SIZE OF MOTOR CONDUCTOR

[espinoza569]

In class we were discussing the sizing of conductors feeding a motor. In the example, the FLC was figured out to be 23 amps using THWN wire. Then going to 310.15 (B)(16) #12 THWN is rated for 25 Amps. However, the ** refers you to 240.4(D) which states "small conductors" such as 12AWG will be rated for 20 AMPS. The instructor says based on this, you have to run 10 AWG.

I think you can still use 12AWG. According to 240.4(D) - "Unless specifically permitted in 240.4(E) or (G)....." continue to the next page in section (G) - "Over current Protection for Specific Conductor Applications"....Motors is in that "specific application". I argue that because it falls into this category, 240.4(D) does not apply which will allow you to use the full ampacity of 12AWG according to 310.15 (B)(16).

 

[Dennis Alwon]

In class we were discussing the sizing of conductors feeding a motor. In the example, the FLC was figured out to be 23 amps using THWN wire. Then going to 310.15 (B)(16) #12 THWN is rated for 25 Amps. However, the ** refers you to 240.4(D) which states "small conductors" such as 12AWG will be rated for 20 AMPS. The instructor says based on this, you have to run 10 AWG.

I think you can still use 12AWG. According to 240.4(D) - "Unless specifically permitted in 240.4(E) or (G)....." continue to the next page in section (G) - "Over current Protection for Specific Conductor Applications"....Motors is in that "specific application". I argue that because it falls into this category, 240.4(D) does not apply which will allow you to use the full ampacity of 12AWG according to 310.15 (B)(16).

Good for you... You are correct and your instructor is misinformed.

 

[Jraef]

Hmmm... not so fast. On motor circuits, article 430 trumps.

You don't size motor leads to the motor nameplate FLA, you size the motor leads based on the FLC in the appropriate article 430 TABLE in the NEC relating to your motor in accordance with the nameplate HP rating; Table 430.152 for 3 phase, table 430.148 for single phase. Neither of those tables list any HP having an FLC of 23A, so you must be using the FLA from the nameplate, which is not per code (unless under certain circumstances where a motor does not have a HP rating, or the FLA is significantly higher than the FLC).

Even then, the conductors need to be sized at 125% of the FLC value in the table (430.22). So if we assume the FLC was for a 2HP 115V motor, which was actually 24A in Table 430.148, the conductor ampacity would need to be a minimum of 30A, ergo #10 would be the correct answer.

 

[espinoza569]

23 AMPS was just an example, I couldn't find the actual issue. However I have found it. The real problem in question was the branch circuit for a one 3-hp, 240 volt, single phase. Table 430.248 gives an FLC of 17A. Multiply that by 125% and you get 21.25A. Either way, it's higher than the 20A rating in question and still covered by the 25A I discussed earlier.

 

[Ingenieur]

OCP can't exceed 20
not the ampacity
in this case 240.4(G) (?) allows higher

 

[ActionDave]

OCP can't exceed 20

Yes it can in some cases. I think you know this already so I'm not sure what you mean.

not the ampacity

???

in this case 240.4(G) (?) allows higher

Yes, it does.

 

[kwired]

23 AMPS was just an example, I couldn't find the actual issue. However I have found it. The real problem in question was the branch circuit for a one 3-hp, 240 volt, single phase. Table 430.248 gives an FLC of 17A. Multiply that by 125% and you get 21.25A. Either way, it's higher than the 20A rating in question and still covered by the 25A I discussed earlier.

The only error you may have is whether or not the 23 amps in OP is the motor full load current or not. If it is the FLC then ampacity of your conductor must be 125% of 23 which will be over 25 and require a larger conductor then 12 AWG.

If 125% of the FLC is the mentioned 23 amps - you were correct in the OP, presuming terminations are 75C and not using conductors with 60C insulation.

 

[infinity]

23 AMPS was just an example, I couldn't find the actual issue. However I have found it. The real problem in question was the branch circuit for a one 3-hp, 240 volt, single phase. Table 430.248 gives an FLC of 17A. Multiply that by 125% and you get 21.25A. Either way, it's higher than the 20A rating in question and still covered by the 25A I discussed earlier.

That is correct. Start with the value in the appropriate Article 430 table multiply by 125% and that's your minimum conductor size. From table 310.15(B)(16) #12 THHN at 75 degrees is good for 25 amps so at 21.25 you're good.

 

[Ingenieur]

Yes it can in some cases. I think you know this already so I'm not sure what you me

???

Yes, it does.

the footnote says ocp can't exceed 20 (not the ampacity)...unless an exception applies, as in the case of a motor feeder
the footnote applies to the ocp, not the ampacity