I am powering a lighting system with a mixture of SCR dimmers and HID lamps. I am pulling approx 285 amps/leg at 120/208V through a 150 kVA transformer. The 120/208V service coming from that transformer is indicated to be 400A/leg, but it seems to get pretty hot at 285A.
From your readings ,the transformer is within it's rating. Power Transformers are usually rated to operate at 75 Deg. C and above. Sometimes up to 95 Deg. C. The data and limit will be on the nameplate. 75 Deg. C is uncomfortably hot. 95 Deg. C. will burn you. Note; SCR dimmers WILL produce harmonic currents that will increase the 'neutral load' well above 'phase load' on 'common neutral' circuits, and might be producing some high frequency heating in that T/F.
Three phase transformers are given a power rating which is the sum of all three powers on each phase. So 285 amps on each phase at 120 line to neutral (assuming this is how youve them connected) is about 35kva, so about 100kva all up. What is actually on the name plate of the transformer? How hot is 'pretty hot'? Is it running as it was desinged (oil, dry, forced air cooling etc). Also how are your lamps/dimmers wired? The other answer is correct, the dimmers will produce alot of harmonics. The meter your are using to read the current is most likely designed to read a 60hz sine wave, any other stuff there will cause a wrong reading (they usually measure the average value, then apply a correction factor to obtain the rms value. But this factor depends on the wave shape, if its not a sine wave it will be wrong). See if you can obtain a meter that will measure the true RMS current. Transformers of that size are usually heavily over engineered, it should be fine as long as the tempertature doesnt continue to increase, ie to the point where the case is too hot to touch. As the other answer said it is probably rated for continuous operation at about 75 degrees. Id not worry too much as long as the temperature doesnt continue increasing.
