Electrical Engineering is a free introductory textbook to the basics of electrical engineering. See the editorial for more information....

The Three-Phase, Four-Wire System

Author: E.E. Kimberly

When both single-phase load and three-phase load must be taken from the same power system with probability of considerable load unbalance, it is advantageous to add a fourth wire connected to the neutral of the power supply. The polyphase loads are connected to the three line wires, and the single-phase loads are connected between line and neutral on the various phases. This system is particularly advantageous in business sections of large cities where the load per square mile is very high. This type of distribution is called a grid network, because the layout of the system somewhat resembles a gridiron.

In low-voltage grid networks the most common power voltage is 208 volts and the most common lamp voltage is 120 volts. If the line-to-line voltage in the system were 220 volts, the line-to-neutral voltage would be


Lamps are not commonly available for 127 volts, and a compromise voltage must be used. It is common practice to reduce the more usual line-to-line voltage of 220 volts to 208 volts, which gives 120 volts line-to-neutral for lamps.

Last Update: 2010-10-06