OCP xfmer

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jimmac49

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Is the following installation a NEC violation of 240.4 (B) (3).

A 13.8kv fused primary, 750 kva transformer, has a 480/ 277 3 phase secondary, with a 1200 amp frame / 1200 amp trip main breaker feeding a distribution bus. I am thinking it should be a 1000kva transformer to comply with Table 450.3(A) Thanks
 
We had a lively discussion of this very topic in our office last week. I was surprised, as were other engineers in the office, to discover that the NEC did not say what we thought it said (imagine that, having to invoke "Charlie's Rule" on myself! ;) :grin: ).

I believe that what you describe is not a violation. Please note that 240.4(B)(3) is talking about protecting the conductors, not the transformer itself. Note also that the protection of the transformer windings, as provided for in Table 450.3(A), does not include the provision that you use the next lower, and not the next higher, standard size overcurrent device, when the device rating exceeds 800 amps. That was the thing that surprised us all. We had thought that you needed to use the next lower size breaker, when you are over 800 amps, but Note 1 to Table 450.3(A) does not say that.

So you can use a 1200 amp main breaker, and not violate the rules for protecting the transformer secondary windings. You will have to make sure that the secondary conductors have an ampacity of at least 1200 amps, and that the calculated load does not exceed 1200 amps.
 
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