split-phase power
A type of electrical power distribution system using two line wires connected to the ends of a distribution transformer and a grounded neutral wire connected to the center tap of the transformer, allowing loads to be connected between line and neutral and receive the normal line voltage (120 or 240 volts), or between the two opposite-phase line wires and receive twice the normal line voltage (240 or 480 volts).
split-phase power: one transformer, two voltage options
Split-phase power is a single-phase distribution system that creates two usable voltage levels from a single transformer secondary winding. The transformer's secondary is center-tapped, meaning a connection point exists at the electrical midpoint. This creates three available conductors: two line wires at opposite ends of the winding, and a neutral wire at the center tap, which is grounded. The result is a three-wire service that supplies both 120 volts (between either line and neutral) and 240 volts (between the two line wires).
Residential buildings in North America depend almost entirely on split-phase service, typically rated at 100 to 200 amperes at the main panel. A 240-volt, 60-hertz transformer delivering 10 kVA or more can support the heating, water heating, air conditioning, and cooking loads that require full voltage, while simultaneously serving lighting and outlet circuits at 120 volts through the neutral. This flexibility is what makes split-phase practical for mixed loads in a single building.
The standard voltage pairs are 120/240 volts in the United States and Canada, and 110/220 volts in some other regions, though 120/240 is now dominant across North America. Industrial sites sometimes use split-phase as a backup or supplementary supply, but three-phase power is preferred for motors and heavy machinery because it delivers smoother power transfer and no pulsation of instantaneous power.
Common problems and limits
Imbalanced loading between the two phases causes excessive neutral current and voltage sag on the more heavily loaded side. If one phase carries 150 amps and the other carries 50 amps, the 100-amp imbalance flows through the neutral conductor back to the transformer. Undersized neutral conductors can overheat and cause fires. Building codes and electrical standards require the neutral to be the same size as the phase conductors, and many jurisdictions now require a separate equipment ground as well.
The term 'split-phase' reflects the transformer's split secondary winding; it is sometimes called single-phase three-wire service to distinguish it from single-phase two-wire (which has only one line and one neutral, delivering one voltage). Split-phase is fundamentally different from true three-phase power, which uses three separate windings 120 degrees apart and requires three or four wires. For industrial work involving motors larger than roughly 5 horsepower, three-phase service is superior and nearly always specified.