Editing Three-phase electric power (section)

== Three-wire and four-wire circuits ==
[[File:The basic 3-phase configurations (mul).svg|thumb|Wye&nbsp;(Y) and delta&nbsp;(Δ) circuits]]

There are two basic three-phase configurations: wye&nbsp;(Y) and delta&nbsp;(Δ). As shown in the diagram, a delta configuration requires only three wires for transmission, but a wye (star) configuration may have a fourth wire. The fourth wire, if present, is provided as a neutral and is normally grounded. The three-wire and four-wire designations do not count the [[ground wire]] present above many transmission lines, which is solely for fault protection and does not carry current under normal use.

A four-wire system with symmetrical voltages between phase and neutral is obtained when the neutral is connected to the "common star point" of all supply windings. In such a system, all three phases will have the same magnitude of voltage relative to the neutral. Other non-symmetrical systems have been used.

The four-wire wye system is used when a mixture of single-phase and three-phase loads are to be served, such as mixed lighting and motor loads. An example of application is local distribution in Europe (and elsewhere), where each customer may be only fed from one phase and the neutral (which is common to the three phases). When a group of customers sharing the neutral draw unequal phase currents, the common neutral wire carries the currents resulting from these imbalances. Electrical engineers try to design the three-phase power system for any one location so that the power drawn from each of three phases is the same, as far as possible at that site.<ref>{{cite web |url=http://www.rapid-tech.com.au/Fluke-2_Saving%20energy%20through%20load%20balancing.pdf |title=Saving energy through load balancing and load scheduling |access-date=2014-08-03 |url-status=dead |archive-url=https://web.archive.org/web/20140911172708/http://www.rapid-tech.com.au/Fluke-2_Saving%20energy%20through%20load%20balancing.pdf |archive-date=2014-09-11 }}</ref> Electrical engineers also try to arrange the distribution network so the loads are balanced as much as possible, since the same principles that apply to individual premises also apply to the wide-scale distribution system power. Hence, every effort is made by supply authorities to distribute the power drawn on each of the three phases over a large number of premises so that, on average, as nearly as possible a balanced load is seen at the point of supply.
[[File:Delta-Wye Transformer.png|thumb|A delta-wye configuration across a transformer core (note that a practical transformer would usually have a different number of turns on each side)]]
For domestic use, some countries such as the [[United Kingdom|UK]] may supply one phase and neutral at a high current (up to 100&nbsp;[[Ampere|A]]) to one property, while others such as [[Germany]] may supply 3&nbsp;phases and neutral to each customer, but at a lower fuse rating, typically 40&ndash;63&nbsp;A per phase, and "rotated" to avoid the effect that more load tends to be put on the first phase.{{citation needed|date=September 2016}}

[[File:High leg delta transformer.svg|thumb|right|A transformer for a "[[high-leg delta]]" system used for mixed single-phase and three-phase loads on the same distribution system. Three-phase loads such as motors connect to L1, L2, and L3. Single-phase loads would be connected between L1 or L2 and neutral, or between L1 and L2. The L3 phase is 1.73&nbsp;times the L1 or L2 voltage to neutral, so this leg is not used for single-phase loads.]]

Based on wye (Y) and delta (Δ) connection. Generally, there are four different types of three-phase transformer winding connections for transmission and distribution purposes:
* wye (Y) – wye (Y) is used for small current and high voltage,
* Delta (Δ) – Delta (Δ) is used for large currents and low voltages,
* Delta (Δ) – wye (Y) is used for step-up transformers, i.e., at generating stations,
* wye (Y) – Delta (Δ) is used for step-down transformers, i.e., at the end of the transmission.

In North America, a [[high-leg delta]] supply is sometimes used where one winding of a delta-connected transformer feeding the load is center-tapped and that center tap is grounded and connected as a neutral as shown in the second diagram. This setup produces three different voltages: If the voltage between the center tap (neutral) and each of the top and bottom taps (phase and anti-phase) is 120&nbsp;[[Volt|V]] (100%), the voltage across the phase and anti-phase lines is 240&nbsp;V (200%), and the neutral to "high leg" voltage is ≈&nbsp;208&nbsp;V (173%).<ref name="Fowler" />

The reason for providing the delta connected supply is usually to power large motors requiring a rotating field. However, the premises concerned will also require the "normal" North American 120&nbsp;V supplies, two of which are derived (180&nbsp;degrees "out of phase") between the "neutral" and either of the center-tapped phase points.