ECN Forum Forums General Topics of the Trade General Discussion Area 3 Phase Power (490 volts on a 480 volt system)

Doing the math isn't that bad.

1% is 1/100, i.e. 1% of 460V is 460/100=4.6.

So 10% is 4.6*10=46.

460V -10% is 460-46=414.
460V +10% is 460+46=506.

That's not the shortest way, but the easiest to explain and remember.

iwires figures confuse me a bit though, since 432 is 480 - 10% and 508 is close to 460V + 10%, but only close.

Be sure to check the control transformer, many have taps on them that need to be set for the line voltage.

If the voltage is too high it usually can be corrected by reconnecting the primary taps on the transformer which feed the system. It is common for transformers to have +(2)2-1/2%FCAN (full capacity above normal)and (2) or (4)2-1/2% FCBN taps (full capacity below normal.)
Taps would be +5%, +2-1/2%, 0%, -2-1/2%, -5%, -7-1/2%, and -10%.
490v is very close to 2-1/2% high so reconnection of the primary tap to the +2-1/2% position would bring the secondary voltage to slightly below 480v. Remember that a transformer does not regulate and the voltage often varies as the load changes. And also the utility supply voltage may also especially at different times of the day.