A high leg delta (also known as wild-leg or red-leg delta) is a type of electrical service connection sometimes found in older three-phase electric power installations. It was used where both lighting and three-phase motor loads were to be fed from the same distribution system.
This type of service is supplied by a transformer having four wires coming out of the secondary: the three phases, plus a neutral that is used to center-tap one of the windings. The voltages between the three phases are the same in magnitude, however the voltage magnitudes between a particular phase and the neutral vary. The phase-to-neutral voltage of two of the phases will be half of the phase-to-phase voltage. The remaining phase-to-neutral voltage will be √3 times half the phase-to-phase voltage. Typically, the transformer is connected such that the 'B' phase is the 'high' leg. According to Article 110.15 of the 2005 National Electrical Code, panelboards connected to this type of transformer must explicitly identify the high leg, preferably by coloring it orange.
Advantages
This type of services is usually supplied using 240V line-to-line and 120V phase to neutral. In some ways, the high leg delta service provides the best of both worlds: a line-to-line voltage that is higher than the usual 208V that most three-phase services have, and a line-to-neutral voltage (on two of the phases) sufficient for connecting appliances and lighting. Thus, large pieces of equipment will draw less current than with 208V, requiring smaller wire and breaker sizes. Lights and appliances requiring 120V can be connected to phases 'A' and 'C' without requiring an additional step-down transformer.
Disadvantages
Since one phase-to-neutral voltage (phase 'B') is higher than the others, no single phase loads can be connected to this phase. This essentially eliminates one third of the breakers in a panel if there are many single-phase loads.
power distribution transformers
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