Chapter 10 The Service Entrance
171
Copyright Goodheart-Willcox Co., Inc.
When in conduit, all the conductors are
insulated. The neutral is white, yellow, or gray.
The hot wires are black or red. Service entrance
cable is similar, but the neutral is bare.
Whether conduit or cable is used will
depend upon customer preference, local codes,
building structure, environment, utility speci-
fi cations, and the electrician’s experience. In
either case, the conduit or cable must be securely
fastened to the building with the proper clamps
or supports as shown later in this chapter.
Sizing service entrance conductors
In addition to proper types of service conduc-
tors, the right conductor size is essential. As
mentioned earlier, Table 310.15(B)(16) of the NEC
establishes the allowable ampacities of insulated
conductors. Section 310.15(B)(7) instructs us on
proper types and sizes of conductors for single-
phase service for a dwelling. See Figure 10-11.
Service Drop
Mast and Insulator
Very rarely are the service drop mast (or
riser) and insulators supplied and installed
by the power company. In most situations, the
electrician makes the installation. The service
drop mast is usually a length of rigid metal
conduit or, if allowed, rigid nonmetallic
conduit. This conduit contains the service
entrance cable and is mounted between the
service head and the meter.
An insulator is a nonconductive device
that supports the service drop wires near the
service head. It can be clamped to the conduit
or attached to the exterior of the building.
Insulators are placed approximately 12″ below
the service head. See Figure 10-12.
Service Head
A service head is a fi tting installed at the
top of the conduit or service entrance cable to
prevent water from entering the meter socket and
shorting out the conductors. Service heads are
designed to be used with either conduit or cable.
See Figure 10-13. The service conductors should
extend through the head for approximately 3′ to
provide a suitable drip loop. A drip loop is merely
a formed curvature of the service conductor to
prevent water from entering the service head
around the cable or conduit. See Figure 10-14.
Metering Equipment
The amount of electrical power used within
a residence must be measured, so the utility
knows how much to charge the customer. The
meter is the device that measures and records
the amount of electricity used. The meter socket
is the receptacle that the meter plugs into. This
Figure 10-11. Service entrance conductor sizes are
specifi ed by the NEC.
service drop mast: A length of rigid conduit that
contains the service entrance cable and is mounted
between the service head and the meter.
insulator: A nonconductive device that supports the
service drop wires near the service head.
service head: A fi tting installed at the top of the service
entrance cable to prevent water from entering the
meter socket.
drip loop: A formed curvature of the service conductor
to prevent water from entering the service head.
meter: The device that measures and records the
amount of electricity used.
meter socket: The receptacle that a meter plugs into.
Minimum Service Entrance Conductor Sizes
Service
Rating
(Amperes)
Minimum Size (AWG or kcmil)
Copper
Aluminum or
Copper-Clad
Aluminum
100 4 2
110 3 1
125 2 1/0
150 1 2/0
175 1/0 3/0
200 2/0 4/0
225 3/0 250
250 4/0 300
300 250 350
350 350 500
400 400 600