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Two-phase electrical power was an early 20th century polyphase alternating current electric power distribution system. Two circuits, or "phases", were used, with voltages 90 electrical degrees apart in time. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines. Some early two-phase generators had two complete rotor and field assemblies, mechanically shifted by 90 mechanical degrees to provide two-phase power.

The advantage of two-phase electrical power was that it allowed for simple, self-starting electric motors. In the early days of electrical engineering, it was easier to analyze and design two-phase systems where the phases were completely separated, since this avoided the need for the effect of unbalanced loads. It was not until the invention of symmetrical components that three-phase power systems had a convenient mathematical tool for describing unbalanced load cases. The revolving magnetic field produced with a two-phase system allowed electric motors to provide torque from zero motor speed, which was not possible with a single-phase induction motor (without extra starting means). Induction motors designed for two-phase operation use the same winding configuration as capacitor start single-phase motors.

Three-phase electric power requires less conductor mass for the same voltage and overall amount of power. It has all but replaced two-phase power for commercial distribution of electrical energy, but two-phase circuits are still found in certain control systems.

Two-phase power can be derived from a three-phase source using two transformers in a Scott connection. One transformer primary is connected across two phases of the supply. The second transformer is connected to a center-tap of the first transformer, and is wound for 86.6% of the phase-to-phase voltage on the 3-phase system. The secondaries of the transformers will have two phases 90 degrees apart in time, and a balanced two-phase load will be evenly balanced over the three supply phases.

Three-wire, 120/240 volt single phase power used in the USA and Canada is sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase.

• Polyphase system

• Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN 0-07-013932-6
• Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN 0-07-020974-X
• Edwin J. Houston and Arthur Kennelly, Recent Types of Dynamo-Electric Machinery, copyright American Technical Book Company 1897, published by P.F. Collier and Sons New York, 1902