Hello,

I am an electrician working in a small industrial plant. I was asked to look into a possible transformer overheating problem and had a few questions about the transformer ratings in general. I apologize for the length of the post, but I just wanted to be as complete as possible.

What we have is a bank of three single phase transformers hooked up as a 4 wire delta system (with a 208 high-leg). The transfers are each rated at 150kVa at 240 Volts with a 65 degree C temperature rise.

The first question I have is just on how you would determine permissible line current for the system. This is what I figured:

Total bank capacity = 150+150+150 = 450 kVa
Current = 450 *1000/(1.732*240) = 1083 amps.

So the line current would be 1083 amps at full load. And would be fused at 125% of this or 1356 amps

The current in the winding would be 1083/1.732 = 625 amps.
This could also be found by using the individual kVa ratings by:
150*1000/240 = 625 amps

Are the above calculations correct?

The second question I have is on the temperature rating of the transformer. If it is rated at 150 kVa with a 65 degree temperature rise wouldn't that mean that when under full load the transformer would be running at 65 degrees above ambient temperature? Or if I convert to farenheight and figured ambient to be 70 degrees F, the rise would be 150 degrees F, and the transformer would be running at about 220 degrees F.

If this is correct how is the temperature normally measured, we currently just shoot the transformer case with a infrared temperature gun. The temperature of the center transformer (the highest) is about 120 degrees F. Our high voltage maintenance company has my supervisor concerned, but I don't think this is a problem based on what I posted above.

We also measured the current in each of the transformer windings and the center transformer was running at 380 amps. The high voltage guy told my supervisor it is only rated at 360 amps,which is the rating for a 3 phase 150 kVa transformer, but wouldn't are system be rated at 450 kVa? Am I doing the above calculations wrong, or is the high voltage maintenance guy giving us incorrect information?

Thanks for any help you can give,

Jamey