450.3.B

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raven

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I have a 3 phase, 45kva xfmr - 480 - 120/208. I have overcurrent protection on the primary and secondary. Based on 450.3.B, I see the maximum protection on the secondary as 150A. Does this have to be 150A. or can it be less? My intent is to install a 100A. main breaker in the panel the xfmr is feeding since a 150A. will not fit. I realize by doing this I am not using the full output of the xfmr.
Thanks
 
Like mentioned you can go next standard size higher.

45kVA is 124.9 amps, you can protect it at 156, but since that is not a standard size you can protect it at 175.

If you don't have the load you can protect it at anything you want below 175, even all the way down to 15 amps. Will need conductor sized for whatever you choose.
 
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