Steve,

The reading shows that the 120 and 277V legs in question are not in phase.

It's a similar principle to the readings you get on a single 3-phase system. If you measure between the hots of two 120V receptacles that happen to be on the same phase of a 120/208 system, the sinewaves are in phase and the measured voltage between them will be zero. If the two receptacles are on different phases, then there is a 120-degree phase difference between them and you read 208V.

You have a similar thing happening between your two systems, except that it is complicated by the two different voltage levels in use. If the 120 receptacle circuit were exactly in phase with the 277V lighting circuit, then you would just measure the difference, approx. 157 volts (RMS).

If the two are exactly 120 degrees out of phase, the difference comes out at 352 volts. Allowing for the fact that the exact line voltages may not be exactly 120 and 277, and that the xfmr is bound to introduce some phase shift, your reading of 310V is quite reasonable. The highest reading you could get in theory would be if the two lines were 180 degrees out of phase, in which case the difference would simply be the sum of the voltages: 397V.

By the way, is the xfmr wired 277/480 Y or just as a 480 delta primary?


[This message has been edited by pauluk (edited 05-01-2002).]