3+ Amps Per +

[Tscott8201]

Ok, I am a bit embarrassed to ask this but I am totally confused. I recently installed a 500 KVA 277/480 padmount transformer. The question arose when I needed to size the CT loops for the new load. How many amps per phase is a 500KVA Tx capable of. The formula I use to find amps is:

I=VA/V(line to line) x 3^(1/2)

In this case the above would yield:

500,000/(480 x 1.73)= 602 Amps

Is the amps per phase 1/3 of this? or is 600A the amps per phase?

Any insight on this would be appreciated, 3Ø has always confused me.

Also, if the customer has an 800 amp 3Ø main disconnect, is that the rating per Ø or total of the Ø's?

Tom

 

[rbulsara]

The amps you calculated is for each line or phase. The formula you used is for a system with a balanced three phase load.

You can arrive at the same conclusion, if you do a single phase calc using the load per phase and the voltage per phase.

3-phase 500 kva is 167kVA per phase and the voltage per phase would be 480/1.732 or 277V. Do the math for per phase or single phase current.

Current rating for three phase equipment is for the current in each phase or line.

 

[waross]

What rbulsara said.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tscott8201]

Thanks, This clears up a lot of confusion. I had the math correct, but was unclear of the rating methods used for the equipment.

Tom

 

[jghrist]

Confusion lies in the term amps per phase. This implies that there will be more amps in three phases. Better to say amps in each phase.

 

[NAZ55]

3 phase power = SQRT(3) x V (line to line) x I (line)
or
3 phase power = 3 x V (line to neutral) x I (line)
or
1 phase power = V(line to neutral) x I (line)

look at it either way, current is always per phase.