High-leg delta

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Example

Phasor diagram showing three 240V phases, and center-tapped 120V lines.

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: emily crib

Vab = Vbc = Vac = 240V baby convertible crib

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

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.

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 eliminates the ability to use one third of the breakers in a panel for single-phase loads. Further, the lack of loading on the high leg will result in an unbalanced load. However, many new fluorescent ceiling fixtures utilize ballasts which automatically sense line voltage. These fixtures are designed to operate on voltages from 110 to 277 volts. Use of modern fixtures in conjunction with code compliant wiring may allow selected lighting to be placed on the high leg, thereby eliminating this drawback.

References

http://www.sea.siemens.com/step/templates/lesson.mason?pan:3:2:3

NFPA 70: National Electrical Code, 2005 Ed.

Categories: Electrical engineering | Power engineering | Electric power