High leg delta

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.

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.

Example

Consider the low voltage side of a 480/240 V high leg delta connected transformer, where the 'B' phase is the 'high' leg. The line to line voltages are all the same:

$V\_\{ab\}\; =\; V\_\{bc\}\; =\; V\_\{ac\}\; =\; 240\; V$

Because the winding between the 'A' and 'C' phases is center-tapped, the line-to-neutral voltages for these phases are as follows:

$V\_\{an\}\; =\; V\_\{cn\}\; =\; 120\; V$

But the phase-neutral voltage for the 'B' phase is different:

$V\_\{bn\}\; =\; 120sqrt\{3\}\; V\; approx\; 208\; V$

This can be proven by writing a KVL equation starting from the grounded neutral:

$0\; +\; 120\; angle\; 0^circ\; +\; 240\; angle\; 120^circ\; =\; 120sqrt\{3\}\; angle\; 90^circ$

Note: Writing the KVL equation going the other way, the same magnitude is found, though the phase angle will of course be different.

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.

References

*http://www.sea.siemens.com/step/templates/lesson.mason?pan:3:2:3
*NFPA 70: National Electrical Code, 2005 Ed.