Split-phase electric power

Lore & Background

The split-phase system is the alternating current (AC) equivalent of the original three-wire DC system developed by the Edison Machine Works. In the United States and Canada, the practice originated with the DC distribution system developed by Thomas Edison. By connecting pairs of lamps in series and doubling the supply voltage, conductor size was reduced substantially. The common wire for two circuits was replaced by a single conductor returned to the center tap of the supply voltage, stabilizing branch circuit voltages. This arrangement reduced the number of conductors from four to three, saving labor and material.

A transformer supplying a three-wire distribution system has a single-phase input winding and a secondary winding with a center tap connected to a grounded neutral. Either end to center has half the voltage of end-to-end. The two phasors do not define a unique direction of rotation for a revolving magnetic field, so a split single-phase is not a two-phase system. In North America, standard household circuits for lighting and small appliances are connected between one line and the neutral, providing 120 V. Higher-demand appliances such as ovens, dryers, or water heaters are powered by 240 V circuits, connected between the two 120 V lines.

In the United Kingdom, a centre-tapped system with 55 V between live conductors and earth (CTE or 55–0–55) is used at larger construction sites to reduce shock hazard. Portable transformers transform single-phase 240 V to this 110 V split-phase system. A balanced power system, sometimes called 'technical power', uses an isolation transformer with a center tap to create a separate supply with conductors at balanced voltages with respect to ground, minimizing noise coupled into sensitive equipment. This system is used only for specialized distribution in audio and video production studios, broadcasting, and sensitive scientific instruments.

Reader's Guide

The split-phase system's significance lies in its efficient use of conductor material, reducing copper requirements by up to 75% for short runs and 3/8 for long runs compared to a two-wire single-phase system. This efficiency made it the standard for residential and light commercial power distribution in North America, where it supplies both 120 V for lighting and small appliances and 240 V for high-demand loads. The system's legacy includes its origin in Thomas Edison's DC distribution, adapted to AC. In specialized applications, split-phase reduces electric shock risk (as in UK construction sites with 55–0–55 V) or minimizes electromagnetic noise (as in balanced power systems for studios and labs). Wiring regulations govern shared neutral protection, requiring circuit breakers from opposite lines to trip simultaneously. The system's design allows neutral current to carry only the imbalance between loads, stabilizing voltages. While not a two-phase system, its practical advantages in material savings and voltage flexibility have ensured its continued widespread use.

Did You Know?

• The split-phase system is the AC equivalent of the three-wire DC system developed by the Edison Machine Works.
• In North America, the two 120 V lines are 180 degrees out of phase with each other relative to the neutral.
• For short wiring runs, three half-sized conductors can substitute for two full-sized ones, using 75% of the copper of an equivalent single-phase system.
• A balanced power system uses only line-to-line connections at 120 V and is not distributed with a grounded neutral.