186
Section 2 Installation
Copyright Goodheart-Willcox Co., Inc.
Delta, four-wire, three-phase system
Another service alternative is the delta-
connected, four-wire, three-phase, 120/240-volt
system. A schematic for this type is illustrated
in Figure 10-37. It can provide 120-volt single-
phase 240-volt single-phase, and 240-volt three-
phase power.
The panel connections, as shown in
Figure 10-38, are made as follows:
1. For 120-volt single-phase circuits, connect
one circuit wire (white) to the neutral and
the other (black) to an ordinary single-pole
breaker attached to phase A or C only. Note:
the breaker must not be attached to phase B
for this type of circuit since this would
provide 208 volts. Equipment designed for
120-volt operation would be ruined.
The phase B wire of this system is often
called the “high leg” or “wild leg” and
must be identifi ed as such at all accessible
points with an orange-colored indicator. See
NEC, Section 110.15. At the panel, wire B is
connected to the center bus bar.
2. To derive power at 240-volt single-phase,
you simply connect both circuit wires to
a double-pole circuit breaker attached to
any two phases. Do not include a neutral
grounded wire in this circuit.
3. For 240-volt three-phase circuits, run three
circuit wires to a three-pole breaker attached
to all three phases. Again, ignore the neutral
for this type of circuit.
Other variations of multiphase service
systems are possible and, in fact, available in
many locations:
• Four-wire, wye-connected, three-phase,
277/480-volt systems are very similar to
the 120/208-volt system already discussed.
The major difference is that the transformer
secondary supplying such a system has
a higher voltage. Since most major appli-
ances, lighting fi xtures, and heating units
do not operate at more than 240 volts, this
system has limited use.
• Three-wire, delta-connected, three-phase,
240-volt systems have severe limitations. It
may supply 240-volt three-phase or 240-volt
single-phase circuits only.
Phase A
Phase B
(wild)
Phase C
Neutral N
Utility supply and
ground at pole
120 V
240 V
240 V
208 V
240 V
120 V
Figure 10-37. Popular delta-connected, three-phase, four-wire service supplies 120 V single-phase, 240 V single-
phase, and 240 V three-phase circuits. The “wild” phase B must always be identifi ed at all terminations and acces-
sible points. Usually it is indicated by the color orange.