NEC Motor conductor sizing 1

[brinkmann27]

I have looked at previous threads on this subject and didn't find an answer to this. Or I missed it.

In a control panel we have a 3 HP 230/3/60 motor rated at 9.6 FLA. If we use NEC for Motor Circuit Conductors Sizing(Article 430-21.) For the conductor size we have FLA * 125 % or 12 amps or # 14 AWG.

The Circuit Breaker is 20 amps.

If we go to the wire capacity table in the appendix, we see # 14 THHN copper is good for 21 amps. But at the bottom of the table you have the:

"Unless otherwise specfically permitted elsewhere in this Code, the overcurrent protection for conductor types marked with an obelisk (which it is) shall not exceed 15 amperes for No.14, 20 amperes for No. 12 and 30 amperes for No. 10 copper".... and so on.

So do we use # 12 wire or is Article 430 the "Unless otherwise specfically permitted elsewhere in this Code" ?

Which size is correct??

Thanks!!

 

[rbulsara]

In NEC (read article 430 compeltely), "motor branch circuit" conductors needs to be sized for 125% of FLA, and as long as you have a separate overload protection the motor, the branch circuit overcurrent protection can be up to 250% of FLA using a thermal magnetic circuit breaker.

So in your case you can apply that exception and use 20A CB with #14 wire. If this were a lighting circuit, for example, the max. CB size permitted for #14 condutors would be 15A.

 

[weh3]

Use #12. You're being penny wise and pound foolish. The incremental cost of #12 is very small compared to your engineering time to justify #14. Most of my jobs require at least #12 for power wiring anyway, and #14 for control.

EC readers: #12 and #14 correspond to roughly 4 and 2.5 mm2, respectively.

Regards,
William

 

[brinkmann27]

William, my question was which size was legally correct ( # 14 or # 12). We had an inspector demand we change to # 12 based on the wire capacity table. We had sized it in accordance with Article 430.(#14). We did change it to make him happy. But who was correct in the first place? In your jobs what code requires you to use # 12?

 

[weh3]

There is apparently a way to justify #14, according to rbulsara, although I was not able to put enough code sections together to see this for myself. The code clearly requires in general that #14's be protected by at most 15A. If you start pulling obscure references from the code, you will just get into a shouting match with the AHJ, and they have the trump card--that they are the AHJ.

On the jobs I was referring to, the #12 minimum was a client's particular requirement for branch circuit wiring.

William

 

[IASMike]

Article 430 says that the wire size must be at least 125% of the motor FLA. By this article being present, this makes it an exception to the note in the ampacity chart. But the AHJ has the final authority to over rule anything in the code.

Mike McCowan
Automation Specialist
Industrial Automated Systems
mvmccowan@iasengineering.com