Minimum Conductor Size for Power Cables

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drktmplr12

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Electrical Engineer
Using NEC 2011.

I noticed almost all 120V branch circuits use 2#12 AWG with another #12G. Is #12 used because the OCPD is a 20-amp breaker feeding a 20-amp receptacle? If that is the case, can I use #14 for feeding a 15A GFI so long as I use a 15A breaker in the panelboard? I don't see any reason why not..

How about a small (<1HP) motor operating at 230V, 3-ph? We always use #12 minimum, regardless of the OCPD (which could be a 15A MCP or even a 7A MCP). I can't seem to find the requirement in Article 310. Although I did find according with 430.22(G), #14 is permitted.

Any insights are appreciated.

Brandon
 
You could use #14 in both cases you describe. I usually don't. Most of my clients over the past 10 years or more have had standards that specify a minimum of #12 and a minimum of 20 amp breakers. I can't give you a reason, other than it is their design choice.
 
drktmplr12 said:
Using NEC 2011.

I noticed almost all 120V branch circuits use 2#12 AWG with another #12G. Is #12 used because the OCPD is a 20-amp breaker feeding a 20-amp receptacle?
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No, the wire size is chosen first to provide the required ampacity. Tables in 310.15(B) are used to select wire size based on wire type and temperature. Temperature corrections are applied to the ampacities 310.15(B)(1) followed by adjustment for wire fill or spacing table 310.15(B)(3)(a).

In the case of small wires, OCPD ratings are sized according to 240.4(D) based on the ampacity of the conductors they protect.

If that is the case, can I use #14 for feeding a 15A GFI so long as I use a 15A breaker in the panelboard? I don't see any reason why not..
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Unless the load is among the equipment listed in Table 240.3 or the application is one of those listed in 240.4(G). Then you have to reference those sections. Other factors are temperature, terminal ratings, and number of conductors in a raceway.
How about a small (<1HP) motor operating at 230V, 3-ph?
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A motor for what application? Duty cycle would be important here.

We always use #12 minimum, regardless of the OCPD (which could be a 15A MCP or even a 7A MCP). I can't seem to find the requirement in Article 310. Although I did find according with 430.22(G), #14 is permitted.

Any insights are appreciated.

Brandon
Click to expand...

Motor conductors are sized according to the motors locked rotor Full Load Amps (FLA).

You seem to be asking us for some kind of rule-of-thumb for conductor sizing and we can't give you one. You have to work the tables and do the maths. That's what the Engineer in your title means.
 
Thanks for your response, charlie b

starbolin said:
No, the wire size is chosen first to provide the required ampacity. Tables in 310.15(B) are used to select wire size based on wire type and temperature. Temperature corrections are applied to the ampacities 310.15(B)(1) followed by adjustment for wire fill or spacing table 310.15(B)(3)(a).

In the case of small wires, OCPD ratings are sized according to 240.4(D) based on the ampacity of the conductors they protect.


Unless the load is among the equipment listed in Table 240.3 or the application is one of those listed in 240.4(G). Then you have to reference those sections. Other factors are temperature, terminal ratings, and number of conductors in a raceway.

A motor for what application? Duty cycle would be important here.

Motor conductors are sized according to the motors locked rotor Full Load Amps (FLA).

You seem to be asking us for some kind of rule-of-thumb for conductor sizing and we can't give you one. You have to work the tables and do the maths. That's what the Engineer in your title means.
Click to expand...

My first question is perhaps reversed. I understand you first select the conductor according to ampacity, type of load and environment. Then select OCPD, then ECG based on breaker size.

Your tone is quite rude to be honest and you appear to be splitting hairs on details I'm not asking about to puff out your chest. You even glanced my profile to question my title. A fine example of how not to act in a community forum. Please read my post again and understand that I am not looking for anyone to tell me a rule of thumb, but instead looking for a specific code reference or insight to understand why a perceived rule-of-thumb exists. charlie b got it.

It seems that industry practice is to just specify #12s for transmitters, PLCs, and other similar branch circuits to simplify design as opposed to specific code requirement. Same goes for small motors single or three phase.

Thanks for your thorough response, however. It is appreciated.
 
starbolin said:
... Motor conductors are sized according to the motors locked rotor Full Load Amps (FLA). ....
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Motor conductors are sized based on 125% of the motor full load current from the tables at the end of Article 430. They are not normally sized for locked rotor current.
 
drktmplr12 said:
Thanks for your response, charlie b



My first question is perhaps reversed. I understand you first select the conductor according to ampacity, type of load and environment. Then select OCPD, then ECG based on breaker size.

Your tone is quite rude to be honest and you appear to be splitting hairs on details I'm not asking about to puff out your chest. You even glanced my profile to question my title. A fine example of how not to act in a community forum. Please read my post again and understand that I am not looking for anyone to tell me a rule of thumb, but instead looking for a specific code reference or insight to understand why a perceived rule-of-thumb exists. charlie b got it.

It seems that industry practice is to just specify #12s for transmitters, PLCs, and other similar branch circuits to simplify design as opposed to specific code requirement. Same goes for small motors single or three phase.

Thanks for your thorough response, however. It is appreciated.
Click to expand...

I apologize if my tone came off as anything but careful and cautious. It was not my intention. Deeply sorry if I caused any offense.
 
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