Transformer Sizing for Load and Fusing for Primary Current

Thread Starter


Joined Sep 20, 2009
I had some questions about power , current, and fusing sizes for transformers.

If you have a 150 KVA transformer with 3 phase 480 Delta to Y and 480VAC output, what is the draw on the primary side (needed for fusing) if the secondary has both 3phase and single phase loads? How many amps available on the secondary does it have?

Is it easier to think of the secondary in terms of available power rather than available amps, so if you have a load that is 120VAC and 9 amps, just convert that to single phase power but if something is 3phase and X amount of amps, that has to be (volts x amps x sqrt(3) ) ?

If you are supplying 3 phase power to a device with it's own internal multiple winding transformer, what is the FLA considered to be on that branch circuit? do you need to know the loads of that device, or does it not matter?


Joined Mar 30, 2018
The easiest way to calculate the current per phase is to divide the total rated KVA by 3 and then divide the result by the volts/phase (phase to neutral).

So at full load your 150KVA three phase transformer will be drawing 50KVA/(480/sqrt(3)) = 180A per phase.

Depending on what the transformer is driving, you might need to configure the protection circuit such that should one phase be overloaded, then all three phases trip – otherwise the transformer will be supplied by only two phases which might do strange things to the output voltage/phase.