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Three-phase transformer with four-wire output for 208Y/120 volt service: one wire for neutral, others for A, B and C phases. Three-phase electric power (abbreviated 3ϕ [1]) is a common type of alternating current (AC) used in electricity generation, transmission, and distribution. [2]
High-leg delta service is supplied in one of two ways. One is by a three-phase transformer (or three single-phase transformers), having four wires coming out of the secondary, the three phases, plus a neutral connected as a center-tap on one of the windings. Another method (the open delta configuration) requires two transformers.
Lighting and general purpose receptacles are at 120 volts AC, with larger devices fed by three wire single-phase circuits at 240 volts. In commercial construction, three-phase circuits are often used. Common 3 phase configurations within a building are 208v/120 wye, 120/240 center tapped delta and 480v/277v wye. Lighting is usually fed by 277 V ...
Delta-wye transformers introduce a 30, 150, 210, or 330 degree phase shift. [3] Thus they cannot be paralleled with wye-wye (or delta-delta) transformers. However, they can be paralleled with identical configurations and some different configurations of other delta-wye (or wye-delta with some attention) transformers. [citation needed]
NEMA 12 series devices are three wire, three-pole, non-grounding devices for 3-phase, 480-volt equipment. According to NEMA, this is "reserved for future configurations", so no designs for this series exist and no devices have been manufactured.
For example, balanced two-phase power can be obtained from a three-phase network by using two specially constructed transformers, with taps at 50% and 86.6% of the primary voltage. This Scott T connection produces a true two-phase system with 90° time difference between the phases.
It is designed for three phase applications and is rated at 10 A, 250 V/440 V. The socket will also accept types 11 and 12 plugs, and the Europlug. The type 25 plug and socket has 3 rectangular pins, 4 mm x 5 mm, plus 2 flat pins (for L2 and L3). It is designed for three phase applications and is rated at 16 A, 250 V/440 V.
Where two voltages are given below separated by "/", the first is the root-mean-square voltage between a phase and the neutral connector, whereas the second is the corresponding root-mean-square voltage between two phases (exception: the category shown below called "One Phase", where 240 V is the root-mean-square voltage between the two legs of a split phase).