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In electrical engineering, 'single-phase electric power' refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison.
Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120°. Standard frequencies of single-phase power systems are either 50 or 60 Hz.

Contents

Splitting out

Where 3 phase stops and single phase begins

Grounding

Further notes

See also

Splitting out

A single-phase load may be powered from a three-phase distribution system either by connection between a phase and neutral or by connecting the load between two phases. The load device must be designed for the voltage in each case. The phase-to-phase voltage is $sqrt(3)$ times the phase-to-neutral voltage. In places using a 415 volt 3 phase system, the phase-to-neutral voltage is 240 volts, allowing single-phase lighting to be connected phase-to ground and three-phase motors to be connected to all three phases. In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between phase and ground. If heating equipment designed for the 240-volt three-wire single phase system is connected to two phases of a 208 volt supply, it will only produce 75% of its rated heating effect. Single-phase motors may have taps to allow their use on either 208 V or 240 V supplies.
On higher voltage systems (kilovolts) where a single phase transformer is in use to supply a low voltage system the method of splitting seems to vary by country. In North America the primary of the step-down transformer is wired across a single high voltage feed wire and ground, at least for smaller supplies (see photo of transformer on right). In Britain the step-down primary is wired phase-phase.
No arrangement of transformers can convert a single-phase load into a balanced load on a polyphase system.

Where 3 phase stops and single phase begins

Single-phase power distribution is used especially in rural areas, where the cost of a three-phase distribution network is high and motor loads are small and uncommon.
High power systems are nearly always three phase. But the largest supply normally available as single phase varies considerably by country and in some cases region. In the UK it is often as high as 100A or even 125A meaning that there is little need for 3 phase in a domestic or small commercial environment. Much of the rest of Europe has traditionally had much smaller limits on the size of single phase supplies resulting in even houses being supplied with 3 phase (in urban areas with three-phase supply networks).
In North America, individual residences and small commercial buildings with services up to about 100 kV·A (400 amperes at 240 volts) will usually have three-wire single-phase distribution, often with only one customer per distribution transformer. Larger consumers such as large buildings, shopping centres, factories, office blocks, and multiple-unit apartment blocks will have three-phase service. In densely-populated areas of cities, network power distribution is used with many customers and many supply transformers connected to provide hundreds or thousands of kV·A load concentrated over a few hundred square metres.
Three-wire single-phase systems are rarely used in the UK where large loads are needed off only two high voltage phases.
A single-phase supply connected to a pure single-phase induction motor does not produce a revolving magnetic field, and so practical single-phase motors always have some means of producing a revolving field to generate starting torque. Aside from certain traction power applications, single-phase induction motors greater than 10 or 20 kW are very uncommon.

Grounding

Typically a third conductor, called 'ground' (U.S.) or 'protective earth' (Europe, IEC), is used as a protection against electric shock, and ordinarily only carries significant current when there is a circuit fault. Several different earthing systems are in use.

Further notes

Note that true two phase power, meaning the simultaneous provision of sine wave and cosine wave electricity (that is, 90 degrees out of phase) is no longer widely used. But some people incorrectly describe split single phase services as "two phase", when in fact such services are really still single phase power.
True two-phase power uses two completely independent pairs of wires. It lost out to three phase power due to the fact that two phase requires four wires total to function, while three-phase only needs three wires. Copper was expensive to manufacture when electrification first started, and the expense of the extra wire needed for two phase to work was a major cost concern.

See also

★ single wire earth return