Split phase

Split phase is a mains electrical supply system mainly used in North America ( which may be incorrectly referred to as two phase). It is used occasionally in the UK in distribution situations where only two 11 kilovolt phases are available, but it is rarely taken into buildings.

A transformer providing split phase has a single phase input, and the output is centre-tapped with the neutral on the centre tap.

This means that there are two phase conductors both with equal voltages to the neutral and the voltage between the two phase conductors is twice the phase-neutral voltage. The neutral conductor ensures that the voltages on the two legs do not get (far) out of balance. Appliances operating at the normal mains voltage (120 V in the us) can be directly supplied, with better voltage regulation and smaller conductors than a single ended single phase system would allow for. It is also possible to supply appliances that need twice the normal voltage (240V in the us) directly by connecting them between the two phase conductors. In the us this is the normal method of connecting larger appliances such as cooking, space heating and air conditioning equipment that are designed to run at 240 V. No individual conductor will be at more than 120 V potential with respect to ground (earth), which reduces insulation rquirements in all apparatus over a 240-volt single ended system.

The practice originated with the DC distribution system developed by Thomas Edison. For example, by dividing a lighting load into two equal groups of lamps connected in series, the total supply voltage can be doubled and the size (and cost) of conductors can be cut in half. By connected the two lamp groups to a neutral, intermediate in potential between the two live legs, any imbalance of the load will be supplied by a current in the neutral, giving substantially constant voltage across both groups. The total mass of conductors required to supply a given load is reduced, compared to a system operating only at the utilization voltage.

A further variation of "split phase" is a three-phase 240 volt delta connected system in which one pair of phases has a centre tap. This allows a single service to supply 120 V for lighting, 240 V single phase for heating appliances, and 240 V three-phase for motor loads (such as air conditioning compressors).

A split phase motor is a type of single-phase electric motor. A split-phase motor runs on a single phase and does not require a split-phase distribution system.