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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. Three-phase systems may have a fourth wire, common in low-voltage distribution. Transformers may be wired to have a four-wire secondary and a three-wire primary, while allowing unbalanced loads and the associated secondary-side neutral currents. The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load, which allows a reduction of the size of the neutral conductor because it carries little or no current. When a group of customers sharing the neutral draw unequal phase currents, the common neutral wire carries the currents resulting from these imbalances. Further up the distribution system, the currents are usually well balanced. Transmission & Distribution World. 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). As shown in the diagram, a delta configuration requires only three wires for transmission, but a wye (star) configuration may have a fourth wire.

Inspection of a phasor diagram, or conversion from phasor notation to complex notation, illuminates how the difference between two line-to-neutral voltages yields a line-to-line voltage that is greater by a factor of √3. A direct connection between two different phases is a short circuit and leads to flow of unbalanced current. That leads to higher efficiency, lower weight, and cleaner waveforms. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, but are occasionally used on long lines with light loads. For domestic use, some countries such as the UK may supply one phase and neutral at a high current (up to 100 A) to one property, while others such as Germany may supply 3 phases and neutral to each customer, but at a lower fuse rating, typically 40-63 A per phase, and "rotated" to avoid the effect that more load tends to be put on the first phase. If you live in an urban area, usually the transformer is quite "large" and has a high capacity but in a rural area, the transformer can be a small one on a pole and only capable of supplying a few homes.

The reason for providing the delta connected supply is usually to power large motors requiring a rotating field. 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. Except in a high-leg delta system and a corner-grounded delta system, single-phase loads may be connected across any two phases, or a load can be connected from phase to neutral. However, most loads are single-phase. However, the premises concerned will also require the "normal" North American 120 V supplies, two of which are derived (180 degrees "out of phase") between the "neutral" and either of the center-tapped phase points. Additionally, in some practical generators and motors, two phases can result in a less smooth (pulsating) torque. A closed delta made of three single-phase transformers can operate as an open delta if one of the transformers has failed or needs to be removed. As compared to a single-phase AC power supply that uses two current-carrying conductors (phase and neutral), a three-phase supply with no neutral and the same phase-to-ground voltage and current capacity per phase can transmit three times as much power by using just 1.5 times as many wires (i.e., three instead of two).

There are two basic three-phase configurations: wye (Y) and delta (Δ). In Europe, three-phase power is normally delivered to the panelboard and further to higher powered devices. In other words obtaining a 400 v load source, using two single 230v line feeds - what power in kwh is measured and which single phase meter measures that power used, or is any power used measured at all? Maintaining the identity of phases is required if two sources could be connected at the same time. In such a system, what are electric cables all three phases will have the same magnitude of voltage relative to the neutral. The neutral allows three separate single-phase supplies to be provided at a constant voltage and is commonly used for supplying multiple single-phase loads. The ratio of capacity to conductor material increases to 3:1 with an ungrounded three-phase and center-grounded single-phase system (or 2.25:1 if both use grounds with the same gauge as the conductors).