320v leg on 480v 3ph.

What is the voltage of the other two phases?

If one of the other two phases were much lower, it would indicate a heavy neutral current from a single phase load on that phase.

PW , thanks for the response . The other 2 legs have 240vac to ground on them

Your using a delta high- leg system. This is usually the "B" phase or T-2

I got a couple of guesses. First, it does not sound like a 3 phase 4wire wye, otherwise you would be measuring somewhere around 277V from phase to ground. It sounds like a center tapped delta, but on a 480V system the high leg would read around 416V. It may be that the phase you were measuring was the residual voltage from an open leg being back fed by the motor.

Ya I agree... you get a point.

Was the motor single phasing, A+B or A+C or B+C?

You should measure the voltage between all 3 phases A-B; B-C; C-A if you read 480v between all phases then it is because of where the transformer secondary upstream is grounded (or possibly floating). If you do not read 480v between all phases then it could indicate an overloaded phase with too much current flowing thru it to the Neutral or another phase.

Thanks everyone for the replies ! I did a little more research on this and this is what I found . Between any 2 phases I read 472 vac . Phase A to ground = 250vac , phase B to ground = 257 vac and phase C to ground 316 vac . The current is the same on all three phases , around 7-1/2 amps. I hope this will shed some light for some of you more experienced pros out there because this is making no sense to me . Going camping tomorrow so I'll check back next week sometime . Thanks again for all the help !

As of yet, you have not mentioned a neutral. If you have a neutral, what are the line to neutral voltage readings?

You need to understand what type of winding, either delta or wye, it is that is supplying the 480 volt system and where that system is grounded.

If you do not see a neutral and if you do not have any line to neutral loads, which gives you 277v which is normally used for lighting loads, your neutral may just have been left at the transformer and not brought any farther.

There has to be a neutral at the transformer, other wise it would be a corner grounded system or not grounded at all. If it was not grounded, the line to ground voltage should be zero and if it was corner grounded, 1 of the phases should show zero volts to ground.

As North of 60 pointed out, more info is needed on your distribution system. From what you stated here, is sounds like a 3 phase 3 wire system that is not grounded or improperly grounded. The readings look like something you would see from a floating ground with the exception that they seem to remain constant.