I'm hooking up some 3 phase transformers that have multiple taps and I need to know the exact service voltage. Phase to ground averages 290 volts. 1 -2 = 498 volts, 1 - 3 = 501 volts and 2 - 3 = 506 volts.
@NickAlexeev yeah, hrm...it's answerable in the state it's in, but the only answer that can be given is "call whoever is providing that service or feeder and ask them for the nominal service voltage"