3 Phase Electric WiringElectricity generation in the United States is typically 3 phase generation at 60Hz, and that 3 phase power is used for long distance transmission of power and for high power industrial use. For household wiring it is usually supplied as 240 volt single phase with a center tap dividing it into two 120 volt supplies. While lighting, normal appliances, etc. are supplied with 120 volts AC, larger appliances may be supplied with 240 volts, single phase. Note that the labeled voltage is typically the rms value of the voltage, and that a 120 volt (rms) household circuit has a peak voltage of 170 volts.
Such voltages from Wye configurations of 3 phase circuits are commonly encountered. If 120 volts is supplied to each supply coil of a Wye, the voltage obtained by bridging two coils is 208 volts. This is less than the 240 volts obtained from a single phase circuit with two 120 volt coils in series, but some equipment can be operated with either 208V or 240V. With the 208V for a given load, the supplied power is only 75% as large since it is proportional to the square of the voltage.
Advantages of 3 phase power distribution:
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