3 Phase Electric Wiring

Electricity 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.

Powering a three-part load, like three identical coils in a 3 phase motor, is a common application and provides an efficient use of 3 phase power.

The three coils of a 3 phase voltage source can be configured as a Wye (sometimes called Y or Star). If the coil voltage is 277V, then a connection to the ends of two adjacent coils would produce 480 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.

Three phase circuits can also be wired in the delta configuration (sometimes called mesh), where any two connections will give the coil voltage as an output voltage.

Advantages of 3 phase power distribution:

• For a balanced 3 phase supply, the total current is zero.
  • This minimizes magnetic fields surrounding the transmission lines.
  • The three phases provide a naturally rotating magnetic field for three phase motors. This simplifies design and enhances efficiency.
  • When connected to large motors, it provides constant power and minimizes vibration. This leads to longer equipment life.