Chapter 10 The Service Entrance
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Copyright Goodheart-Willcox Co., Inc.
on how the branch circuit is connected within
the distribution panel.
Multiphase Systems
There may be situations where a homeowner
requests a system with more than one phase.
This is a multiphase system that is usually
installed to power heavy-duty or special equip-
ment such as a milling machine or a lathe. Be
sure to review local codes for any restrictions
regarding installing a multiphase system.
Wye, four-wire, three-phase system
Figure 10-35 illustrates a wye-connected,
four-wire, three-phase, 120/208-volt arrange-
ment. Such a system can supply both single-
phase 120-volt circuits and three-phase 208-volt
circuits.
Study the main-panel wiring diagram
shown in Figure 10-36. Note particularly the
different circuit arrangements that are possible.
In this type of panel, there are three hot bus bars.
Each has 120 volts to ground when connected
to the neutral. For 120-volt circuits, the circuit
Service entrance
conductors
Service drop
Secondary lines
Primary lines
Transformer
Drip loop
Figure 10-32. The fi nal step-down occurs at the local
pole transformer near the structure it serves. Overhead
conductors provide the last link to the customer’s
service wires at the service point.
Figure 10-33. The meaning of phase. Electrical energy
may be expressed graphically by using sine-wave
forms. A—Single-phase electrical energy generation.
B—Two-phase current is accomplished by simultane-
ously generating two single-phase currents 90° apart.
C—By generating three, single-phase currents 120°
apart, three-phase current is created.