99NEC§424-3 “…ampacity of the branch-circuit…shall not be less than 125 percent…[but] rating or setting of overcurrent protective devices shall be permitted in §240-3.”
Doesn’t that mean 110-amp overcurrent protection and 2AWG 75°C Cu conductors?
He did say THHN. I show 90 degrees C with an ampacity of 95 amps. Of course that does not take voltage drop or the fact that you would probably but a 100 amp breaker on the circuit. More than likely he would need but a 100 amp breaker and use #2 awg. Wirenut
I would agree with busbar, at least for a U.S. installation. The breaker MUST be rated for 125% of the rating of the heater for electric heating. This would be 107.5A so a 110 A breaker is the minimum allowable size.
This, in turn, would require #2 AWG, assuming that the terminations at **both ends** of the circuit are rated for 75 deg C. But this is not a good assumption for end-use equipment 100 amps or less.
Safest approach is to assume 60 deg C terminations and use 60C insulation ampacity for all circuits up through 100A. This would require #1 AWG.
This is an NEC-UL centered analysis and probably would be much different in other parts of the world.
Can you please further explain your calculation. Is the 480V a line-line or phase voltage from a delta or a star. If I assume the 480V is line-line and that the 72kW heater is a delta, I get 29A. However, if I assume it is a star, my calculation is 29A.
Which is it?
For a delta,
72000=3(VLL)(ILL)
72000=3(480)(Ip/1.73)
Ip=29A
Look in any first-year electrical text. Shouldn't be too hard to find one. Since this is an engineering forum for engineering professionals, I assume you are an engineering professional, and already have one or more textbooks from whatever technical training course you took.
I know the formula, I wanted to know how wirenut assumed that it was a delta? It could have been a star? I don't work with motors and nameplates on an everyday basis so that is why I am asking?
I was assuming you wanted the current from the source, which is the line current. I doesnt matter how the load is connected, the current for the feeder breaker will be the same.
Although I have my PE, and have sized cable for industry and Naval ships for years, I am no expert in the NEC. Perhaps these other contributers are better sources for the complexities of the various standards and codes.
This could already have been assumed or is mute however I will toss it in the pot - the wire ampacity might need to be further derated depending upon how many other motor/heating circuits are in the raceway/conduit due to heating from the multiple load conductors within the same raceway. For phase angle controlled loads I will sometimes (depending upon the controller load type) further derate the conductor ampacity because of the heating from the "harmonincs".
