Nec Article 424

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bradp

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Dallas, Texas
I have seen several different methods for circuiting electric fan powered mixing boxes, some of which I don't feel are correct.
  • I have an open office space with (2) FPMB at 480,3PH each with a 1/8 HP fan at 277,1PH
  • FPMB-1 and FPMB-2 both have 8kW heaters
  • Both boxes are specified with factory interlocking main disconnect switch.
NEC 424.3 allows branch circuits supplying two or more outlets for fixed electric space-heating equipment to be rated at 15, 20, 25, or 30 amperes.

If I put both of these boxes on a 30A-3P circuit with (4)#10,(1)#10G-3/4"C am I in violation of the NEC? I have also been told that I need a fused disconnect at each FPMB. Is this correct? If so, where in the code is this stated?

If I were to provide power for a single FPMB at 20kW with a 1/2 HP fan I would provide a 40A-3P circuit with (4)#8,(1)#10G.-3/4"C to the unit. Would I need a fused disconnect at this unit?

Hopefully this leads to some good discussion. I searched for several hours, but could find a definitive answer.
 
If the heaters are 8kw 3phase at 480V then each heater is 9.6 amps. According to 424.11(E) you cannot protect this over 150%. 9.6*1.5= 14.4 but you can go to the next size breaker according to the same article. So unless the unit calls for fuses, then a 15 amp cb or 15 amp fuse disconnect would be needed. At least that is how I see it
 
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