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A split-phase or single-phase three-wire system is a type of
single-phase electric power 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 alter ...
distribution. It is the
alternating current Alternating current (AC) is an electric current An electric current is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'natu ...
(AC) equivalent of the original
Edison Machine Works The Edison Machine Works was a manufacturing company set up to produce dynamo A dynamo is an electrical generator that creates direct current using a commutator (electric), commutator. Dynamos were the first electrical generators capable of deli ...
three-wire
direct-current Direct current (DC) is the one directional flow of electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charge: ''posit ...
system. Its primary advantage is that, for a given capacity of a distribution system, it saves conductor material over a single-ended single-phase system, while only requiring a single phase on the supply side of the distribution transformer. This system is common in North America for residential and light commercial applications. Two 120 V AC lines are supplied to the premises which are out of phase by 180 degrees with each other (when both measured with respect to the neutral), along with a common neutral. The neutral conductor is connected to ground at the transformer center tap. Circuits for lighting and small appliance power outlets (ie.
NEMA 1 NEMA connectors are power plug AC power plugs and sockets connect electric equipment to the alternating current (AC) mains electricity, power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in ...
and
NEMA 5 NEMA connectors are power plugs and receptacles used for alternating current, AC mains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association. NEMA wiring devices ar ...
) use 120 V circuits - these are connected between one of the lines and neutral using a single-pole circuit breaker. High-demand applications, such as air conditioners, are often powered using 240 V AC circuits - these are connected between the two 120 V AC lines. These 240 V loads are either hard-wired or use
NEMA 10 NEMA connectors are power plug AC power plugs and sockets connect electric equipment to the alternating current (AC) mains electricity, power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in ...
or NEMA 14 outlets which are deliberately incompatible with the 120 V outlets. Other applications of a split-phase power system are used to reduce the electric shock hazard or to reduce electromagnetic noise.


Connections

A
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any one coil of the transformer produces a varying magnetic flux ...

transformer
supplying a three-wire distribution system has a single-phase input (primary) winding. The output (secondary) winding is center-tapped and the
center tap In electronics, a center tap (CT) is a contact made to a point halfway along a winding of a transformer A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another, or multiple Electr ...
connected to a grounded neutral. As shown in Fig. 1, either end to center has half the voltage of end-to-end. Fig. 2 illustrates the
phasor In and , a phasor (a of phase vector), is a representing a whose (''A''), (''ω''), and (''θ'') are . It is related to a more general concept called ,Bracewell, Ron. ''The Fourier Transform and Its Applications''. McGraw-Hill, 1965. p2 ...

phasor
diagram of the output voltages for a split-phase transformer. 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 Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation Electricity generation is the process of generating electric power from s ...

Thomas Edison
. By connecting pairs of lamps or groups of lamps on the same circuit in series, and doubling the supply voltage, the size of conductors was reduced substantially. The line to neutral voltage is half the line-to-line voltage. Lighting and small appliances requiring less than 1800 watts may be connected between a line wire and the neutral. Higher wattage appliances, such as cooking equipment, space heating, water heaters, clothes dryers, air conditioners and electric vehicle charging equipment are connected across the two line conductors. This means that (for the supply of the same amount of power) the current is halved. Hence, smaller conductors may be used than would be needed if the appliances were designed to be supplied by the lower voltage.Gonen, Turan. ''Electric Power Distribution System Engineering, 2nd ed.'' CRC Press, 2007, p. 284. If the load were guaranteed to be balanced, then the neutral conductor would not carry any current and the system would be equivalent to a single-ended system of twice the voltage with the line wires taking half the current. This would not need a neutral conductor at all, but would be wildly 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. By connecting 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 current carried in all three wires (including the neutral) will always be twice the supply current of the most heavily loaded half. For short wiring runs limited by conductor
ampacity Ampacity is a portmanteau A portmanteau (, ) or portmanteau word (from "Portmanteau (luggage), portmanteau") is a Blend word, blend of words
, this allows three half-sized conductors to be substituted for two full-sized ones, using 75% of the copper of an equivalent single-phase system. Longer wiring runs are more limited by voltage drop in the conductors. Because the supply voltage is doubled, a balanced load can tolerate double the voltage drop, allowing quarter-sized conductors to be used; this uses 3/8 the copper of an equivalent single-phase system. In practice, some intermediate value is chosen. For example, if the imbalance is limited to 25% of the total load (half of one half) rather than the absolute worst-case 50%, then conductors 3/8 of the single-phase size will guarantee the same maximum voltage drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two single-phase conductors.


Balanced power

In a so-called ''balanced power'' system, sometimes called "technical power", an isolation
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any one coil of the transformer produces a varying magnetic flux ...

transformer
with a center tap is used to create a separate supply with conductors at balanced voltages with respect to ground. The purpose of a balanced power system is to minimize the noise coupled into sensitive equipment from the power supply. Unlike a three-wire distribution system, the grounded neutral is not distributed to the loads; only line-to-line connections at 120 V are used. 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. The U.S.
National Electrical Code The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical wiring Electrical wiring is an electrical Electricity is the set of physical phenomena associated with the pr ...
provides rules for technical power installations. The systems are not to be used for general-purpose lighting or other equipment, and may use special sockets to ensure only approved equipment is connected to the system. Additionally, technical power systems pay special attention to the way the distribution system is grounded. A risk of using a balanced power system, in an installation that also uses "conventional" power in the same rooms, is that a user may inadvertently interconnect the power systems together via an intermediate system of audio or video equipment, elements of which might be connected to different power systems. The chance of this happening may be reduced by appropriate labelling of the balanced power outlets and by the use of a type of power outlet socket for the balanced system that is physically different from that of the "conventional" power system to further differentiate them.


Applications


Europe

In
Europe Europe is a continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of scienc ...

Europe
,
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 ...
230/400 V is most commonly used. However, 230/460 V, three-wire,
single-phase Image:Single-phase-pole-transformer-d335.jpg, 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-phas ...
systems are used to run farms and small groups of houses when only two of the
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 ...
high-voltage conductors are used. A split-phase final step-down transformer is then used, with the centre-tap earthed and the two halves usually supplying different buildings with a single phase supply, although in the UK a large farm may be given a 230-0-230 (nominal) supply. In the
UK
UK
, electric tools and portable lighting at larger construction and demolition sites are governed by BS7375, 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 reduced low voltage system is used with 110 V equipment. No neutral conductor is distributed. In high hazard locations, additional double pole RCD protection may be used. The intention is to reduce the electrocution hazard that may exist when using electrical equipment at a wet or outdoor construction site, and eliminate the requirement for rapid automatic disconnection for prevention of shocks during faults. Portable transformers that transform single-phase 240 V to this 110 V split-phase system are a common piece of construction equipment. Generator sets used for construction sites are equipped to supply it directly. An incidental benefit is that the filaments of 110 V
incandescent lamp image of the tungsten filament of an incandescent light bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire #Filament, filament heated until it glows. The filament is enclosed in a gla ...

incandescent lamp
s used on such systems are thicker and therefore mechanically more rugged than those of 240 V lamps.


North America

This three-wire single phase system is common in North America for residential and light commercial applications. Circuit
breaker panel A distribution board (also known as panelboard, breaker panel, electric panel, DB board or DB box) is a component of an electricity supply system that divides an electrical power feed into subsidiary circuitsCircuit may refer to: Science an ...
s typically have two live (hot) wires, and a neutral, connected at one point to the grounded center tap of a local transformer. Usually, one of the live wires is black and the other one red; the neutral wire is always white. Single pole
circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit An electrical network is an interconnection of electrical component An electronic component is any basic discrete device or physical entity ...

circuit breaker
s feed 120 volt circuits from one of the 120 volt buses within the panel, or two-pole circuit breakers feed 240 volt circuits from both buses. 120 V circuits are the most common, and used to power
NEMA 1 NEMA connectors are power plug AC power plugs and sockets connect electric equipment to the alternating current (AC) mains electricity, power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in ...
and
NEMA 5 NEMA connectors are power plugs and receptacles used for alternating current, AC mains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association. NEMA wiring devices ar ...
outlets, and most residential and light commercial direct-wired lighting circuits. 240 V circuits are used for high-demand applications, such as
air conditioner Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 ...

air conditioner
s,
space heater A space heater is a device used to heat a single, small to medium sized area. This contrasts with central heating A central heating system provides warmth to the number of spaces within a building and optionally also able to heat domestic ho ...

space heater
s,
electric stove An electric stove or electric range is a stove 200px, 18th-century Neoclassical cocklestove in the Palais Rohan ( Strasbourg, France) A stove is a device that burns fuel or uses electricity to generate heat inside or on top of the apparatu ...

electric stove
s, electric
clothes dryer A clothes dryer, also known as tumble dryer or simply dryer, is a powered household appliance that is used to remove moisture from a load of clothing, bedding and other textiles, usually shortly after they are washed in a washing machine. Many dr ...

clothes dryer
s, water heaters, and electric vehicle charge points. These use
NEMA 10 NEMA connectors are power plug AC power plugs and sockets connect electric equipment to the alternating current (AC) mains electricity, power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in ...
or NEMA 14 outlets that are deliberately incompatible with the 120 V outlets. Wiring regulations govern the application of split-phase circuits. Since the neutral (return) conductor is not protected by a fuse or circuit breaker, a neutral wire can be shared only by two circuits fed from opposite lines of the supply system. Two circuits from opposing lines may share a neutral if both breakers are connected by a bar so that both trip simultaneously (http://ecmweb.com/code-basics/branch-circuits-part-1NEC 210.4), this prevents 120 V from feeding across 240 V circuits.


Railways

In
Sweden Sweden ( sv, Sverige ), officially the Kingdom of Sweden ( sv, links=no, Konungariket Sverige ), is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that ...
split-phase electric power is also used on some railways. The center tap is grounded, one pole is fed with an overhead wire section, while the other wire is used for another section. Amtrak's 60 Hz traction power system in the
Northeast Corridor The Northeast Corridor (NEC) is an electrified railroad line in the Northeast megalopolis of the United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Con ...
between New York and Boston also uses split-phase power distribution. Two separate wires are run along the track, the contact wire for the locomotive and an electrically separate feeder wire. Each wire is fed with 25 kV with respect to ground, with 50 kV between them.
Autotransformer An autotransformer is an electrical 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 in any one coil of ...

Autotransformer
s along the track balance the loads between the contact and feeder wires, reducing resistive losses. In the UK
Network Rail Network Rail Limited is the owner (via its subsidiary Network Rail Infrastructure Limited, which was known as Railtrack plc before 2002) and Railway infrastructure manager, infrastructure manager of most of the railway network in Great Britain. ...

Network Rail
are using autotransformers on all new 50 Hz electrification, and (as of 2014) are converting many old booster transforme

installations to autotransformers, to reduce energy losse

and exported electromagnetic interference, both of which increase when longer, heavier, or faster trains are introduced, drawing higher peak current from the supply. Note that booster transformers only "boost" the return of traction current through its intended path, the "return conductor", rather than randomly through the earth, and do not boost, but rather reduce, the available voltage at the train, and introduce additional losses. The autotransformer system enforces the traction return current taking its intended path, while reducing the transmission losses, and therefore achieves both required objectives, of controlling return current leakage to earth and ensuring low energy loss, simultaneously. There is an initial cost penalty, because the previous return conductor, insulated to a fairly modest voltage, must be replaced by an anti-phase feeder, insulated to 25 kV, and the autotransformers themselves are larger and more expensive than the previous booster transformers; but over time the lower loss of energy results in overall costsavings.


See also

*
Shared neutral As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equ ...


References

Electric power distribution Electric motors AC power