Split-phase electric power
It is the alternating current (AC) equivalent of the original Edison Machine Works three-wire direct-current system.
[1] The system is common in North America for residential and light commercial applications.
A transformer supplying a three-wire distribution system has a single-phase input (primary) winding.
Since the two phasors do not define a unique direction of rotation for a revolving magnetic field, a split single-phase is not a two-phase system.
In the United States and Canada, the practice originated with the DC distribution system developed by Thomas Edison.
Connection of the junction point of each parallel branch of two series lamps to a common neutral, returned to the center tap of the supply voltage, stabilized the branch circuit voltages from changes when loads were switched on and off.
This would not need a neutral conductor at all, but would be impractical for varying loads; just connecting the groups in series would result in excessive voltage and brightness variation as lamps are switched on and off.
A balanced power system is used only for specialized distribution in audio and video production studios, sound and television broadcasting, and installations of sensitive scientific instruments.
However, 130/225 V, three-wire, two-phase electric power discontinued systems called B1 are used to run old installations in small groups of houses when only two of the three-phase high-voltage conductors are used.
In the United Kingdom, electric tools and portable lighting at larger construction and demolition sites are governed by BS 7375, and where possible are recommended to be fed from a centre-tapped system with only 55 V between live conductors and the earth (so-called CTE or centre-tap earth, or 55–0–55).
This three-wire single-phase system is common in North America for residential and light commercial applications.
Circuit breaker panels typically have two live (hot) wires, and a neutral, connected at one point to the grounded center tap of a local transformer.
Wiring regulations govern the application of split-phase circuits so that the shared neutral can be protected from excess current.
Split-phase distribution is used on Amtrak's 60 Hz traction power system in the Northeast Corridor between New York and Boston.
Autotransformers along the track balance the loads between the contact and feeder wires, reducing resistive losses.
