Transformer Overcurrent Rating

[Telaid]

I have a 45 kva 3 phase, 480Y/220 transformer, i calculate 54 amps. Would a 60 amp breaker be sufficient? Or should it be larger due to inrush current?

 

[cpal]

Is this pri OCP?
What is the Pri Voltage?


Have you consulted 450.3(B)???

Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.

 

[charlie b]

45,000 divided by (480 times 1.732) is 54.1.

I'd multiply by 1.33 to allow for inrush, and get about 72 amps. You could go as high as 250%, if you have secondary protection as well. Table 450.3(B).

I would choose a 70 amp breaker, but anything up to 150 amps would be allowable.

By the way, I've never come across a three phase transformer with 220 volts on the secondary. May I ask what the application might be?

 

[kingpb]

If you want to make sure the protection is correct, the only way to know for sure is to consult the manufacturer and get the ANSI transformer damage curve. The curve also shows the inrush. In lieu of that, you can use the NEC as a guidelines; it may or may not be correct, since transformer protection in the NEC is based only on typical equipment. Granted a 45KVA transformer is small potatoes, in the grand scheme of things.

Now, if your interested in transformer protection check out the SEL 387, granted overkill for your application, but still fun to know about.:roll:

http://www.selinc.com/sel-387a.htm

 

[Telaid]

By the way, I've never come across a three phase transformer with 220 volts on the secondary. May I ask what the application might be?[/QUOTE]

Sorry I meant 480Y/277 primary, 208Y/120 secondary.