Sizing motor overcurrent-protection of Design-B motor

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DigiMan

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Scranton/Wilkes-barre, PA
I have a squirrel-cage, resistor starting Design-B, 3 phase motor. It has a FLA of 30A. The overcurrent protection device is a non time delay fuse. What is the maximum branch circuit overcurrent-protection device I can use on this motor?

333% (100A) of FLA? 366% (110A) of FLA? 410% (125A) of FLA? I cant find a clear answer in the code for this one... Is it based on Table 430.52? or am I missing something?
 
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DigiMan

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Scranton/Wilkes-barre, PA
I know Table 430.52, non-time-delay fuse, for Short-Circuit protection allows 300% of FLA... 30FLA x 300% = 90A... is there some other multiplier that I'm missing to calculate the overcurrent-protection maximum for the branch-circuit that this motor would be on?

I see that 430.32(A)(1) under "all other motors" allows for 115% for continuous duty motos as a multiplier, but this is of the nameplate rating, not the short-circuit load calc. Am I missing something being that it is for the branch?
 
david luchini

david luchini

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The answer is 110A.

Exception #2(a) to Table 430.52 allows a non-time delay fuse (not exceeding 600A) to be increased to not more than 400% of the full load current when the value in the table is insufficient for the starting current.

30x400%=120A. The next standard size up (125A) would exceed 400% of FLC, so the next standard size down (110A) would be the maximum allowable overcurrent protection in your scenario.
 
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