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1002 ⏐⏐⏐ POLYPHASE SYSTEMS
Secondary
3 φ transformer
208 V ∠0°
I L1
Line
Line
Neutral
Line
Because of the variety of equipment found in an industrial environment,
both three-phase power and single-phase power are usually provided
with the single-phase obtained off the three-phase system. In
addition, since the load on each phase is continually changing, a fourwire
system (with a neutral) is normally employed to ensure steady
voltage levels and to provide a path for the current resulting from an
unbalanced load. The system of Fig. 22.35 has a three-phase transformer
dropping the line voltage from 13,800 V to 208 V. All the lowerpower-demand
loads such as lighting, wall outlets, security, etc., use the
single-phase, 120-V line to neutral voltage. Higher power loads, such as
air conditioners, electric ovens or dryers, etc., use the single-phase,
208 V available from line to line. For larger motors and special highdemand
equipment, the full three-phase power can be taken directly off
the system, as shown in Fig. 22.35. In the design and construction of a
commercial establishment, the National Electric Code requires that every
effort be made to ensure that the expected loads, whether they be single-
208 V ∠120°
E L
208 V ∠–120°
1 φ
120 V
E L
E L
I N
1 φ
280 V
I L2
I L3
I fL3
+
Z 3
+
V f1 Z 1
V f3
FIG. 22.34
Unbalanced Y-connected load.
1 φ
120 V
FIG. 22.35
3f/1f, 208-V/120-V industrial supply.
1 φ
120 V
–
–
3φ
208 V
balanced
load
–
I fL1
V f2
Z 2
I fL2
+