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What is two-phase electrical service?

Residential electric service in the United States (120/240 Vac) is sometimes called two-phase service but this is NOT correct. It is only single-phase, since both line voltages are derived from a single phase of a distribution transformer with a center tapped neutral and are 180° out of phase with each other.

Two-phase service is an obsolete style of electrical power distribution where two phases are provided that are 90° out of phase with each other. There were two line wires and one neutral, so two-phase service was commonly a two-phase three-wire service.

Another variation used four line wires and one neutral for a two-phase five-wire service. This was sometimes incorrectly called four-phase five-wire service.

We were not aware of any two-phase services still in use until a visitor contacted us in 2020 to report that Peco Energy in Philadelphia still has 4 kV two-phase three-wire service distribution transformers in the Old City section. The 90 degree difference between the 2 phases resulted in 125 volts phase to neutral and 175 volts phase to phase.

WattNode meters should have no trouble monitoring a two-phase three-wire service.

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