Two-phase electrical power was an early 20th-century
polyphase alternating current electric power distribution system. Two circuits were used, with voltage
phases differing by one-quarter of a cycle, 90°. Usually circuits used four wires, two for each phase. Less frequently, three wires were used, with a common wire with a larger-diameter conductor. Some early two-phase
generators had two complete rotor and field assemblies, with windings physically offset to provide two-phase power. The generators at
Niagara Falls installed in 1895 were the largest generators in the world at that time and were two-phase machines. As of 21st century, two-phase power was superseded with
three phases and is not used in the industry. There remains, however, a two-phase commercial distribution system in
Philadelphia,
Pennsylvania; many buildings in Center City are permanently wired for two-phase and
PECO (the local electric utility company) has continued the service. This type of service happens to exist in Hartford, Connecticut. It does serve a few buildings in that city.