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NEC 670.4(C)  and NEC 409.21 Overcurrent Protective Device Calculation

Question asked by ntan on Dec 19, 2017
Latest reply on Apr 11, 2018 by ntan

I am in the process of designing an electrical control panel for industrial machinery and am following NEC 2017 670.4(C)  and NEC 409.21 to size the disconnect OCPD for the panel. The loads in the panel are as follows:
Motors:

HPFLA
34.8
1.53

 

Other Loads:

FLABreaker (A)
1.356
3.24
810
810
810

 

When I calculate the OCPD according to the articles, I get 36.35 amps FLA (sum of the loads: 4.8 + 3 + 1.35 + 3.2 + 8 + 8 + 8) and a maximum overcurrent protective device rating of 38.35A (sum of the loads plus the largest OCPD: 4.8 + 3 + 1.35 + 3.2 + 8 + 8 + 10). This means the largest breaker I can use for the panel is 35 amps since I must use the next smaller size of an available breaker; however, this is below the FLA of the panel. Am I interpreting the NEC article calculations correctly? Can anyone provide insight how to handle this situation? Thank you all for your help.

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