any other nondwelling type buildings
 Receptacles with grade-level access and at rooftop locations
 Commercial garages
 Elevator pits
 Agricultural buildings
 Aircraft hangers
 Wet locations in health care facilities
 Boathouses, marinas, and boatyards
 Receptacle outlets on roofs (except dwelling units)
 Circuits to resistance (impedance) heating units such swimming pool, spa, hot tub, deicing and snow-melting heaters;
 Receptacle and lighting outlets near swimming pools, spas, hot tubs, and fountains
 Receptacles in temporary locations (i.e., on construction sites, carnivals, circuses, and
fairs)
Arc Fault Circuit Protection Requirements
 The NEC requires AFCIs for bedroom circuits in new residential construction (starting in
January 2002).
 Future editions of the Code will likely expand coverage to include commercial and
industrial applications such as use in fire station sleeping areas, military housing,
hospitals, outpatient clinics, rest homes, retirement homes, and in other locations where
extension cords or cord-connected equipment may be used and where the general
occupancy may be at risk from arc faults.
 Older homes with ordinary circuit breakers may benefit from the added protection against
arcing faults that can occur in aging wiring systems.
 Buildings wired with solid aluminum conductors used in the late 1960s and early 1970s
are prone to poor connections. Such building electrical systems can also be retrofitted
with AFCI protection to identify faulty connections.
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3.12 ELECTRICAL SYSTEM DESIGN
Preliminary Design Guidelines
 Before actually beginning the design layout of the project, the designer will need to
accumulate certain information:
1. Determine whether electrical service is available at the site, and what type of system
voltage is available (e.g., 120/240 V AC, 1phase-3W, 208 Y/120 V AC, 3phase-4W,
and so forth). If service is not available, arrangements must be made with the power
company to extend service to the building site. Large projects may require more
power or a different system voltage than the existing service can supply. Coordination
with the power company is desirable as early in the design stage as possible. Costs
that must be paid by the owner should be thoroughly discussed, written, and received
by the owner.
2. Obtain a list from the owner of all the types and locations of equipment and
appliances to be used in the building that will require electricity. Although the
electrical designer may be aware of electrical requirements for much of the
equipment, it may be necessary to find the manufacturer’s specifications for certain
equipment (e.g., motor sizes, power and system voltage required).
3. Work with the architectural designer to best locate all of the electrical equipment and
appliances on the floor plan. On commercial projects, this sometimes takes several
meetings with the architects, engineers, consultants, owner’s representatives, and
manufacturer’s representatives. There are times when the type of equipment used and
its location must be approved by governmental agencies.
4. Review with the architect where the basic mechanical equipment (e.g., HVAC and
plumbing), the service entrance equipment, the power and lighting panels, and the
conduit or cable will be located.
5. Discuss with the owners any future plans for changing or expanding the facilities
(e.g., remodeling, constructing additions or other buildings, future equipment
requirements) and anything else that could potentially affect the size and location of
the electrical service. Many times the service entrance must be sized to anticipate
future expansion as well as present building plans. Once the basic information has
been gathered, the designer can begin to design the system itself.
Basic Design Considerations
 In electrical design there are numerous possible solutions. Experience guides the designer
to a solution that best suits the need of the building occupant. The designer achieves a
good solution by:
 Observing and evaluating existing installations and adapting them to meet the project
requirements.
 Applying electrical systems theory.
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 Applying Code requirements.
 Design of an electrical system begins with the layout of all outlets or outlet connections.
Design Guidelines for Common Spaces
Convenience Outlets and Switches
1. The number and type of lighting outlets should be fitted to the various seeing tasks.
Lighting outlets should be located to meet the desired lighting effects and fixtures to be
used. Refer to Chapter 2.
2. All convenience receptacles on 15 A and 20 A general purpose circuits should be of the
grounding type, minimizing the hazard of shock from short circuits.
3. GFCI protection should be provided on convenience receptacles where required by local
code (e.g., where the occupant is exposed to water).
4. AFCI protection should be used on convenience receptacles where required by local code
(e.g., in bedroom circuits).
5. All rooms that have more than one entrance should be equipped with multiple-switch
controls (e.g., two-way or three-way switching) at each principal entrance. Principal
entrances are those commonly used for entry to the room when going from a normally
lighted to an unlighted condition. If this recommendation would result in the placing of
switches controlling the same lighting installation within 8 ft (2.5 m) of each other,
multiple-switch controls may not be required.
6. Wall switches should be located at the lockset or latch side of doors or at the traffic side
of arches, and within the room or area where the lighting outlets are located.
7. Convenience receptacles in living rooms, bedrooms, dining areas, and other habitable
spaces should be placed so that no point along the floor line in any usable wall space is
more than 6 ft (1.8 m) from an outlet in that space. Any wall 2 ft (0.6 m) or more in
length must have a convenience receptacle. Preferably, convenience receptacles should
be located near the ends of a wall space, rather than near the center of the wall, to reduce
the likelihood of being concealed behind large pieces of furniture. Outlets should not be
placed above electrical baseboards, hot air registers, and hot water or steam registers. The
intent is to eliminate cords having to pass over hot or conductive surfaces wherever
possible.
Building Exterior
1. One or more lighting outlets should be located at or near all exterior entrances. Outlets
should be switched or automatically controlled.
2. For each single-family dwelling, at least one duplex receptacle shall be installed outdoors
to be readily available from ground level. Weatherproof convenience receptacles should
be provided on exterior walls for outside work. GFCI protection is required for outdoor
receptacles.
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3. One or more outlets may be required for exterior equipment (e.g., swimming pool pump,
well pump, and so on).
Common Areas and Living Rooms
1. Outlets for ambient and task lighting should be provided. General illumination outlets
should be wall switch controlled.
2. Convenience receptacles in living areas should be placed so that no point along the floor
line in any usable wall space is more than 6 ft (0.6 m) from a receptacle outlet in that
space.
3. One or more receptacles for entertainment equipment should be provided at bookcases,
shelves, or other suitable locations.
4. When general illumination is to be provided from portable lamps, then at least two
separate wall-switched plug-in positions should be provided. These can be provided with
two switched regular duplex receptacles or one switched plug-in position in each of two
split-receptacle outlets.
5. An outlet for a fireplace may be required.
6. A smoke detector/alarm on a 120 V circuit should be considered.
Food Preparation/Kitchen/Cooking Areas
1. Lighting design should provide for ambient and local/task illumination of the work areas,
sink, range, counters, and tables. Lighting outlets should be switch controlled.
2. Special purpose receptacles should be provided for all fixed appliances (e.g., range,
built-in microwave, exhaust hood, dishwasher, trash disposal unit, waste disposal, and so
on).
3. At least two 20 A small appliance circuits for kitchen countertops should be provided.
These circuits are in addition to those required for refrigerators, ranges, microwaves,
lighting, and so forth. Outlets on these circuits should serve only the kitchen, pantry,
and/or dining room areas. The following convenience receptacles should be connected to
small appliance circuits:
 One receptacle for each 2 linear ft (0.6 m) of work-surface face.
 At least one receptacle to serve each separate work surface. Any counterspace
wider than 12 in should have a convenience receptacle.
4. Convenience receptacles serving countertop areas (e.g., except behind refrigerator and
those serving fixed appliances) should be GFCI protected. It is recommended that
receptacles serving countertop areas be split wired.
5. A smoke detector/alarm on a 120 V circuit should be considered.
Sleeping Areas/Bedrooms
1. General illumination should be provided from either ceiling or wall outlets, controlled by
one or more wall switches.
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2. A convenience receptacle should be placed on each side and within 6 ft (1.8 m) of the
centerline of each probable individual bed location. Preferably, convenience receptacles
should be located near the ends of a wall space, rather than near the center of the wall, to
reduce the likelihood of being concealed behind large pieces of furniture.
3. All 120 V branch circuits that supply outlets in dwelling unit bedrooms must be protected
by an AFCI device.
4. A smoke detector/alarm must be provided on a 120 V circuit and should be AFCI
protected.
5. In master bedrooms, outlets should be considered for a television and entertainment
equipment (e.g., DVD/VCR player).
6. In master bedrooms, an outlet for a fireplace may be required.
Bathrooms/Restrooms
1. Lighting sources at the mirror should be capable of illuminating both sides of the face.
2. At least one GFCI-protected receptacle within 3 ft of the outside edge of each lavatory
basin should be provided. A receptacle that is a part of a bathroom lighting fixture is not
typically suitable for this purpose. Placing each bathroom on a separate circuit should be
considered because of the heavy use and demands of these receptacles. The circuit
serving bathrooms should have no other outlets (e.g., it cannot supply power to an outside
receptacle or a garage receptacle). Where the 20 A circuit supplies only a single
bathroom, it can supply power to outlets for other equipment within the same bathroom
(i.e., lighting outlets
3. or an exhaust fan).
4. A wall-switched or timer-operated, built-in ventilating fan capable of providing a
minimum of 8 to 10 air changes per hour per water closet (50 cfm/water closet) should be
provided where no natural ventilation through windows is included.
5. Wall switches should be located so as not to be readily accessible while standing in the
tub or shower stall.
Laundry Areas
1. Outlets for fixed lights should be installed to provide illumination of work areas, such as
laundry tubs, sorting tables, washing, ironing, and drying centers. Lighting outlets should
be wall-switched controlled.
2. In the laundry area, one 20 A receptacle for the clothes washer and a special receptacle
for the clothes dryer are required. Outlets for other workstations (e.g., sewing. ironing,
repairing, and so on) should be provided.
3. One outlet and one switch for a ventilation fan should be provided.
4. A smoke detector/alarm on a 120 V circuit should be considered.
3. 56
Halls/Corridors
1. Ceiling fixtures should be installed for proper illumination of the entire area with
particular attention paid to irregularly shaped spaces.
2. Convenience receptacles in hallways within a dwelling unit should be placed so that no
point in the hallway shall be more than 10 ft (1.0 m) from a duplex receptacle as
measured by the shortest path that the supply cord of an appliance connected to the
receptacle would follow without passing through an opening fitted with a door
(the“vacuum-cleaner” rule). Each hall over 25 ft2 (2.3 m2) in floor space should have at
least one receptacle.
3. In entrance foyers, convenience receptacles should be placed so that no point along the
floor line in any usable wall space is more than 10 ft (3.1 m) from a receptacle in that
space.
Stairways
1. Fixed wall or ceiling lighting outlets should be installed to provide adequate illumination
of each stair flight.
2. Outlets should be so arranged that the stair system can be fully illuminated from either
floor.
3. A smoke detector/alarm on a 120 V circuit should be considered at the top of the stairs.
Utility Rooms
1. Lighting outlets should be placed to illuminate the furnace/ boiler area and work area. At
least one lighting outlet should be wall-switch controlled.
2. Convenience receptacles should be provided.
3. Outlets should be provided for each piece of mechanical equipment requiring electrical
connections such as the boiler, chiller, furnace, water pump, or compressor.
4. A special purpose outlet may be required for an electric resistance water heater, and/or an
electric-resistance furnace.
Shops/Garages
1. Lighting outlets should be placed to illuminate the work areas. Task lighting should be
provided above workbenches. Lighting outlet should be wall-switch controlled.
2. At least one duplex receptacle should be provided for each space in a garage or carport.
3. Convenience outlets should be provided at workbenches. In garages or near water, these
outlets should be GFCI protected.
4. Outlets should be provided for automatic overhear (garage) door operators in the ceiling
above each bay.
5. Special purpose outlets should be provided for special equipment such as air
compressors, welding equipment, tire changer, dust collection equipment, machining
3. 57
equipment (e.g., table saws, drill presses, milling machines, lathes, machining centers),
and so on. Check with owner on equipment voltage requirements, load, and placement.
6. A smoke detector/alarm on a 120 V circuit should be considered.
Storage Rooms/Closets
1. Generally, one lighting outlet for each large closet or storage room should be provided.
Where shelving or other conditions make the installation of lights within a closet
ineffective or unsafe, convenience receptacles in the adjoining space should be so located
as to provide light within the closet.
2. Wall switches or automatic door switches are preferred, but pull switches are acceptable.
Electrical/Telecommunications Closets
1. Lighting outlets should be placed to illuminate the area.
2. A minimum of two dedicated convenience receptacles on separate circuits is required.
Additional duplex convenience receptacles should be placed at 6 ft (1.8 m) intervals
around the perimeter.
3. Rooms should be located away from sources of electromagnetic interference (e.g.,
transformers, motors, x-ray equipment, induction heaters, arc welders, radios, radar
systems, and so forth).
4. Emergency power should be considered and supplied.
5. A smoke detector/alarm on a 120 V circuit should be considered.
3.13 DESIGN EXAMPLE
An example of system design of a 61 ft-6 in by 36 ft, single story and single-family
residence is used in the explanation. This residence contains general lighting (lights and
receptacle outlets), equipment, and appliances, including what follows: water heater (3800 VA),
clothes dryer (4400 VA), dishwasher (1000 VA), range (11 700 VA), waste disposal (1000 VA),
air conditioner (6300 VA), and garage door opener (1000 VA). Loads used are from
manufacturers’ data.
Circuit Design
1. Locate the convenience receptacles on the floor plan with symbols used to represent each.
Receptacles should be located conveniently following the guidelines provided. All
switching, sizing of conductors, and circuit layout will be done later. GFCI- and AFCIprotected receptacles should be located following the requirements specified. See Figure
3.12.
2. Locate all appliance and equipment outlets on the floor plan using the appropriate
symbols for the various receptacles required. See Figure 2.12. This process calls for
coordination with the architectural designer because each appliance and piece of
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3.
4.
5.
6.
7.
3. 59
equipment must be known to locate the outlets properly. Many times, the architectural
plans do not convey each appliance and piece of equipment that must be connected. For
example, a kitchen waste disposal must be connected with a switch on the wall, yet the
waste disposal and switch are seldom shown on the floor plan. The electrical designer
should request that the architectural designer list all appliances and equipment in writing
or on the drawing for complete coordination. The electrical designer should also check
the number of appliances or equipment required. For example, two furnaces may be used
in large residences and both will need connections. Also, in commercial buildings,
typically more than one heating and/or cooling unit may be used. The electrical designer
will need to know the specified voltage and amperage requirements for equipment and
appliances. A list of typical appliances that require electrical connections and their
requirements are listed in Tables 3.12 through 3.17. These requirements will vary among
different manufacturers. Manufacturer nameplate ratings should be confirmed and used in
computations.
Locate the lighting fixtures, using the appropriate symbols to represent each. See Figure
3.13. The electrical designer should make a list of types of luminaires and the load
requirements. This list will be used later when grouping circuits. It may also be included
in the specifications or on the drawings as a luminaire (light fixture) schedule. The fixture
schedule will be used by the electrical estimator when determining the cost to be charged
for the work; next by the electrical purchasing agent when the fixtures and materials are
ordered for the project; and then by the electrician who actually installs the work. The
architectural designer or even the building owner may decide the fixtures that are to be
used, often with assistance of the electrical designer. See Figure 19.14. On commercial,
industrial, and institutional projects, the electrical designer will need to determine the
number and types of lighting fixtures required to provide adequate lighting levels. A
discussion of lighting system design is provided in Chapter 4.
Lay out the switches required to control the lights, appliances, equipment, and any
desired receptacles. The discussion of switches outlines where they are most commonly
used and the symbols used. See Figure 3.13.
Locate the panelboard in a convenient location. The location must be accessible (e.g., not
in a closet or storage room, unless dedicated). Follow the requirements outlined earlier.
See Figure 3.12.
ayout circuiting for large appliances and equipment served by an individual branch
circuit. Examples of individual branch circuits would be circuits from the panelboard to a
dishwasher (120 V), electric clothes dryer (120/240 V), an electric oven or range
(120/240 V), or an electric water heater (240 V).
Layout circuiting for lighting and convenience receptacles on general purpose (lighting)
branch circuits. Usually 15 A and 20 A general purpose branch circuits are used for
convenience receptacles, luminaires, and small appliances. See Figure 3.15. The designer
must comply with code requirements. In practice, a good designer tends to be a little
more conservative, generally limiting a 15 A branch circuit to 1000 to 1200 VA and a 20
A branch circuit to 1300 to 1600 VA. Convenience receptacles are generally limited to
about 6 to 8 on a circuit. This allows the circuit to take additional loads, such as when
higher wattage lamps are used to replace those originally installed and calculated.
Because more and more small appliances and equipment are being purchased and
connected to receptacles, the designer must anticipate future requirements. Such a layout
allows for the extension of a circuit if it is necessary to add a light or a convenience
receptacle instead of adding a whole new circuit from the panelboard. If it is possible that
the occupant will desire to install individual air conditioners, an individual branch circuit
may be desired.
8. Lay out the panel circuits, either on the drawing or in a table as shown in Figure 3.16. In
large designs, with more than one panel, this provides the electrician with a schedule of
what circuits will be served from what box. Although a panelboard layout is not often
done for a residence, it is helpful to both the electrician and the designer if one is
included. For commercial projects, a panel layout is almost always included.
3. 60
3. 61
Load Computations
Load computations involve computing the demand load for a building system or a
distribution system extending from a panelboard. This load includes the total of all general
lighting, appliance, and equipment loads in the building. The demand load is not a total of all
connected loads, but rather a fraction of the connected loads. General considerations in demand
load computations follow.
1. Compute the general lighting load. This is calculated for all types of occupancies based
on the unit load given in the table (in watts) times the square footage of the building. (For
this exercise it is based upon Code specified 3 VA/ft 2 for residences). The floor area is
determined using the outside dimensions of the building involved and the number of
stories. For dwellings, do not include any open porches, garages, or carports. Any
unfinished or unused spaces do not have to be included in the square footage unless they
are adaptable for future use. For the (hypothetical) residential system example, the
outside dimensions (excluding garage) of the garage are:
61 ft 6 in * 36 ft, there are 2214 ft2 of floor area
The minimum general lighting load, based on the specified 3 VA/ft2 for residences:
3
VAft2 * 2214 ft2 _ 6642 VA
2. Compute the appliance and laundry circuit load. Code requires at least two 20 A
appliance branch circuits for the kitchen. The load is based on 1500 VA (from Code) for
each appliance branch circuit in the kitchen. In addition, one 20 A circuit is required for
laundry room appliances. For the residential system example, this results in a total of
three 20 A branch circuits for appliances:
Appliance and laundry load = 3 circuits x1500 VA
= 4500 VA
3. Subtotal the general lighting, appliance, and laundry branch circuit loads.
For the residential system example:
General lighting
Appliance and laundry circuits
Subtotal
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6642 VA
4500 VA
11 142 VA
4. The demand load allowed by the Code takes into account that all of the electrical
connections will not be in use at one time. Although there are limits to this reduction for
certain types of occupancies, in a dwelling the first 3000 VA are taken as 100%, and from
3000 to 120 000 VA, only 35% of the load is calculated.
For the residential system example, the load subtotal is 11 142 VA, so:
First 3000 VA at 100%
Remaining 8142 VA at 35%
Total demand load
3000 VA
2850 VA
5850 VA
The loads of all other appliances and equipment (motors) must be added to this demand load to
determine the total service load on the system.
5. To determine the appliance and equipment load, all appliances and equipment that will
not be on the lines discussed above must be listed along with their electrical
requirements. Although typical ratings are given in Tables 19.12 through 19.16,
nameplate ratings from manufacturers’ data should be used in the design. For the
residential system example, the following is a list of fixed appliances and equipment and
their ratings from manufacturer’s data:
3. 63
Water heater
Clothes dryer
Dishwasher
Range
Waste disposal
Air conditioner
Garage door opener
3800 VA
4400 VA
1000 VA
11 700 VA
1000 VA
6300 VA
1000 VA
The demand load for an electric range, consisting of an oven and a cooktop unit, is taken from
manufacturer’s data.
For the residential system example, an electric range with a rating of 11.7 kW, the demand load
would be 8 kW (or 8000 VA):
Electric range demand load = 8000 VA
The demand load for the clothes dryer is the total amount of power required according to the
manufacturer’s data.
For the residential system example, the full 4400 VA must be used in the calculation:
Clothes dryer demand load = 4400 VA
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The demand for fixed appliances (other than the range, clothes dryer, and air conditioning and
space heating equipment) is taken as 100% of the total amount required, except that when there
are four or more of these fixed appliances (other than those omitted), the demand load can be
taken as 75% of the fixed appliance load.
For the residential system example, there are three fixed appliances: the water heater at 3800
VA, the dishwasher at 1000 VA, and the waste disposal at 1000 VA. The total of the ratings is
4800 VA, also the demand load.
Fixed appliances demand load = 5800 VA
Motors, such as those used in central air conditioners, have their demand loads calculated as
125% of the motor rating.
For the residential system example, the air conditioner is rated at 6300 VA and the garage door
opener is at 1000 VA. The total demand load will be 7300 VA x 125% = 9125 VA.
Motor (air conditioner/opener) demand load = 9125 VA
The total demand load for all of the lighting and appliances is then tabulated.
For the residential system example:
General lighting, appliances, and laundry
Electric range
Clothes dryer
Fixed appliances
Motor (air conditioner/opener)
Total demand load
5850 VA
8000 VA
4400 VA
5800 VA
9125 VA
33 175 VA
Service Entrance Design
 The service entrance conductors and equipment are designed based on the computed total
demand load.
1. The minimum service entrance size is found by dividing the demand load for the building
by the voltage serving the building. Most commonly, 240 V service is used. For the
residential system example, the total demand load of 33 175 VA is divided by the 240 V
service for a minimum service entrance of 143 A:
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minimum service entrance = 33 175 VA/240 V
= 138.2 or 138 A
The 138 A computed demand load would be rounded up to the nearest commercially
available panelboard rating. From Table 2.4 (Chapter 2), a panelboard with a 150 A
2. The next step is sizing the service entrance conductors and any conductors between the
service equipment and the branch overcurrent device (circuit breaker or fuse). The feeder
size is based on the computed demand load. The size is then selected from Table 2.12 (in
Chapter 2). For the residential system example, using XHHW (75°C,167°F) aluminum
service entrance conductors and a 150 A load, as calculated:
150 A feeder demand load, 3/0 AWG, XHHW, aluminum
3. The size of the neutral conductor may be determined as 70% of the demand load
calculated for the range plus all other demand loads on the system.
For the residential system example, the neutral feeder demand load would be:
Range load (8000 W x 70%)
5600 VA
All other demand loads
25 175 VA
Neutral demand load
30 778 VA
Neutral net computed demand load = 30 778 VA/240 V
= 128.2 A
Select the size of the neutral conductor for a 128 A neutral feeder demand load from
Table 2.12 (in Chapter 2): 2/0 AWG, XHHW, aluminum conductor For the residential
system example, the three service conductors are: Two No. 3/0 AWG and one 2/0 AWG
aluminum conductors.
Circuit Design
 Circuit design involves ascertaining the number and rating of circuits needed in the
panelboard. It generally involves the following steps:
1. Determine the minimum number of lighting circuits by dividing the general lighting load
by the voltage, finding the amperage required and dividing the amperage into circuits.
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For the residential system example, the general lighting load was calculated above as
6642 VA and the voltage used for the lighting is 120 V. Therefore:
6642 VA/120 V = 56 A
Because the general purpose branch circuit size is limited to 80% of the rating, four 20 A branch
circuits for a total of 64 A, or five 15 A branches for a total of 60 A, are needed.
For 20 A circuits: 56 A/ (20 A * 80%) = 3.5
= 4 circuits
For 15 A circuits: 56 A/ (15 A * 80%) = 4.7
= 5 circuits
This is the minimum number of branch circuits required to serve the residence. In laying out the
circuits, almost all designs will have more circuits than the minimum number required. This is
because most designers will limit each circuit to fewer receptacles, lights, or a combination of
receptacles and lights.
2. Lay out and number all branch circuits on the drawing. Bear in mind that, in most cases, all of
these general use receptacles and all lighting will use 120 V service. In large commercial
applications, 277 V may be used for interior lighting. Note that each circuit is numbered. For
the residential system example, there are a total of 14 circuits.
Panelboard Selection
 Select the panelboard based on the number of circuits and the required amperage. Be
certain that all the pole space is not taken up so there is room for expansion. Keep in
mind that each 120 V circuit takes up one pole (for a one-pole circuit breaker), while
each 240 V or 120/240 V circuit takes up two poles.
For the residential system example, there are:
14-120-V circuits 14 poles
4-240-V circuits (4 * 2 poles)
8 poles
Total poles required
22 poles
A 150 A, 24-pole panelboard should be selected (from Table 2.4, Chapter 2).
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REVIEW QUESTIONS
1. What electrical code is universally used in the Philippines?
2. Describe the following branch circuits and where each is used:
a. Individual
b. General purpose
c. Appliance
3. Describe a split wired circuit.
4. How many circuits are required to accommodate kitchen appliances in an average size
home?
5. When selecting the panelboard size, what considerations for the future should be taken
into account?
6. How is the general lighting load for a building determined?
7. How is the minimum service entrance determined?
8. In selecting the service entrance size, what should be considered? On a 120/240 V,
single-phase, three-wire system, identify the following:
a. Voltage between one grounded conductor and one ungrounded conductor
b. Voltage between two ungrounded conductors
9. On a 208 Y/120 V, three-phase, four-wire system, identify the following:
a. Voltage between one grounded conductor and one ungrounded conductor
b. Voltage between two ungrounded conductors
10. On a 240 Y/120 V, three-phase, four-wire system, identify the following:
a. Voltage between one grounded conductor and one ungrounded conductor
b. Voltage between two ungrounded conductors
11. On a 480 Y/277 V, three-phase, four-wire system, identify the following:
a. Voltage between one grounded conductor and one ungrounded conductor
b. Voltage between two ungrounded conductors
12. Identify five types of equipment that may require voltages higher than 120 V AC.
13. Identify three types of equipment that may require three-phase power.
14. Identify the type of cable permitted in single- and multifamily dwelling units but that
cannot be used underground, nor in buildings that are more than three stories above
grade, nor in commercial garages, motion picture studios, theaters, places of assembly,
elevator hoist ways, and other corrosive or hazardous locations.
15. Identify the type of cable permitted in direct-burial applications such as a feeder or
branch circuit provided it is protected by an overcurrent protection device (fuse or circuit
breaker) before if leaves the panelboard.
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PROBLEM SOLVING
1. Determine the ampacity of a No. 12 AWG copper conductor with THHN insulation that
will be used in a 120 V, two-wire circuit in an environment with an average ambient air
temperature of no greater than 86°F (30°C). Neglect temperature and bundling correction
factors.
2. Determine the ampacity of a No. 14 AWG copper conductor with THHN insulation that
will be used in a 120 V, two-wire circuit in an environment with an average ambient air
temperature of no greater than 86°F (30°C). Neglect temperature and bundling correction
factors.
3. Determine the ampacity of a No. 12 AWG copper conductor with THHN insulation that
will be used in a 277 V, two-wire circuit in an environment with an average ambient air
temperature of no greater than 86°F (30°C). Neglect temperature and bundling correction
factors.
4. Determine the ampacity of a No. 4 AWG copper conductor with THHN insulation that
will be used in a 120 V, two-wire circuit in an environment with an average ambient air
temperature of no greater than 86°F (30°C). Neglect temperature and bundling correction
factors.
5. Determine the ampacity of a No. 1/0 AWG copper conductor with underground feeder
(UF) insulation that will be used in a 120 V, two-wire circuit in an environment with an
average ambient air temperature of no greater than 86°F (30°C). Neglect temperature and
bundling correction factors.
6. Determine the maximum one-way distance two conductors can carry a current of 20 A on
a 120 V, single phase circuit based on a maximum voltage drop of 3%.Base your analysis
on the following conductor sizes:
a. No. 14 AWG—copper
b. No. 14 AWG—aluminum
c. No. 12 AWG—copper
d. No. 12 AWG—aluminum
e. No. 8 AWG—copper
f. No. 8 AWG—aluminum
7. Determine the maximum distance two conductors can carry a current of 20 A on a 277 V,
single-phase circuit based on a maximum voltage drop of 3%. Base your analysis on the
following conductor sizes:
a.
b.
c.
d.
e.
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No. 14 AWG—copper
No. 14 AWG—aluminum
No. 12 AWG—copper
No. 12 AWG—aluminum
No. 8 AWG—copper
f. No. 8 AWG—aluminum
8. A set of conductors will feed power to a well station with pump having a load of 30 A.
The building is 150 ft from the 120 V power source. Determine the required wire size for
copper and aluminum conductors at a maximum voltage drop of 3%.
9. A set of conductors will feed power to an outbuilding with equipment having a total load
of 12 000 VA. The building is 200 ft from the 120 V power source. Determine the
required wire size for copper and aluminum conductors at a maximum voltage drop of
3%.
10. Determine the circuit rating for the following appliances or equipment on a 120/240 V
circuit:
a. Household range
b. Trash compactor
c. Household clothes washer
d. Household clothes dryer (electric)
e. Central air conditioner (5 ton)
f. Eight luminaires with 4-32 W fluorescent lamps per luminaire (138W including ballast)
11. Determine the circuit rating for the following appliances or equipment on a 277 V circuit:
a. Eight luminaires with 4-32 W fluorescent lamps per luminaire (138W including
ballast)
b. Sixteen luminaires with 4-32 W fluorescent lamps per luminaire (138W including
ballast)
c. Twenty luminaires with 4-32 W fluorescent lamps per luminaire (138W including
ballast)
12. Design the electrical service entrance system for the residence in Appendix D. Base your
analysis on:
a. Water heater 3800 W
b. Clothes dryer 4400 W
c. Dishwasher 1000 W
d. Range 11 700 W
e. Air conditioner 9000 W
13. Design the electrical service entrance system for one of the apartments in Appendix A.
Calculate the total load and service for the 38 ft x 28 ft (11.6 m x 8.55 m) apartment building.
Base your analysis on:
a. Air conditioner, 2000 W
b. Electric range, 10 500 W
c. Water heater, 3500 W
d. Dishwasher, 1000 W
e. No clothes dryer
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