High leg delta
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A high leg delta (also known as wild-leg or red-leg delta) is a type of transformer winding connection sometimes found in older electrical installations. A transformer wound in this fashion will have 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 designed 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.
[edit] 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:
Vab = Vbc = Vac = 240V
Because the winding between the 'A' and 'C' phases is center-tapped, the line-to-neutral voltages for these phases are as follows:
Van = Vcn = 120V
But the phase-neutral voltage for the 'B' phase is different:
This can be proven by writing a KVL equation starting from the grounded neutral:
Note: Writing the KVL equation going the other way, the same magnitude is found, though the phase angle will of course be different.
[edit] References
- http://www.sea.siemens.com/step/templates/lesson.mason?pan:3:2:3
- NFPA 70: National Electrical Code, 2005 Ed.
