transformers

[Mister Kool]

How is the overcurrent protection sized for the primary side of a transformer?

lets say I have a 45KVA transformer 480V/208V, 3phase. I know that the secondary side would be fused at 125A

BUT

what is the rule of thumb for the primary side????

Thank you

 

[GaTech04]

You'd probably go by 450.3(B):

45,000 / (480 * 1.73) = 54.2A * 125% = 67.7A.

Note 1 allows you to go up to the next standard size, so a 70/3 breaker for the MOCP.

 

[Mister Kool]

thanx a lot