Explanation of 3-Phase AC Power

Updated April 17, 2017

Three-phase power systems are commonly used to transmit power across great distances and to power large motors. Power is evenly distributed in a three-phase system, meaning an even power load is carried among the three wires.

How Three-Phase Power Works

Power is generated at a power station, where mechanical energy is converted into electricity with a transformer. The transformer has three coils; each coil produces an alternating current at the same frequency as the other two, but each current is shifted one-third of the period. The three currents are carried along separate wires until they are transformed into a single-phase current and connected to a building.

Neutral Wire

A fourth wire is often used in low-voltage situations. This carries a neutral current that is equal to the sum of the other three currents. Single-phase loads, such as connections to residential buildings, are often connected between the neutral current and one of the three phases.


Three-phase systems create a rotating magnetic field, which is used in induction motors. Large appliances and farming equipment operate with a three-phase design because net mechanical torque generated by the three currents is much less than that of single phase power. A three-phase system is an excellent choice for long distance transmission because it uses less conductor material than single-phase or two-phase systems of the same voltage.

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About the Author

A professional travel writer since April 2010, Doug Leenhouts has written for and He has a Bachelor of Science in management information systems from the Worcester Polytechnic Institute and three years of service in a consulting firm.