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[[File:Threephasepolemountclose.jpg|thumb|350px|right|Three-phase transformer with four wire output for 208Y/120 volt service: one wire for neutral, others for A, B and C phases]]
[[File:Three Phase Electric Power Transmission.jpg|thumb|right|350px|Three-phase electric power transmission]]

'''Three-phase electric power''' is a common method of [[Alternating current|alternating-current]] [[electric power]] [[Electric power generation|generation]], [[Electric power transmission|transmission]], and [[Electric power distribution|distribution]].<ref>William D. Stevenson, Jr. ''Elements of Power System Analysis Third Edition'', McGraw-Hill, New York (1975). ISBN 0-07-061285-4, p. 2</ref> It is a type of [[polyphase system]] and is the most common method used by [[electrical grid]]s worldwide to transfer power. It is also used to power large [[Electric motor|motor]]s and other heavy loads. A [[three-phase]] system is usually more economical than an equivalent [[Single-phase electric power|single-phase]] or [[Two-phase electric power|two-phase]] system at the same [[voltage]] because it uses less conductor material to transmit electrical power.<ref>http://www.allaboutcircuits.com/vol_2/chpt_10/2.html</ref>
The three-phase system was independently invented by [[Galileo Ferraris]], [[Mikhail Dolivo-Dobrovolsky]] and [[Nikola Tesla]] in the late 1880s.

== Details ==

In a three-phase system, three circuit conductors carry three [[alternating current]]s (of the same frequency) which reach their instantaneous peak values at one third of a cycle from each other. Taking one current as the reference, the other two currents are delayed in time by one third and two thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an [[electric motor]].

Three-phase systems may have a [[ground and neutral|neutral]] wire.  A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage [[Single-phase electric power|single-phase]] loads.  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 electric power systems:
*The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load.  This makes it possible to reduce the size of the neutral conductor because it carries little to no current; all the phase conductors carry the same current and so can be the same size, for a balanced load.
*Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
*Three-phase systems can produce a [[rotating magnetic field]] with a specified direction and constant magnitude, which simplifies the design of electric motors.

Most household loads are single-phase. In North American residences, three-phase power might feed a multiple-unit apartment block, but each unit is only fed by one of the phases. In lower-density areas, only a single phase might be used for distribution. Some large European appliances may be powered by three-phase power, such as electric stoves and clothes dryers.

Wiring for the three phases is typically identified by color codes which vary by country. Connection of the phases in the right order is required to ensure the intended direction of rotation of three-phase motors. For example, pumps and fans may not work in reverse. Maintaining the identity of phases is required if there is any possibility two sources can be connected at the same time; a direct interconnection between two different phases is a short-circuit.

== Generation and distribution ==
[[File:3-phase flow.gif|thumb|right|352px|Animation of three-phase current flow]]
{{double image|right|Hawkins Electrical Guide - 3phase Elementary 6wire.jpg|250|Hawkins Electrical Guide - 3phase Elementary 3wire.jpg|100|Left: Elementary six-wire three-phase alternator, with each phase using a separate pair of transmission wires.<ref>''[[Hawkins Electrical Guide]]'', Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p. 1026, fig. 1260.</ref> Right: Elementary three-wire three-phase alternator, showing how the phases can share only three wires.<ref>''[[Hawkins Electrical Guide]]'', Theo. Audel and Co., 2nd ed., 1917, vol. 4, Ch. 46: Alternating Currents, p. 1026, fig. 1261.</ref>
}}

At the [[power station]], an [[electrical generator]] converts mechanical power into a set of three [[Alternating current|AC]] [[electric current]]s, one from each coil (or winding) of the generator. The windings are arranged such that the currents vary [[Sine wave|sinusoidally]] at the same frequency but with the peaks and troughs of their [[wave|wave forms]] offset to provide three complementary currents with a phase separation of one-third cycle ([[Sine wave|120° or {{frac|2π|3}} radians]]).  The [[utility frequency|generator frequency]] is typically 50 or 60 [[Hertz|Hz]], varying by country.
{{further2|[[Mains power systems]]}}

At the power station, [[transformer]]s change the voltage from generators to a level suitable for [[Electric power transmission|transmission]] minimizing losses.

After further voltage conversions in the transmission network, the voltage is finally transformed to the standard utilization before power is supplied to customers.

Most automotive alternators generate three phase AC and rectify it to DC with a [[diode bridge]].<ref>http://www.rle.mit.edu/per/conferencepapers/cpconvergence00p583.pdf</ref>

== Transformer connections ==
A "delta" connected transformer winding is connected between phases of a three-phase system. A "wye" ("star") transformer connects each winding from a phase wire to a common neutral point.

In an "open delta" or "V" system, only two sets of transformers are used. A closed delta system can operate as an open delta if one of the transformers has failed or needs to be removed.<ref name="Fowler"/> In open delta, each transformer must carry current for its respective phases as well as current for the third phase, therefore capacity is reduced to 87%. With one of three transformers missing and the remaining two at 87% efficiency, the capacity is 58% ({{math|(2/3) × 87%}}).<ref name=power>{{cite journal|last=McGraw-Hill|journal=Power|year=1920|volume=51|issue=17|url=http://books.google.com/books?id=u91QAAAAYAAJ&pg=PA673&lpg=PA673|accessdate=21 December 2012}}</ref><ref>H. W. Beaty, D.G.Fink (ed) ''Standard Handbook for Electrical Engineers Fifteenth Edition'',McGraw-Hill, 2007 ISBN 0-07-144146-8, p. 10–11</ref>

Where a delta-fed system must be grounded for protection from surge voltages, a grounding transformer (usually a [[zigzag transformer]]) may be connected to allow ground fault currents to return from any phase to ground. Another variation is a "corner grounded" delta system, which is a closed delta that is grounded at one of the junctions of transformers.<ref>[http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/2700DB0202.pdf Schneider]</ref>

== Three-wire and four-wire circuits ==
[[Image:The basic 3-phase configurations.svg|thumb|Wye (Y) and Delta (Δ) circuits]]
There are two basic three-phase configurations: delta and wye (star). Either type can be wired for three or four wires. The fourth wire, if present, is provided as a neutral. The '3-wire' and '4-wire' designations do not count the [[ground wire]] used on many transmission lines which is solely for fault protection and does not carry current under non-fault conditions.

A four-wire system with symmetrical voltages between phase and neutral is obtained when the neutral is connected to the "common star point" of an all supply windings. All three phases will have the same magnitude of voltages to the neutral in such a system. Other non-symmetrical systems have been used. In a [[high-leg delta]] system, one winding of a delta transformer feeding the system is center-tapped and connected to neutral. This setup produces three voltages. If the voltage between center tap and the two adjacent phases is 100%, the voltage across any two phases is 200% and neutral to "high leg" is ≈ 173%.<ref name="Fowler"/>

Three-wire distribution systems need one less conductor and help redistribute unbalanced loading during transformation going back to the generation source.{{citation needed|date=December 2012}}

The four-wire wye system is used when ground referenced voltages or the flexibility of more voltage selections are required. Faults on one phase to ground will cause a protection event (fuse or breaker open) locally and not involve other phases or other connected equipment.{{citation needed|date=December 2012}} An example of application is a local distribution in Europe, where each customer is fed a phase and a neutral.  When a set of customers sharing the neutral draw unequal currents, the common neutral wire carries a current as a result of the imbalance. Electrical engineers try to design the system so the loads are balanced as much as possible. By distributing a large number of houses over all three phases, on average a nearly balanced load is seen at the point of supply.{{citation needed|date=December 2012}}

In a  ''three-phase, four-wire, delta'' ([[high-leg delta]])  system, the neutral is a center tap in one of the delta phase supply windings. This can also be supplied by two single-phase transformers in a ''V'' formation (open delta).

== Balanced circuits ==

In the perfectly balanced case all three lines share equivalent loads. Examining the circuits we can derive relationships between line voltage and current, and load voltage and current for wye and delta connected loads.

In a balanced system each line will produce equal voltage magnitudes at phase angles equally spaced from each other. With V<sub>1</sub> as our reference and V<sub>3</sub> lagging V<sub>2</sub> lagging V<sub>1</sub>, using [[phasor]] notation, we have:<ref name="GloverSarma2011">{{cite book|author1=J. Duncan Glover|author2=Mulukutla S. Sarma|author3=Thomas J. Overbye|title=Power System Analysis & Design|url=http://books.google.com/books?id=HrtXToLEbVoC|date=April 2011|publisher=Cengage Learning|isbn=978-1-111-42579-1|pages=60–68}}</ref>

<math>V_1 = V_{LN}\angle 0^\circ </math>

<math>V_2 = V_{LN}\angle -120^\circ</math>

<math>V_3 = V_{LN}\angle +120^\circ</math>

These Voltages feed into either a wye or delta connected load.

=== Wye ===

[[File:3 Phase Power Connected to Wye Load.svg|thumb|right|Three phase AC generator connected as a wye source to a wye connected load.]]

For the wye case, all loads see their respective line voltages, and so:<ref name = "GloverSarma2011" />

<math> I_1 = \frac{V_1}{|Z_{total}|}\angle (-\theta) </math>

<math> I_2 = \frac{V_2}{|Z_{total}|}\angle (-120^\circ-\theta) </math>

<math> I_3 = \frac{V_3}{|Z_{total}|}\angle (120^\circ-\theta) </math>

where {{math|''Z<sub>total</sub>''}} is the sum of line and load impedances ({{math|''Z<sub>total</sub>'' {{=}} Z<sub>LN</sub> + Z<sub>Y</sub>}}), and ''&theta;'' is the phase of the total impedance ({{math|''Z<sub>total</sub>''}}).

The phase angle difference between voltage and current of each phase is not necessarily 0 and is dependent on the type of load impedance, Z<sub>y</sub>. Inductive and capacitive loads will cause current to either lag or lead the voltage. However, the relative phase angle between each pair of lines (1 to 2, 2 to 3,and 3 to 1) will still be –120 degrees.

By performing [[Kirchhoff's circuit laws#Kirchhoff's current law (KCL)|Kirchhoff's Current Law (KCL)]] on the neutral node, the three phase currents sum up to the total current in the neutral line. In the balanced case:

<math> I_1 + I_2 + I_3 = I_n = 0 </math>

=== Delta  ===

[[File:3 Phase Power Connected to Delta Load.svg|thumb|right|Three phase AC generator connected as a wye source to a delta connected load.]]

In the delta circuit loads are connected across the lines and so loads see line-to-line voltages:<ref name = "GloverSarma2011" />

<math>\begin{align}
V_{12} & = V_1 - V_2 = (V_{LN}\angle 0^\circ) - (V_{LN}\angle -120^\circ) \\
&= \sqrt{3}V_{LN}\angle 30^\circ = \sqrt{3}V_{1}\angle (phase_{V_1}+30^\circ) \\\\

V_{23} & = V_2 - V_3 = (V_{LN}\angle -120^\circ) - (V_{LN}\angle 120^\circ) \\
& = \sqrt{3}V_{LN}\angle -90^\circ = \sqrt{3}V_{2}\angle (phase_{V_2}+30^\circ) \\\\

V_{31} & = V_3 - V_1 = (V_{LN}\angle 120^\circ) - (V_{LN}\angle 0^\circ) \\
& = \sqrt{3}V_{LN}\angle 150^\circ = \sqrt{3}V_{3}\angle (phase_{V_3}+30^\circ) \\\\
\end{align}</math>

Further:

<math> I_{12} = \frac{V_{12}}{|Z_\Delta|} \angle (30^\circ-\theta) </math>

<math> I_{23} = \frac{V_{23}}{|Z_\Delta|} \angle (-90^\circ-\theta) </math>

<math> I_{31} = \frac{V_{31}}{|Z_\Delta|} \angle (150^\circ-\theta) </math>

where ''&theta;'' is the phase of delta impedance (''Z<sub>&Delta;</sub>'').

Relative angles are preserved, so I<sub>31</sub> lags I<sub>23</sub> lags I<sub>12</sub> by 120 degrees. Calculating line currents by using KCL at each delta node gives:

<math> I_1 = I_{12} - I_{31} = I_{12} - I_{12}\angle 120^\circ </math>

{{spaces|6}} <math> = \sqrt{3}I_{12} \angle (phase_{I_{12}}-30^\circ) = \sqrt{3}I_{12} \angle (-\theta) </math>

And similarly for each other line:

<math> I_2 = \sqrt{3}I_{23} \angle (phase_{I_{23}}-30^\circ) = \sqrt{3}I_{23} \angle (-120^\circ-\theta) </math>

<math> I_3 = \sqrt{3}I_{31} \angle (phase_{I_{31}}-30^\circ) = \sqrt{3}I_{31} \angle (120^\circ-\theta) </math>

again, ''&theta;'' is the phase of delta impedance (''Z<sub>&Delta;</sub>'').

== Single-phase loads ==

Single-phase loads may be connected across any two phases, or a load can be connected from phase to neutral.<ref name="IAEI"/> Distributing single-phase loads among the phases of a three-phase system balances the load and makes most economical use of conductors and transformers.

In a symmetrical three-phase four-wire, wye system, the three phase conductors have the same voltage to the system neutral. The voltage between line conductors is [[square root of 3|{{sqrt|3}}]] times the phase conductor to neutral voltage.
:<!--<math>V_\text{L-L} = \sqrt{3} V_\text{L-N}</math> It is the only spot where <math> is preferred.  Should this article load MathJax for this one formula?
 -->{{math|size=120%|1=''V''<sub>L-L</sub> = {{sqrt|3}} ''V''<sub>L-N</sub>}}<ref>The boy electrician by J W Sims M.I.E.E. (Page 98)</ref>
The currents returning from the customers' premises to the supply transformer all share the neutral wire. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero.  Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently.

If the supply neutral is broken, phase-to-neutral voltage is no longer maintained. Phases with higher relative loading will experience reduced voltage and phases with lower relative loading will experience elevated voltage, up to the phase-to-phase voltage.{{citation needed|date=December 2012}}

A [[high-leg delta]] provides phase-to-neutral relationship of {{math|1=''V''<sub>L-L</sub> = 2 ''V''<sub>L-N</sub> }}, however, L-N load is imposed on one phase.<ref name=Fowler>{{cite book|last=Fowler|first=Nick|title=Electrician's Calculations Manual 2nd Edition|year=2011|publisher=McGraw-Hill|isbn=9780071770170|pages=3–5|url=http://books.google.com/books?id=9QBWHJdPGM0C}}</ref> A transformer manufacturer's page suggests that L-N loading to not exceed 5% of transformer capacity.<ref>[http://www.federalpacific.com/university/transbasics/chapter3.html Federal pacific]</ref>

{{sqrt|3}} is ≈ 1.73, so if {{math|''V''<sub>L-N</sub>}} was defined as 100%, {{math|''V''<sub>L-L</sub>}} would be {{math|1=≈ 100% × 1.73 = 173%}}
If {{math|''V''<sub>L-L</sub>}} was set as 100%, then {{math|''V''<sub>L-N</sub> ≈ 57.7%}}

=== Unbalanced loads ===

When the currents on the three live wires of a three-phase system are not equal or are not at an exact 120° phase angle, the power-loss is greater than for a perfectly balanced system.  The degree of imbalance is expressed by [[symmetrical components]].  Three-phase systems are evaluated at generating stations and substations in terms of these three components, of which two are zero in a perfectly balanced system. {{citation needed|date=December 2012}}

=== Non-linear loads ===

With linear loads, the neutral only carries the current due to imbalance between the phases. Devices that utilize rectifier-capacitor front-end such as [[switch-mode power supplies]], computers, office equipment and such produce [[third order harmonic]]s that are in-phase on all the supply phases. Consequently, such harmonic currents add in the neutral which can cause the neutral current to exceed the phase current.<ref name="IAEI">{{cite web|last=Lowenstein|first=Michael|title=The 3rd Harmonic Blocking Filter: A Well Established Approach to Harmonic Current Mitigation|url=http://www.iaei.org/blogpost/890108/159506/The-3rd-Harmonic-Blocking-Filter-A-Well-Established-Approach-to-Harmonic-Current-Mitigation|publisher=IAEI Magazine|accessdate=24 November 2012}}</ref><ref>{{cite web|last=Enjeti|first=Prasad|title=Harmonics in Low Voltage Three-Phase Four-Wire Electric Distribution Systems and Filtering Solutions|url=http://www.pserc.wisc.edu/documents/general_information/presentations/pserc_seminars/pserc_seminars0/enjeti_slides.pdf|publisher=Texas A&M University Power Electronics and Power Quality Laboratory|accessdate=24 November 2012}}</ref>

== Three-phase loads ==
[[File:High leg delta transformer.png|thumb|right| A transformer for a high-leg delta system; 200 V 3-phase motors would be connected to L1, L2 and L3. 200 V Single-phase load would be connected L1 and L2. Single phase 100 V load between either L1 or L2 and neutral (N). L3 (wild or high leg) will be 173.2 V to neutral.]]

An 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.{{citation needed|date=December 2012}} Such motors are applied in industry for many applications. A three-phase motor is 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&nbsp;kW) are uncommon.{{citation needed|date=December 2012}} Three-phase motors also vibrate less and hence last longer than single-phase motors of the same power used under the same conditions.{{citation needed|date=December 2012}}

Line frequency flicker in light can be reduced by evenly spreading three phases across line frequency operated light sources so that illuminated area is provided light from all three phases. The effect of line frequency flicker is detrimental to super slow motion cameras used in sports event broadcasting. Three phase lighting has been applied successfully at the 2008 Beijing Olympics to provide consistent light level for each frame for SSM cameras.<ref name=3phlight>{{cite web|last=Hui|first=Sun|title=Sports Lighting&nbsp;– Design Considerations For The Beijing 2008 Olympic Games|url=http://www.geappliances.com/email/olympic_specifier/downloads/SportsLightinginBeijing2008OlympicGames.pdf|publisher=GE Lighting|accessdate=18 December 2012}}</ref>
Resistance heating loads such as electric [[boiler]]s or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected.

[[Rectifier]]s may use a three-phase source to produce a six-pulse DC output.<ref>[http://www.ewh.ieee.org/soc/es/Nov1998/08/CONVERTR.HTM IEEE]{{full|date=November 2012}}</ref> The output of such rectifiers is much smoother than rectified single phase and, unlike single-phase, does not drop to zero between pulses. Such rectifiers may be used for battery charging, [[electrolysis]] processes such as [[Aluminium#Production and refinement|aluminium production]] or for operation of DC motors. "Zig-zag" transformers may make the equivalent of six-phase full-wave rectification, twelve pulses per cycle, and this method is occasionally employed to reduce the cost of the filtering components, while improving the quality of the resulting DC.

One example of a three-phase load is the [[electric arc furnace]] used in [[steelmaking]] and in refining of ores.

In Germany, a 1965 publication shows some "full size" stoves are designed for a three-phase feed. However, the individual heating units may be connected between phase and neutral to allow for connection by three individual circuits on the same single-phase supply.<ref>[http://books.google.co.uk/books?id=O3gnAAAAMAAJ&q=%22full+size+cookers+for+Europe+have+to+be+connected+to+a+three-phase+star+connected+supply%22&dq=%22full+size+cookers+for+Europe+have+to+be+connected+to+a+three-phase+star+connected+supply%22&hl=en&sa=X&ei=fSipUKXYCfCk0AX4zYDoBA&ved=0CDAQ6AEwAA "British and European practices for domestic appliances compared"], ''Electrical Times'', volume 148, page 691, 1965.</ref>

== Phase converters ==

Phase converters are used when three-phase equipment needs to be operated on a single-phase power source. <!-- what about the example for operating a large single phase load on a 3ph generator and need to balance it across 3 phases?  -->They are used when three-phase power is not available or cost is not justifiable. Such converters may also allow the frequency to be varied (resynthesis) allowing speed control. Some railway locomotives use a single-phase source to drive three-phase motors fed through an electronic drive.<ref>{{cite journal|journal=Japan Railway & Transport Review|date=Oct 2011|url=http://www.jrtr.net/jrtr58/pdf/51-60web.pdf|volume=No. 58|page=58}}</ref>

=== Mechanical ===

One method to generate three-phase power from a single-phase source is the [[rotary phase converter]], essentially a three-phase motor with special starting arrangements and [[power factor]] correction that produces balanced three-phase voltages. When properly designed, these rotary converters can allow satisfactory operation of a three-phase motor on a single-phase source. In such a device, the energy storage is performed by the [[inertia]] (flywheel effect) of the rotating components. An external flywheel is sometimes found on one or both ends of the shaft.

A three-phase generator can be driven by a single-phase motor. This motor-generator combination can provide a frequency changer function as well as phase conversion, but requires two machines with all their expense and losses.  The motor-generator method can also form an [[uninterruptable power supply]] when used in conjunction with a large flywheel and a standby generator set.

=== Non-mechanical ===
{{unreferenced section|date=December 2012}}

A second method that was popular in the 1940s and 1950s was the ''transformer method''. At that time, capacitors were more expensive than transformers, so an [[autotransformer]] was used to apply more power through fewer capacitors. Separated it from another common method, the static converter, as both methods have no moving parts, which separates them from the rotary converters.<!-- This method needs citation  -->

Another method often attempted is with a device referred to as a static phase converter.  This method of running three-phase equipment is commonly attempted with motor loads though it only supplies 2/3 power and can cause the motor loads to run hot and in some cases overheat. This method does not work when sensitive circuitry is involved such as [[computer numerical control|CNC]] devices or in induction and rectifier-type loads.

[[Variable-frequency drive]]s (also known as solid-state [[Inverter (electrical)|inverters]]) are used to provide precise speed and torque control of three-phase motors.  Some models can be powered by a single-phase supply. VFDs work by converting the supply voltage to DC and then converting the DC to a suitable three-phase source for the motor.

[[Digital phase converter]]s are designed for fixed-frequency operation from a single-phase source. Similar to a variable-frequency drive, they use a microprocessor to control solid-state power switching components to maintain balanced three-phase voltages.

== Alternatives to three-phase ==

* [[Split-phase electric power]] is used when three-phase power is not available and allows double the normal utilization voltage to be supplied for high-power loads.
* [[Two-phase electric power]], like three-phase, gives constant power transfer to a linear load. For loads that connect each phase to neutral, assuming the load is the same power draw, the two-wire system has a neutral current that is greater than neutral current in a three-phase system. Also motors are not entirely linear, which means that despite the theory, motors running on three-phase tend to run smoother than those on two-phase. The generators in the [[Adams Power Plant]] at [[Niagara Falls]] that were installed in 1895 were the largest generators in the world at the time and were two-phase machines. True two-phase power distribution is obsolete for "new work" applications, but still exists for "old work" applications, perhaps most particularly in Buffalo and Niagara Falls, NY, Toronto and Niagara Falls, Ontario, Philadelphia and Reading, PA, and Camden, NJ. "New work" three-phase installations may be supplied by old two-phase feeders, and "old work" two-phase installations may be supplied by new three-phase feeders using a [[Scott-T transformer]], invented by [[Charles F. Scott (engineer)|Charles F. Scott]].<ref name="Brittain (2007)">{{cite journal |last=Brittain |first=J. E. |title=Electrical Engineering Hall of Fame: Charles F. Scott |journal=[[Proceedings of the IEEE]] |year=2007 |volume=95 |issue=4 |pages=836–839 |doi=10.1109/JPROC.2006.892488}}</ref> Special-purpose systems may use a two-phase system for frequency control.
* ''Monocyclic power'' was a name for an asymmetrical modified two-phase power system used by [[General Electric]] around 1897, championed by [[Charles Proteus Steinmetz]] and [[Elihu Thomson]].  This system was devised to avoid patent infringement. In this system, a generator was wound with a full-voltage single-phase winding intended for lighting loads and with a small fraction (usually 1/4 of the line voltage) winding that produced a voltage in quadrature with the main windings.  The intention was to use this "power wire" additional winding to provide starting torque for induction motors, with the main winding providing power for lighting loads.  After the expiration of the Westinghouse patents on symmetrical two-phase and three-phase power distribution systems, the monocyclic system fell out of use; it was difficult to analyze and did not last long enough for satisfactory energy metering to be developed.
* High-phase-order systems for power transmission have been built and tested. Such transmission lines typically would use six phases or twelve phases. High-phase-order transmission lines allow transfer of slightly less than proportionately higher power through a given volume without the expense of a [[high-voltage direct current]] (HVDC) converter at each end of the line. However, they require correspondingly more pieces of equipment.

== Color codes ==
{{See also|Electrical wiring#Color code}}
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 [[electric motors|induction motors]].  Colors used may adhere to International Standard [[IEC 60446]], older standards or to no standard at all and may vary even within a single installation. For example, in the U.S. and Canada, different color codes are used for grounded (earthed) and ungrounded systems.
{| class="wikitable"
! Country
! L1
! L2
! L3
! Neutral
! Ground / protective earth
|-
| Australia and New Zealand as per AS/NZS 3000:2007 Figure 3.2 (or as per IEC 60446 as approved by AS:3000)
| bgcolor="red"|Red (or brown) <ref group="note" name="note1">In Australia and New Zealand, active conductors can be any color except green/yellow, green, yellow, black or light blue. Yellow is no longer permitted in the 2007 revision of wiring code ASNZS 3000.  European color codes are used for all IEC or flex cables such as extension leads, appliance leads etc. and are equally permitted for use in building wiring per AS/NZS 3000:2007.</ref>
| bgcolor="white"|White (or black)<ref group="note" name="note1" /> (prev. yellow)
| bgcolor="darkblue" style="color:white"|Dark blue (or grey)<ref group="note" name="note1" />
| bgcolor="black" style="color:white"|Black (or blue)<ref group="note" name="note1" />
| bgcolor="green"  style="color:yellow"|Green/yellow striped (green on very old installations)
|-
| Canada (mandatory)<ref>Canadian Electrical Code Part I, 23rd Edition, (2002) ISBN 1-55324-690-X, rule 4-036 (3)</ref>
| bgcolor="red"|Red
| bgcolor="black" style="color:white"|Black
| bgcolor="blue" style="color:white"|Blue
| bgcolor="white"|White or Grey
| bgcolor="green" style="color:white"|Green or bare copper
|-
| Canada (isolated three-phase installations)<ref>''[[Canadian Electrical Code]]'' 23th{{Clarify|date=September 2013|reason=Should this be 19th or is the date wrong and it should be 22nd in 2012?}} edition 2002, rule 24-208(c)</ref>
| bgcolor="orange"|Orange
| bgcolor="brown" style="color:white"|Brown
| bgcolor="yellow"|Yellow
| bgcolor="white"|White
| bgcolor="green" style="color:white"|Green
|-
| [[European Union]] and all countries who use European [[CENELEC]] standards April 2004 ([[IEC 60446]]), Hong Kong from July 2007, Singapore from March 2009
| bgcolor="brown" style="color:white"|Brown
| bgcolor="black" style="color:white"|Black
| bgcolor="gray" style="color:white"|Grey
| bgcolor="blue" style="color:white"|Blue
| bgcolor="green"  style="color:yellow"|Green/yellow striped<ref group="note" name="note2">The international standard green-yellow marking of protective-earth conductors was introduced to reduce the risk of confusion by [[color blindness|color blind]] installers. About 7% to 10% of men cannot clearly distinguish between red and green, which is a particular concern in older schemes where red marks a live conductor and green marks protective earth or safety ground.</ref>
|-
| Older European ([[IEC 60446]], varies by country<ref group="note" name="note3">In Europe, there still exist installations with older colors for protective earth but, since the early 1970s, all new installations use green/yellow according to [[IEC 60446]].</ref>
| bgcolor="black" style="color:brown"|Black or brown or grey
| bgcolor="black" style="color:brown"|Black or brown or grey
| bgcolor="black" style="color:brown"|Black or brown or grey
|bgcolor="blue" style="color:white"| Blue
| bgcolor="green"  style="color:yellow"|Green/yellow striped<ref group="note" name="note3" />
|-
| UK until April 2006, Hong Kong until April 2009, South Africa, Malaysia, Singapore until February 2011
| bgcolor="red"|Red
| bgcolor="yellow"|Yellow
| bgcolor="blue" style="color:white"|Blue
| bgcolor="black" style="color:white"|Black
| bgcolor="green"  style="color:yellow"|Green/yellow striped (green on installations before c. 1970)
|-
| India and Pakistan
| bgcolor="red"|Red
| bgcolor="yellow"|Yellow
| bgcolor="blue" style="color:white"|Blue
| bgcolor="black" style="color:white"|Black
| bgcolor="green" style="color:yellow"|Green/yellow striped, or green
|-
| Former USSR (Russia, Ukraine, Kazakhstan) and People's Republic of China (per GB 50303-2002 Section 15.2.2)
| bgcolor="yellow"|Yellow
| bgcolor="green"|Green
| bgcolor="red"|Red
| bgcolor="#0099FF" style="color:white"|Sky blue
| bgcolor="green"  style="color:yellow"|Green/yellow striped
|-
| Norway
| bgcolor="black" style="color:white"|Black
| bgcolor="white"  style="color:grey"|White/Grey
| bgcolor="brown" style="color:white"|Brown
| bgcolor="blue" style="color:white"|Blue
| bgcolor="yellow"  style="color:green"|Yellow/green striped, older may be only yellow or bare copper
|-
| United States (common practice)<ref group="note" name="note4">See Paul Cook: [http://www.theiet.org/publishing/wiring-regulations/colour/col-harm.cfm Harmonised colours and alphanumeric marking]. IEE Wiring Matters, Spring 2006.</ref>
| bgcolor="black" style="color:white"|Black
| bgcolor="red"|Red
| bgcolor="blue" style="color:white"|Blue
| bgcolor="white"  style="color:gray"|White, or grey
| bgcolor="green"  style="color:yellow"|Green, green/yellow striped,<ref group="note" name="note7">In the U.S., a green/yellow striped wire may indicate an [[isolated ground]].{{Citation needed|date=August 2007}} In most countries today, green/yellow striped wire may only be used for protective earth (safety ground) and may never be unconnected or used for any other purpose.</ref> or a bare copper wire
|-
| United States (alternative practice)<ref group="note" name="note5">Since 1975, the U.S. National Electric Code has not specified coloring of phase conductors. It is common practice in many regions to identify 120/208Y conductors as black, red, and blue. Local regulations may amend the N.E.C. The U.S. National Electric Code has color requirements for grounded conductors, ground and grounded-delta 3-phase systems which result in one ungrounded leg having a higher voltage potential to ground than the other two ungrounded legs. Orange is only appropriate when the system has a grounded delta service, regardless of voltage.</ref>
| bgcolor="brown" style="color:white"|Brown
| bgcolor="purple"  style="color:orange"|Orange (delta), violet (wye)
| bgcolor="yellow"|Yellow
| bgcolor="gray"  style="color:white"|Grey, or white
| bgcolor="green" style="color:white"|Green
|}

== See also ==
* [[Three-phase AC railway electrification]]
* [[Charging station]]
* [[Frequency converter]]
* [[Industrial & multiphase power plugs & sockets]]
* [[International Electrotechnical Exhibition]]
* [[John Hopkinson]]
* [[Y-Δ transform]]

== Notes ==

{{reflist|30em|group="note"}}

== References ==

{{reflist|30em}}

{{Nikola Tesla}}

[[Category:Electric power]]
[[Category:Electrical engineering]]
[[Category:Electrical wiring]]
[[Category:Nikola Tesla]]