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 requires a lower investment. Standard power transmission systems are 3-phase, 3-conductor,
overhead lines with or without a ground conductor. Transmission lines are classed as
unregulated because the voltage at the generating station is controlled only to keep the lines
operating within normal voltage limits and to facilitate power flow.
1.4 PRIMARY DISTRIBUTION SYSTEMS. The transmission system voltage is stepped-down
to lower levels by distribution substation transformers. The primary distribution system is that
portion of the power network between the distribution substation and the utilization transformers.
The primary distribution system consists of circuits, referred to as primary or distribution feeders,
that originate at the secondary bus of the distribution substation. The distribution substation is
usually the delivery point of electric power in large industrial or commercial applications.
1.4.1 Nominal System Voltages. Primary distribution system voltages range from 2,400 V to
69,000 V. Some of the standard nominal system voltages are:
Volts
Phase
Wire
4,160Y/2,400
Three
Four
4,160
Three
Three
6,900
Three
Three
12,470Y/7,200
Three
Four
12,470
Three
Three
13,200Y/7,620
Three
Four
13,200
Three
Three
13,800Y/7,970
Three
Four
13,800
Three
Three
24,940Y/14,400
Three
Four
34,500
Three
Three
69,000
Three
Three
The primary distribution voltages in widest use are 12,470 V and 13,200 V, both three wire and
four wire. Major expansion of distribution systems below the 15 kV nominal level (12 kV - 14.4
1-4
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