NEC Issue
NEC Issue
(OP)
Hello all.
I have an issue with article 430, I read and read... and read and read, but I cannot find the answer to my question. I think the NEC is very clear when it comes to branch circuit conductor rating and protection against SC and GF rating, even feeder protection rating its quite clear.
But, how do you rate the conductor ampacity for the feeder?
Do you use 430.24?
Let me se if I get things right...
Say you have a feeder supplying 2 motors and lighting load.
Motor 1 = 32 FLA
Motor 2 = 5 FLA
Lload = 5 Amp
So the breaker rating would be:
80+5+5 = 90 Amp
And the feeder conductor ampacity would be:
32*1.25+5+5 = 50 Amp
That would give me a combination of a 90 Amp breaker with a 6 AWG wire, which would not be protected by the breaker.
So...
What do you do?
Am I getting this wrong?
Does the NEC requires an overload protection for the feeder? (although I can't find anything related to this matter)
Any help would be appreciated, thanks in advance :).
I have an issue with article 430, I read and read... and read and read, but I cannot find the answer to my question. I think the NEC is very clear when it comes to branch circuit conductor rating and protection against SC and GF rating, even feeder protection rating its quite clear.
But, how do you rate the conductor ampacity for the feeder?
Do you use 430.24?
Let me se if I get things right...
Say you have a feeder supplying 2 motors and lighting load.
Motor 1 = 32 FLA
Motor 2 = 5 FLA
Lload = 5 Amp
So the breaker rating would be:
80+5+5 = 90 Amp
And the feeder conductor ampacity would be:
32*1.25+5+5 = 50 Amp
That would give me a combination of a 90 Amp breaker with a 6 AWG wire, which would not be protected by the breaker.
So...
What do you do?
Am I getting this wrong?
Does the NEC requires an overload protection for the feeder? (although I can't find anything related to this matter)
Any help would be appreciated, thanks in advance :).





RE: NEC Issue
Do you just sum all the amps and check if they not exceed the 15 or 20amp protection?
Hehehe, thanks :).
RE: NEC Issue
Normally feeder loads and panel sizes are figured at 125% of the largest motor load + the sum of the motors + 125% of the continuous lighting loads+ the sum of all other none continuous loads including none continuous lighting. If you have some large motor load as in the example then adjust the wire and breaker size to fit it.
There is a lot more NEC code rules for cable and breaker sizing which makes it complicated. So check other sections of the code. I would also recomment that you attend NEC code classes on a regular bases and call your local building department for questions like these. Post this question on the ELectrical Contruction and Maintenance magazine web site and you will get a ton of answers and I suppose more questions for clarification.
good luck
RE: NEC Issue
RE: NEC Issue
No, you are not wrong. In a motor branch circuit the conductor (and the motor) overload protection is provided by the overload protection used for the motor. Yes, 90A CB and #6 AWG conductor is acceptable in this case. That is why NEC is so explicit about those requirements.
Make sure that the equipment grounding conductor is sized per the 90A breaker (250.122).
Rafiq Bulsara
http://www.srengineersct.com
RE: NEC Issue
Rafiq Bulsara
http://www.srengineersct.com
RE: NEC Issue
Also, sizing a breaker at 80A for a 32A motor is high. Try about 50A to 60A for that motor.
RE: NEC Issue
David Baird
Sr Controls Designer
EET degree.
Journeyman Electrician.