GOALS
• analyze the requirements of a singlefamily dwelling.
• determine the size of serviceentrance conductors.
• select the proper material and demonstrate the proper methods for the
service entrance installation.
The service entrance for most presentday lighting installations is a
singlephase, threewire service.
ill. 1 Singlephase, threewire system.
The middle wire is called the neutral wire and is grounded. Therefore,
this neutral wire (grounded conductor) is the identified or white wire
of any singlephase, threewire installation. The two outside wires are
known as the hot wires (conductors). The voltage between the neutral wire and either of the two hot wires is 115 volts, and the voltage between the
two hot wires is 230 volts.
It is an advantage to have both 115 and 230 volts available. Many types
of loads, such as electric water heaters, electric ranges and fractional
horsepower motors, operate on 230 volts.
Serviceentrance voltages have gradually increased in many parts of the
country. Therefore, it's not uncommon to find 120 volts and 240 volts
as standard voltages.
TYPICAL SINGLEFAMILY DWELLING
It is the intent of this unit to present the fundamental installation
rules for a service entrance insofar as they concern the calculations that
affect the serviceentrance switch, serviceentrance conductors, and grounding.
The branch circuits supplying the various items of electrical equipment
in a residence are covered briefly. The metering facilities for electrical
spaceheating equipment as well as electric water heaters vary according
to the requirements of the local utility company. As a result, it's not
possible to cover all of these metering methods in detail. The electrician
must check the requirements for these installations with the utility company
serving the area in which the wiring is to be installed.
A typical application for a 115/230volt, singlephase, threewire service
installation is a singlefamily dwelling. The residence considered in this
unit's a sixroom house (including three bedrooms) with an area of 1,500
square feet (1,500 ft^2) The residence contains a 3kW water heater; a
5.0kW clothes dryer; an 8A, 230V room air conditioner; a dishwasher
rated at 11.1A, 115 V; a 4kW wallmounted oven; a 6kW countermounted cooking
unit; a garbage disposal rated at 7.5 A, 115 V; and 12 kW of electric spaceheating
equipment installed for all six rooms. Each room has individual thermostatic
control.
Determination of Number of Lighting Circuits
To determine the number of lighting circuits, the lighting load of the
residence must be calculated. The calculations are based on the watts per
square foot method outlined in the National Electrical Code (referred to
here as “the Code”). In general, the outside dimensions of the building
are used, not including open porches, garages, unfinished attics, or basements.
For the residence in question, the area is 1,500 square feet. The recommended
unit load is 3 watts per square foot.
Therefore, the total lighting load in watts is:
1,500 square feet x 3 watts per square foot = 4,500 watts
To determine the minimum number of 115volt branch lighting circuits:
amperes = watts/volts = 4,500/115 = 39.1 or 40 amperes
In general, 15ampere lighting circuits using No. 14 AWG conductors are
installed in residential occupancies. Some electrical specifications require
a minimum conductor size of No. 12 AWG on all circuits.
Thus 40/15 = 2 plus or 3 lighting circuits (minimum)
However, a residence of this type may have as many as 60 outlets, including
ceiling fixtures, porch fixtures, and wall convenience receptacles located
throughout the living area, basement, garage, and grounds. As a result,
most electricians prefer to limit the number of outlets per circuit to
8 or 10. This results in a more adequate number of lighting circuits. For
the residence covered in this unit, at least 6 lighting circuits will be
installed even though the minimum number of circuits required is 3.
Determination of Number of Small Appliance Circuits
The Code specifies that for small appliances an additional load of not
less than 1,500 watts shall be included for each circuit for the receptacle
outlets. These circuits shall feed only receptacle outlets in the kitchen,
pantry, family room, dining room, and breakfast room of a dwelling. Two
or more 20ampere branch circuits shall be provided, and such circuits
shall have no other outlets. No. 12 AWG wire is used instead of No. 14
AWG wire to minimize the voltage drop in the circuit. Thus, by using the
larger wire, the performance of appliances is improved and the danger of
overloading circuits is decreased.
Since automatic washing machines draw a large amount of current during
certain portions of their operating cycles, the Code requires that a separate
20ampere circuit for the laundry outlets be installed. All convenience
receptacles must be of the grounding type.
To determine the serviceentrance requirements of this dwelling, the small
appliance load is assumed to be 4,500 watts, based on three 20ampere circuits
at 1,500 watts per circuit.
Garbage Disposal. The garbage disposal unit's rated
at 7.5 amperes, 115 volts, and will be supplied by a separate 15ampere
circuit which requires No. 14 AWG conductors.
The garbage disposal load is:
7.5 amperes x 115 volts = 862 watts
Dishwasher. The dishwasher is rated at 11.1 amperes,
115 volts, and will be supplied by a separate 15ampere circuit using No.
14 AWG wire. (It is possible to supply the garbage disposal unit and the
dishwasher with one 115/230volt, threewire circuit.)
The dishwasher load in watts is:
11.1 amperes x 115 volts = 1,276 watts
Dryer Circuit. The electric clothes dryer is rated at
5.0 kW, 230 V. The current it draws is:
5,000 watts/230 volts = 21.7 amperes
The Code states that the branchcircuit rating for continuous duty loads
shall be not less than 125 percent of the rating of the appliance.
21.7 amperes x 1.25 = 27.1 amperes
The circuit to the dryer will be a 30ampere, 230volt, threewire circuit
consisting of two No. 10 AWG conductors for the hot wires and one No. 10
AWG conductor for the neutral conductor.
Wallmounted Oven. The oven is rated at 4 kW, 115/230
V and will be connected to a separate circuit. In watts, the load is 4,000
watts; in amperes, the load is equal to:
amperes = watts/ volts=4000/230=17.4 amperes
No. 12 AWG wire may be used for this circuit.
Countermounted Cooking Unit. The surface cooking unit's rated at 6 kW, 115/230 V. The unit will be connected to a separate circuit
consisting of three No. 10 AWG conductors supplied by a 30ampere, twopole
overcurrent device.
The load, in amperes, is:
amperes = watts/volts = 600/230 = 26A
Air Conditioner. The room air conditioner draws 8 amperes
at 230 volts and will be connected to a separate 15ampere, 230volt circuit
with No. 14 AWG conductors (15 amperes).
The air conditioner load, in watts, is:
8 amperes x 230 volts = 1,840 watts
Water Heater. Many utility companies furnish current
for residential electric water heater loads at a power consumption rate
lower than the regular lighting rate. In such installations, some utility
companies require a separate offpeak meter, while other companies predetermine
a fixed portion of the monthly light bill to cover the power consumption
of the water heater.
In general, for the offpeak metering circuit, the top element of the
heater is connected to a twopole, 230volt circuit supplied through the
house meter. The bottom element of the heater is connected to a twopole,
230volt circuit supplied through the offpeak meter. These elements can
be connected for limited demand, in which case both elements cannot be
energized simultaneously; or they may be connected for unlimited demand,
in which case both elements may be energized simultaneously. The types
of thermostats furnished with the water heater determine how the elements
are connected.
Various types of equipment are manufactured in which both regular and offpeak overcurrent protective devices may be located in the same enclosure.
The offpeak device is called a feedthrough unit and isn't connected
in any manner to the main bus of the panel even though it's located in
the same enclosure. In the feedthrough unit, the two wires from the offpeak
meter are connected to one side of the unit, and the two wires sup plying
the element of the water heater are connected to the other side.
The water heater in this residence is rated at 3 kW. This load, in amperes,
is equal to:
amperes =watts / volts = 3,000 / 230 =13 amperes
When connected for unlimited demand, the maximum current of this water
heater is 13 amperes.
Consult the local utility company for guidelines on the proper connection
of water heaters.
Electric Space Heating. The specified total of 12 kW
of electric space heating units will be installed throughout the residence.
Each of the six rooms will have a thermostat to provide individual heating
control. According to the National Electrical Code, there must be four
or more individually controlled electric space heating units to apply certain
demand factors permitted by the Code. These factors are used to calculate
the ser viceentrance capacity. Approximately 2 kW of space heat will be
provided in each room. Since these units are rated at 230 volts, the load
for each is:
amperes = watts/volts = 2,000 / 230 = 8.7A
The branchcircuit current rating is 125 percent of the load, or, 1.25
x 8.7 = 10.9 amperes.
Therefore, each unit will be connected to a separate 15ampere, 230volt
circuit using No.14 AWG wire.
Some utility companies offer lower rates for electric heating when this
is a residential requirement in addition to general electrical services.
These rates usually are based on special metering methods. The electrician
should consult the utility company supplying power in a local area for
the correct method of connecting heating loads.
Summary of Branch Circuits for the Residence:
No. of Circuits 
Voltage 
Use 
BranchCircuit Ampere Rating 
Poles 
Wire Size (AWG No.) 
6
3
1
1
1
1
1
1
1
6 
115
115
115
115
115/230
115/230
115/230
230
230
230 
General lighting
Small appliance and laundry
Garbage disposal
Dishwasher
Dryer
Oven
Surface cooking unit
Air conditioner
Water heater
Space heat 
15
20
15
15
30
20
30
15
20
15 
1
1
1
1
2
2
2
2
2
2 
14
12
14
14
10
12
10
14
12
14 
SIZE OF SERVICEENTRANCE CONDUCTORS
Section 23042 of the National Electrical Code specifies that serviceentrance
conductors shall have a currentcarrying capacity sufficient to carry the
load as determined by Article 220 For dwelling occupancies, the Code permits
the use of either of two methods to determine the size of these conductors
Method Number 1 (Standard Method) (Article 220, Parts A and B)
General Lighting Load
15,00 square feet at 3 watts per square foot: 4,500 watts
Small Appliance Load (Section 22016):
Three 20ampere appliance circuits at 1,500 watts per circuit: 4,500 watts
Total (without range): 9,000 watts
Application of demand factor (Table 22011):
3,000 watts at 100% = 3,000 watts
9,000  3,000 = 6,000 watts at 35% = 2,100 watts
Net computed load without range 5,100 watts
Wallmounted Oven and Countermounted Cooking Unit (Table 22019, Note 4):
6,000 + 4,000 = 10,000 watts at 80% = 8,000 watts
Net computed load with range (5,100 + 8,000) = 13,100 watts
Electric Space Heating (Section 22015) 12,000 watts
Air conditioning wattage is 8 x 230 = 1,840 watts. This value is less
than the 12,000 watts of space heating; therefore, the air conditioner
load need not be included in the service calculation (Section 22021).
Water Heater .... 3,000 watts
Dryer 5,000 watts
Dishwasher .... 1,276 watts
Garbage Disposal (862 x 1.25)
[Section 21022(a)] 1,077 watts
Total .... 10,353 watts
Since there are four appliances in addition to the cooking units and space
heating, a demand factor of 75 percent may be applied to the fixed appliance
load (Section 22017).
Thus, 10,353 x 0.75 = 7,765 watts
Total Calculated Load: 13,100 + 12,000 + 7,765 = 32,865 watts
Amperes = watts/volts = 32,865/230 = 142.9 amperes
According to Table 31016 Note 3 of the Code, for a load of 142.9 amperes,
No. 1 RHW or THW wire may be installed as the copper serviceentrance conductors.
It should be noted that for singlefamily residences with initial load
of 10 kW or more, computed in accordance with Article 220, there shall
be a minimum of a 100 ampere, threewire service. This minimum also applies
to residences that have six or more twowire branch circuits [23042(b)].
Method Number 2 (Article 220, Part C)
An optional method of determining the load of a singlefamily dwelling
is recognized by the Code in Section 22030. This method simplifies the
calculations and usually results in a smaller size of service entrance
than is permitted by Method 1.
1,500 square feet at 3 watts per square foot .... 4,500 watts
Three 20ampere appliance circuits at 1,500 watts per circuit .... 4,500
watts
Wallmounted oven (nameplate rating) .... 4,000 watts
Countermounted cooking unit (nameplate rating) .... 6,000 watts
Water heater .... 3,000 watts
Dryer .... 5,000 watts
Dishwasher .... 1,276 watts
Garbage disposal (862 x 1.25) [ 21022(a)] .... 1,077 watts
Electric space heating 12,000 watts
Air conditioner wattage is 8 x 230 = 1,840 watts. This value is less than
the 12,000 watts of space heating; therefore, the air conditioner load
need not be included in the service calculation. [22021] _____
Total load.... 41,353 watts
Then: first 10 kWh at 100% =10,000 watts
remainder of load at 40% (31,353 x 0.4) = 12,541 watts
Total Calculated Load.... 22,541 watts
Amperes = watts/volts = 22,541/230 = 98 amperes
According to Table 31016 of the Code, for a load of 98 amperes, No. 4
RHW wire or equivalent may be installed as the copper serviceentrance
conductors.
Both Methods 1 and 2 for determining total load are correct as far as
the Code is concerned. Therefore, the decision as to which method is permitted
in an area is made by the local electrical inspector.
To provide a single panel which will accommodate all the circuits in the
residence, it's necessary to install a 200ampere panel.
Certain localities require that the conductors supplying a panel or switch
must have a currentcarrying capacity equal to the rating of the panel
or switch. Therefore, for the residence covered in this unit, No. 2/0 RHW
or equivalent wire is required for the service entrance. The installation
of No. 2/0 RHW wire or equivalent will give the homeowner full 200ampere
capacity. See Note 3 to Table 3101 6.
ServiceEntrance Switch (Sections 23070 and 23071)
Section 23071 of the National Electrical Code in essence specifies that
the service disconnecting means shall consist of not more than six switches
or six circuit breakers in a single enclosure, in a group of separate enclosures,
or in or on a switchboard. It is the intent of this section to insure that
all electrical equipment within a building can be disconnected with no
more than six manual operations. However, certain local ordinances don't
permit the six subdivision rule but rather require that each service shall
have a single main disconnect.
To accommodate the number of circuits listed in the Summary of Branch
Circuits for the Residence, a 200ampere panel will be installed. This
panel will contain all of the required branch circuits plus a 200ampere
main pullout in one enclosure. This type of enclosure is acceptable as
both the load center and service equipment, and meets Code requirements
in most localities.
Generally, the service switch is located in the basement and the meter
is mounted on the outside of the house for easy access by the utility company.
Ground Connection
Section 2505(b) (1) of the Code requires the grounding of interior alternating
current systems where the system can be grounded so that the maximum voltage
to ground, on the ungrounded conductors, does not exceed 150 volts. Grounding
is accomplished by running a wire from the neutral connection in the main
service switch or meter to the water piping system on the street side of
the water meter. The reason for connecting on the street side of the water
meter is so that the ground circuit remains connected if the meter must
be removed for repair.
Sections 25091 and 25092 set forth the rules governing grounding materials and the installation of the ground wire. The size of ground wire required
is found in Table
25094. It was mentioned previously that the residence covered in this
unit will be sup plied with No. 2/0 RHW serviceentrance conductors. According
to Table 25094, No. 2/0 RHW conductors require a No. 4 AWG grounding conductor.
Note: It is beyond the scope of this unit to illustrate all the methods
of metering the water heater and electrical space heating load. ill
112, therefore, illustrates the entire load connected to a single meter.
This figure is used only to outline the installation requirements.
Bonding
The proper bonding of all serviceentrance equipment is as important as
the use of the proper size of service conductors. Section 25071 lists
the equipment that shall be bonded and Section 25072 lists the methods
approved for bonding this equipment. Bonding jumpers shall have a currentcarrying
capacity not less than is required for the corresponding grounding conductor.
The purpose of bonding on serviceentrance equipment is to assure a low
impedance path to ground should a fault occur on any of the serviceentrance
conductors. Severe arcing, which presents a fire hazard, may occur at a
fault. Proper bonding reduces this hazard to some extent.
The fire hazard exists because the serviceentrance conductors are not
fused at the service head. The shortcircuit current on these conductors
is limited only by the capacity of the transformer or transformers supplying
the service equipment and the distance the service equipment is located
from these transformers. Shortcircuit current can easily reach 10,000
amperes in residential areas and as high as 200,000 amperes in industrial
areas. All overcurrent devices (fuses and circuit breakers) must have adequate
interrupting capacity. See Sections 1109 and 23098 of the Code.
QUIZ
1. State one reason why the singlephase, 115/230volt, threewire system
is used in presentday installations instead of a 115volt, twowire system.
ill. 112 A typical serviceentrance
installation.
 Point of attachment to buildings, section 23026
 In general, service head to be above connections: sect/on
23054
 Clearances of service drops: sect/on 23024
 Size of serviceentrance conductors: sect/on 2304 1 (100
ampere minimum recommended grounded neutral may be bare when300
volts or less to ground.)
 Conduit size: tables3a,3b,3c,4, 5,5a,8
 Mount meter at eye level
 Main switch and overcurrent protection: sections 23070 through
23094
 23065 available shortcircuit current. Service equipment
shall be suitable for the shortcircuit current at its supply
terminals.
 Fuses or circuit breakers must have interrupting capacity
sufficient for voltage employed and current which must be interrupted.
(sections 1109 and 11010).
 Service grounding conductors: table 25094, sections 25091,92
 Recommended height of meter socket
 Bonding section through section also insulating bushings

2. State how the area, in square feet, for a singlefamily dwelling is
determined to arrive at the approximate lighting load.
3. What is the number of watts per square foot allowed by the Code when
determining the lighting load for a singlefamily dwelling?
4. What is the minimum number of appliance circuits permitted by the Code
for a singlefamily dwelling?
5. Explain why the ground wire is connected to the street side of the
water meter in installations where there is a public water system.
In items 610, select the best answer, and place the corresponding letter
in the space provided.
6. A singlephase, 115 /230volt, threewire system shall
a. be grounded.
b. be ungrounded.
c. have one appliance circuit.
d. have No. 12 serviceentrance wires.
7. The wire used for the small appliance circuits cannot be smaller than
a. No. 10.
b. No. 12.
c. No. 14.
d. No. 16.
8. In a normal threewire installation for a singlefamily dwelling, the
voltage from one ungrounded wire to the grounded neutral is approximately
a. 460 volts. c. 150 volts.
b. 230 volts. d. 115 volts.
9. For singlefamily residences with an initial load of 10 kW or more,
computed according to acceptable methods, the service shall be at least
______
a. 50 amperes. c. 100 amperes.
b. 80 amperes. d. 120 amperes.
10. Serviceentrance equipment shall be
a. of a common size for all installations.
b. bonded.
c. stapled.
d. selected before the load is determined.
11. Determine the minimum number of 115volt lighting circuits necessary
in a singlefamily dwelling with an active area of 2,300 square feet.
12. A singlefamily dwelling has a 7.6kW, 115/230volt electric range.
Determine the minimum size of the conductors to be used for the range feeder
from the service entrance cabinet to the range outlet. 