Three-phase electric power

Missing image
Three-phase power transformer which is the sole transfer point for electricity to a suburban shopping mall in Canada. Note the four wires used for the 208V/120Y service: one is for the neutral, and the other three are for the X, Y, and Z phases.

Three-phase is a common method of electric power transmission. It is a type of polyphase system.

This article deals with where, how and why "three phase" is used. For information on the basic mathematics and principles of three phase see three-phase. For information on testing three phase kit please see three-phase testing.

Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

Three phase has properties that make it very desirable in distribution. Firstly all three wires carry the same current. Secondly power transfer into a linear balanced load is constant.

Most domestic loads are single phase. Generally three phase power either does not enter domestic houses at all, or where it does, it is split out at the main distribution board.

The three phases are typically indicated by colors which vary by country. See the table for more information.


Color codes

Conductors of a three phase system are usually identified by a color code, to allow for balanced loading and to assure the correct phase rotation for induction motors. Colors used may adhere to old standards or to no standard at all, and may vary even within a single installation.






North America






North America (newer 277/480 installations)






UK until April 2006





green/yellow striped (green on very old installs)

Europe (including UK) from April 2004





green/yellow striped

Previous European (varies by country)

Brown or black

Brown or black

Brown or black


green/yellow striped

Generation and distribution

At the power station, an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases. In a three-phase system the phases are spaced equally, giving a phase separation of 120°. The frequency is typically 50Hz in Europe and 60Hz in the US (see List of countries with mains power plugs, voltages and frequencies).

Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers "step-up" this voltage to one more suitable for transmission.

After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (the voltage of "house" or "household" current in American English). The power may already have been split into single phase at this point or it may still be three phase. Where the stepdown is 3 phase, the output of this transformer is ususally star connected with the standard mains voltage (120V in North America and 230V in Europe) being the phase-neutral voltage. Another system commonly seen in the USA is to have a delta connected secondry with a centre tap on one of the windings supplying the ground and neutral. This allows for 240V three phase as well as 3 different single phase voltages (120V between two of the phases and the neutral, 208V between the third phase (known as a wild leg) and neutral and 240V between any two phases to be made availible from the same supply.

Single phase loads

Single-phase loads may be connected to a three-phase system, either by connecting across two live conductors (a phase-to-phase connection), or by connecting between a phase conductor and the system neutral, which must be connected to the center of the Y(star)secondary winding of the supply transformer. Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors.

The line-to-line voltage of a three-phase system is √3 times the line to neutral voltage. Where the line-to-neutral voltage is a standard utilization voltage, (for example in a 240 V/415 V system) individual single-phase utility customers or loads may each be connected to a different phase of the supply. Where the line-to-neutral voltage is not a common utilization voltage, for example in a 347/600 V system, single-phase loads must be supplied by individual step-down transformers. In multiple-unit residential buildings in North America, lighting and convenience outlets can be connected line-to-neutral to give the 120 V utilization voltage, and high-power loads such as cooking equipment, space heating, water heaters,or air conditioning can be connected across two phases to give 208 V. This practice is common enough that 208 V single-phase equipment is readily available in North America. Attempts to use the more common 120/240 V equipment intented for three-wire single-phase distribution may result in poor performance since 240 V heating equipment will only produce 75% of its rating when operated at 208V.

Three phase loads

The most important class of three-phase load is the electric motor. A three phase induction motor has a simple design, inherently high starting torque, and high efficiency. Such motors are applied in industry for pumps, fans, blowers, compressors, conveyor drives, and many other kinds of motor-driven equipment. A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating; and single-phase AC motors above 10 HP (7.5 kW) are uncommon.

Large air conditioning equipment (for example, most York units above 2.5 tons (8.8 kW) cooling capacity) use three-phase motors for reasons of efficiency and economy.

Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected. These types of loads do not require the revolving magnetic field characteristic of three-phase motors but take advantage of the higher voltage and power level usually associated with three-phase distribution.

Large rectifier systems may have three-phase inputs; the resulting DC current is easier to filter (smooth) than the output of a single-phase rectifier. Such rectifiers may be used for battery charging, electrolysis processes such as aluminum production, or for operation of DC motors.

An interesting example of a three-phase load is the electric arc furnace used in steelmaking and in refining of ores.

In much of Europe stoves are designed to allow for a three phase feed. Usually the individual heating units are connected between phase and neutral to allow for connection to a single phase supply where this is all that is available.

Phase converters

Occasionally the advantages of three-phase motors make it worth-while to convert single-phase power to three phase. Small customers, such as residential or farm properties may not have access to a three-phase supply, or may not want to pay for the extra cost of a three-phase service, but may still wish to use three-phase equipment.

One method for using three-phase equipment on a single-phase supply is with a rotary converter, essentially a three-phase motor with special starting arrangements that produces a three-phase system. When properly designed these rotary converters can allow satisfactory operation of three-phase equipment such as machine tools on a single phase supply.

Some devices are made which create an imitation three-phase from three-wire single phase supplies. This is done by creating a third "subphase" between the two live conductors, resulting in a phase separation of 180° - 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.

Solid-state inverters also can be used to power three-phase motors from a single-phase supply.

Alternatives to three-phase

  • Three-wire single-phase distribution is useful when high voltage three phase is not available, and allows double the normal utilization voltage to be supplied for high-power loads.
  • Two phase Like three phase, gives constant power transfer to a linear load. But in a three wire system it has a neutral current which is greater than the phase currents. Also motors aren't entirely linear and this means that despite the theory motors running on three phase tend to run smoother than those on two phase. True two-phase power distribution is essentially obsolete. Special purpose systems may use a two-phase system for control.
  • High phase order systems for power transmission have been built and tested. Such transmission lines use 6 or 12 phases and design practices characteristic of extra-high voltage transmission lines. High-phase order transmission lines may allow transfer of more power through a given transmission line right-of-way without the expense of a HVDC converter at each end of the line.

Three phase electrical outlets

see main article Industrial & multiphase power plugs & sockets

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Missing image

The image on the left shows a 3 phase socket. The one on the right shows a triplex outlet with neons in the top sockets for phase indication. Both are American. Underneath the three phase socket on the left there are breaker positions marked in small writing. These are 8, 10 and 12 representing the red, black and blue phases respectively.

Three phase power can be supplied either by the use of a three phase socket, or by triplexing. Most North American receptacles are duplex receptacles. The top and bottom sockets can also be separated, if desired, and, for example, supplied by separate breakers with a common neutral. This is typically done in kitchens where a high load will likely be placed on both sockets. In this case, a common trip 2-pole breaker is often used.

The concept of duplexing can be generalized to triplexing, so that three duplex receptacles can be supplied by a common neutral, from a 3-phase supply. Typically, a 3 pole common trip 15 A breaker is used to supply such a socket. This enables three single phase loads to be supplied in a phase-sequenced manner. An example of such a load is a light fixture having three bulbs. For flicker-free operation, three bulbs are each fitted with a separate plug, and driven 120 degrees out of phase with one another, from a triplex receptacle. The top receptacles shown in the figure, are fitted with neon night lights to indicate phase sequence, for triplex loads where proper phase sequence is desired.

This can, however, create problems in environments that have large quantities of dimmed circuits, such as theatres. Harmonic 'noise' created by dimming equipment can effectively overload a combined neutral and result in a fire. Combined neutrals can also cause inconsistent response from individual circuits in this situation. Although not the case in older facilites, in all new installations of large-scale theatrical dimming equipment, manufacturers require individual neutrals to maintain the warranty status of the equipment.

This fact should also be taken into account in wiring office buildings, as a large number of computer power supplies can also overload a combined neutral.

See also

de:Dreiphasenwechselstrom fi:Kolmivaihevirta fr:triphas ja:三相交流


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