)U have supply 480V.Your building need 120/208v 400a service.what size transformer u must use ?

a.150KVA b.200KVA c.500Kva .d.600kva

Alex, I'll go with a 150KVA secondary transformer and I'd like to bet it all.

Learning and understanding three-phase wiring is not the easiest thing. Once you get it you'll never forget it.

The number you need to remember for three-phase is 1.73. That number is the square root of the number 3. It used wherever a three-phase voltage is used. 208, 240, 480, etc.... are all three-phase voltages and the most common ones.

Next, and very important for any "real electrician" to know is Ohm's Law. You can google this and find out what this is on your own.

Long story short... 208 volts x 1.73 = 360. Use this number whenever you need to find a three-phase voltage for 208 volt services.

Next, 360 volts x 400 amps = 144,000 watts, or commonly known in the trade as 144 kilo-volt amps, KVA for short.

Now, since you need at least 400 amps for this service a 150KVA transformer will be needed.

They don't just give out electrical contracting licenses where I live, here they're earned.