single-phase electric power

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173px, A single-phase polemount stepdown transformer in Canada. One supply phase (phase-to-neutral) from the utility is converted to split-phase for the customers. In electrical engineering, single-phase electric power is the distribution of
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...
electric power Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt The watt (symbol: W) is a unit of Power (physics), power or radiant flux. In the International Sy ...
using a system in which all the
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

s of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current does not produce a rotating magnetic field; single-phase motors need additional circuits for starting (capacitor start motor), and such motors are uncommon above 10 kW in rating. Because the voltage of a single phase system reaches a peak value twice in each cycle, the instantaneous power is not constant. Standard
frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...
of single-phase power systems are either 50 or 60 . Special single-phase
traction power network A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses alternating current Alternati ...
s may operate at 16.67 Hz or other frequencies to power electric railways.

# Splitting out

Single-phase is sometimes divided in half at the
distribution transformer A distribution transformer or service transformer is a transformer A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple Electrical network, circuits. A varying current ...
on the secondary winding to create
split-phase electric power "split-phase" secondary. On the three secondary terminals, the center tap is grounded with a short strap to the transformer case. A split-phase or single-phase three-wire system is a type of single-phase electric power 173px, A single-phase po ...
for household appliances and lighting.Terrell Croft and Wilford Summers (ed), ''American Electricians' Handbook, Eleventh Edition'', McGraw Hill, New York (1987) {{ISBN, 0-07-013932-6, chapter 3, pages 3-10, 3-14 to 3-22.

# Applications

In North America, individual residences and small commercial buildings with services up to about 100 kVA (417 amperes at 240 volts) will usually have three-wire single-phase distribution, especially in rural areas where motor loads are small and uncommon. In rural areas where no three-phase supply is available, farmers or households who wish to use three-phase motors may install a phase converter. Larger consumers such as large buildings, shopping centers, 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 kVA, a load concentrated over a few hundred square meters. High-power systems, say hundreds of kVA or larger, are nearly always three-phase. The largest supply normally available as single-phase varies according to the standards of the electrical utility. In the
United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United Kingdom. Some prefer to use Britain as shortha ...

a single-phase household supply may be rated 100 A or even 125 A, meaning that there is little need for three-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 three-phase (in urban areas with three-phase supply networks). If heating equipment designed for a 240-volt 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-volt or 240-volt supply. A single-phase load may be powered directly from a
three-phase In electrical engineering, three-phase electric power systems have at least three conductors carrying alternating voltages that are offset in time by one-third of the period. A three-phase system may be arranged in delta (∆) or star (Y) (also ...
distribution transformer in two ways: by connection between one phase and neutral or by connection between two phases. These two give different voltages from a given supply. For example, on a 120/208 three-phase system, which is common in North America, the phase-to-neutral voltage is 120 volts and the phase-to-phase voltage is 208 volts. This allows single-phase lighting to be connected phase-to-neutral. Single-phase power may be used for ; the largest
single-phase generator Single-phase generator (also known as single-phase alternator) is an alternating current electrical generator that produces a single, continuously alternating voltage. Single-phase generators can be used to generate power in single-phase electric po ...
in the world, at
Neckarwestheim Nuclear Power Plant s of the two Neckarwestheim-units Neckarwestheim Nuclear Power Station is a nuclear power plant in Neckarwestheim, Germany, sometimes abbreviated GKN (for german: Gemeinschaftskraftwerk Neckar), operated by EnBW Kernkraft GmbH. GKN 1 Unit I, in se ...
, supplied a railway system on a dedicated
traction power network A traction network or traction power network is an electricity grid for the supply of electrified rail networks. The installation of a separate traction network generally is done only if the railway in question uses alternating current Alternati ...
.

# Grounding

Typically a third conductor, called ''
ground Ground may refer to: * Soil, a mixture of clay, sand and organic matter present on the surface of the Earth * Ground (electricity), the reference point in an electrical circuit from which voltages are measured ** Earthing system, part of an elect ...
'' (or "safety ground") (U.S.) or ''protective earth'' (UK, Europe, IEC), is used as a protection against
electric shock Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce a ...
, and ordinarily carries significant current only when there is a circuit fault. Several different
earthing system An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the Ground (electricity), ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of ear ...
s are in use. In some extreme rural areas
single-wire earth return Single-wire earth return (SWER) or single-wire ground return is a single-wire transmission line which supplies single-phase electric power from an electrical grid to remote areas at low cost. Its distinguishing feature is that the earth ...
distribution is used.