What is split phase

Split-phase electricity - Split-phase electric power

Type of single-phase power distribution
Pole-mounted single-phase transformer with three-wire "split-phase" secondary with center tap. The central tap is grounded to the three secondary terminals with a short strap on the transformer housing.

A shared or single-phase three-wire system is a kind of single-phase power distribution. This is the AC equivalent of Edison Machine Works' original three-wire DC system. Its main advantage is that it saves conductor material compared to a one-sided single-phase system, while only a single phase is required on the supply side of the distribution transformer.

This system is common in North America for residential and light commercial applications. Two 120V AC power lines are supplied to the premises, 180 degrees out of phase with each other (when both are measured with respect to the neutral), along with a common neutral. The neutral conductor is connected to ground on the central tap of the transformer. Circuits for lighting and sockets for small appliances (e.g. NEMA 1 and NEMA 5) use 120 V circuits - these are connected with a single pole circuit breaker between one of the lines and the neutral conductor. High demand applications such as Commonly used air conditioners, such as air conditioners, are powered by 240 VAC circuits - these are connected between the two 120 VAC power lines. These 240v loads are either hard-wired or use NEMA 10 or NEMA 14 receptacles, which are intentionally incompatible with the 120v receptacles.

Other applications of a split phase power system are used to reduce the risk of electric shock or electromagnetic interference.


A transformer supplying a three-wire distribution system has a single-phase input winding (primary winding). The output winding (secondary winding) is tapped in the middle and the center tap is connected to a grounded neutral conductor. As shown in Figure 1, each end to center is half the tension from end to end. Fig. 2 illustrates the phasor diagram of the output voltages for a split-phase transformer. Since the two pointers do not define a clear direction of rotation for a rotating magnetic field, a split single-phase system is not a two-phase system.

In the USA and Canada, the practice arose with the DC distribution system developed by Thomas Edison. By connecting pairs or groups of lamps in series on the same circuit and doubling the supply voltage, the size of the conductors has been significantly reduced.

The line-to-neutral voltage is half the line-to-line voltage. Lighting and small appliances that require less than 1800 watts can be connected between a line cable and the neutral conductor. Higher power devices such as cooking appliances, space heating, water heaters, tumble dryers, air conditioners and chargers for electric vehicles are connected via the two line conductors. This means that (with the same power supply) the current is halved. Therefore, smaller conductors can be used than would be required if the equipment were designed to be powered by the lower voltage.

If the load were guaranteed to be balanced, the neutral would not carry current and the system would be a single ended system with twice the voltage, with the lead wires taking half the current. This would not need a neutral conductor at all, but would be extremely impractical for different loads; Simply connecting the groups in series would lead to excessive voltage and brightness fluctuations when the lamps are switched on and off.

By connecting the two groups of lamps with a neutral conductor with medium potential between the two current-carrying legs, an imbalance in the load is provided by a current in the neutral conductor, which results in a substantially constant voltage across both groups. The total current in all three wires (including the neutral conductor) is always twice the supply current of the most heavily loaded half.

For short wiring runs limited by the amperage of the conductor, three half-size conductors can be replaced with two full-size conductors, using 75% of the copper of an equivalent single-phase system.

Longer wiring runs are more restricted by the voltage drop in the conductors. Because the supply voltage is doubled, a balanced load can tolerate twice the voltage drop, so quarter-size conductors can be used. This uses 3/8 the copper of an equivalent single phase system.

In practice, an intermediate value is chosen. For example, if the imbalance is limited to 25% of the total load (half of half) rather than the absolute worst-case value of 50%, conductors 3/8 of the single-phase size guarantee the same maximum voltage drop. 9/8 total of a single phase conductor, 56% of the copper of the two single phase conductors.

Balanced power

In a so-called symmetrical power supply system , sometimes referred to as "engineering feat", a center tap isolating transformer is used to provide a separate supply of conductors at voltages balanced with respect to ground. The purpose of a balanced power system is to minimize the noise that the power supply injects into sensitive equipment.

In contrast to a three-wire distribution system, the earthed neutral conductor is not distributed among the loads. Only line-to-line connections with 120 V are used. A balanced power supply system is only used for specialized sales in audio and video production studios, for audio and television broadcasts, and for the installation of sensitive scientific instruments.

The US National Electrical Code contains rules for technical power installations. The systems must not be used for general lighting or other devices and may use specific electrical outlets to ensure that only approved devices are connected to the system. In addition, technical power supply systems pay special attention to the way in which 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 inadvertently interconnects the power systems through an intermediate system of audio or video equipment, elements of which may be on different power systems connected. The likelihood of this happening can be reduced by properly labeling the balanced outlets and by using some type of socket for the balanced system that is physically different from that of the "traditional" power system to further differentiate them.



In Europe, three-phase 230/400 V is most commonly used. However, 230/460 V, three-wire, single-phase systems are used to run farms and small groups of houses when only two of the three-phase high-voltage conductors are used. A split phase step-down transformer is then used with the center tap grounded and the two halves typically supplying different buildings with a single phase supply, although a large farm in the UK may have a 230-0-230 (nominal) supply.

In the UK, power tools and portable lighting in major construction and demolition sites are subject to BS7375 and should be powered from a center tap system with only 55V between live conductors and earth (called a CTE or center tap earth or 55) -0-55). This reduced, low voltage system is used with 110V equipment. There is no neutral conductor distributed. Additional two-pole residual current protection can be used at locations with high risk. The aim is to reduce the risk of electric shock that can occur when using electrical equipment on a wet construction site or in an outdoor construction site and to eliminate the need for a quick auto shutdown to avoid shock in the event of a failure. Portable transformers that convert single-phase 240V to this 110V split-phase system are a common construction piece of equipment. Generators for construction sites are equipped for direct supply.

A fortuitous advantage is that the filaments of 110V incandescent lamps used in such systems are thicker and therefore more mechanically robust than those of 240V lamps.

North America

This three-phase, single-phase system is common in North America for residential and light commercial applications. Circuit breaker panels usually have two live (hot) wires and one 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 is red. The neutral conductor is always white. Single pole circuit breakers supply 120 volt circuits from one of the 120 volt buses in the switchboard, or double pole circuit breakers supply 240 volt circuits from both buses. 120V circuits are the most common and are used to power NEMA 1 and NEMA 5 electrical outlets, as well as most direct-wired lighting circuits for residential and commercial use. 240 V circuits are used for high demand applications such as B. Air conditioners, space heaters, electric stoves, electric tumble dryers, water heaters and charging stations for electric vehicles. These use NEMA 10 or NEMA 14 sockets, which are intentionally incompatible with the 120v sockets.

The wiring regulations regulate the use of split-phase circuits. Since the neutral conductor (return conductor) is not protected by a fuse or a circuit breaker, a neutral conductor can only be shared by two circuits that are fed from opposite lines of the supply system. Two circuits on opposite lines can share a neutral conductor if both circuit breakers are connected by a rod so that both trip at the same time (NEC 210.4). This prevents 120V from being fed through 240V circuits.


In Sweden, split-phase electricity is also used on some railways. The center tap is grounded, one pole is fed with an overhead line section, while the other wire is used for another section.

Amtrak's 60 Hz traction network in the northeast corridor between New York and Boston also uses split-phase power distribution. Two separate wires run along the rail, the contact wire for the locomotive and an electrically separated supply wire. Each wire is fed at 25 kV with respect to ground with 50 kV between them. Autotransformers along the track balance the loads between the contact and supply lines and reduce resistance losses.

In the UK, Network Rail uses autotransformers for all new 50 Hz electrification and is converting (as of 2014) many old booster transformer installations [1] to autotransformers to reduce energy losses [2] and exported electromagnetic interference, both of which increase If longer, heavier or faster trains are introduced, a higher peak current is drawn from the supply. It should be noted that booster transformers only "amplify" the return of the traction current via their intended path, the "return conductor", and not accidentally through the earth and do not increase the available voltage on the train, but reduce it and introduce additional losses. The autotransformer system forces the traction return current to take its intended route while reducing transmission losses, and therefore achieves both of the necessary goals of controlling return current loss to earth while ensuring low energy loss. There is an initial cost penalty as the previous return, which is isolated to a relatively modest voltage, has to be replaced with an anti-phase branch isolated against 25 kV and the autotransformers themselves are larger and more expensive than the previous booster transformers. However, over time, the lower energy loss translates into total cost savings.

See also


  1. ^ Terrell Croft and Wilford Summers (Eds.), American Electricians' Handbook, Eleventh Edition , McGraw Hill, New York (1987) ISBN 0-07-013932-6, Chapter 3, Pages 3-10, 3-14 through 3-22.
  2. ^ Gonen, Turan. Engineering of power distribution systems, 2nd ed. CRC Press, 2007, p. 284.
  3. ^ NFPA 70, National Electrical Code 2005, National Fire Protection Association, Inc., Quincy, Massachusetts USA, (2005). no ISBN, Articles 640 and 647
  4. ^ http://ecmweb.com/code-basics/branch-circuits-part-1