Circuit for Oversized Transformer
Circuit for Oversized Transformer
(OP)
Assume a balanced connected load of 25 kVA on the secondary of a 45 kVA, 3-phase, 480-208/120V transformer. Does the NEC prohibit less than that of the transformer's primary rating for sizing wiring and CB? For example, can a 45A CB, and 50A ampacity wiring be used to supply the transformer's primary (which has a FLA rating of 54A)?
If there is a code violation, what paragraph/section of the NEC?
If there is a code violation, what paragraph/section of the NEC?






RE: Circuit for Oversized Transformer
RE: Circuit for Oversized Transformer
Also, the transformer inrush problem that David Beach mentioned is a real one. Back when I was doing design work, we used an 80A breaker (and feeder) on a 480V 45 kVA primary. We found smaller sizes would sometimes trip on inrush.
RE: Circuit for Oversized Transformer
RE: Circuit for Oversized Transformer
For wareagle's reply, I DO want to "install the full load capacity"; which is what prompted the thread. But I have been unable to find a code basis that prohibits others from doing otherwise.
By-the-way, maybe you are an Auburn graduate, as myself (Physics, 1987); if so, War Eagle to you!
RE: Circuit for Oversized Transformer
But you will definitely have problems with a 45 A CB on the primary of a 45 kVA 480V primary.
RE: Circuit for Oversized Transformer
(a) General branch-circuit conductors shall have an ampacity not less than the maximum load to be served.
I'd consider the maximum load to be the rating of the transformer. The inspector may also.
yours