Circuit for Oversized Transformer

[rwilsond]

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?

 

[davidbeach]

Without having done a search and looking up the exact wording, I believe the NEC sets maximum protection values without specifying minimums. Where you will get into trouble undersizing the primary to a transformer is on inrush, but slight undersizing as you propose shouldn't be a problem.

 

[dpc]

Article 215.2(B)(1)"Feeder Supplying Transformers. The ampacity of feeder conductors shall not be less than the sum of the nameplate ratings of the transformers supplied when only transformers are supplied."

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.

 

[wareagle]

I don't think the 45 amp CB will with stand the inrush. Usually the primary has a breaker thats 125% of the full load. In your case that would be 75 amps. Why would you not want to install the full load capacity?

 

[rwilsond]

Thanks for the replies. For dpc's reply, note that the section referred to is for 600V, and above (the transformer in question is 480V-208Y/120V).

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!

 

[dpc]

You're right on the NEC reference applying to 600V and above.

But you will definitely have problems with a 45 A CB on the primary of a 45 kVA 480V primary.

 

[waross]

You may be able to use 210-19 (1996 code edition)
(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