NEC Issue

[aeffects]

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 .

 

[aeffects]

I forgot to add, for branch circuit having motors not over 1HP individually, not exceding 6 FLA, and having ligthing loads:

Do you just sum all the amps and check if they not exceed the 15 or 20amp protection?

Hehehe, thanks .

 

[advidana]

The large breaker is to allow motor starting currents. The overloads on the motor are suppose to protect the cable but I would use 90 amp rated cable to protect the cables. I would this just in case the overload fails or if some how future loads are added to the feeder. It would prevent the cables from over heating.

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

 

[advidana]

For small 15 and 20 amp circuits keep it simple, just use 125% of the sum of all the loads to size the wire and breaker.

 

[rbulsara]

aeffects:
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

 

[rbulsara]

See NEC 240.4(G) for specific reference.

Rafiq Bulsara

http://www.srengineersct.com

 

[LionelHutz]

I don' see enough info. Is this a branch circuit or a motor feeder? If you're feeding something like a MCC or a splitter than it's a branch circuit and a 90A breaker would require wire sized for a 90A circuit.

Also, sizing a breaker at 80A for a 32A motor is high. Try about 50A to 60A for that motor.

 

[dbaird]

I agree. Once the feeder conductor has been sized properly the devices become branch circuits and should be protected and sized smaller appropriately per NEC. The conductors for these loads should follow the appropriate NEC tap rules.

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.