in a residential circumstance I ran across a situation where the outdoor compressor unit had a min. circuit ampacity of 21.25 and a max. fuse size of 30 amps this unit is thermally protected can i use a 12 wire and fuse it at 30? I used a 10 wire but could I have used a 12 instead?
a/c wire size
- Thread starter butch
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bphgravity
Senior Member
- Location
- Florida
Re: a/c wire size
Sort of, the note to Table 310.16 that refers you to 240.4(D) does not include all those things listed in (E) through (G). A/C equipment is included in this. Therefore, the actual numbers on 310.16 can be used. THHN is good for thirty amps, but cannot be loaded to more than the terminal ratings, which in most cases is 75 degrees C. In that column, #12 is good for 25-amp. Good enough for the minimum circuit size, but to low for the OCD. #10 is the better choice.
Sort of, the note to Table 310.16 that refers you to 240.4(D) does not include all those things listed in (E) through (G). A/C equipment is included in this. Therefore, the actual numbers on 310.16 can be used. THHN is good for thirty amps, but cannot be loaded to more than the terminal ratings, which in most cases is 75 degrees C. In that column, #12 is good for 25-amp. Good enough for the minimum circuit size, but to low for the OCD. #10 is the better choice.
- Location
- Massachusetts
Re: a/c wire size
Yes you could use #12 on the 30 amp breaker.
Look at NEC 2002 440.22(B) and that will among other things direct you to 430.53 and 440.22(A)
The Short circuit and ground fault protection can be up to 225% of the load current. At 225% your 21.25 load = 69 amps so you could put a 60 amp breaker and still use #12s
The overloads in the unit protect the branch circuit conductors from overload.
[ March 11, 2003, 10:56 AM: Message edited by: iwire ]
Yes you could use #12 on the 30 amp breaker.
Look at NEC 2002 440.22(B) and that will among other things direct you to 430.53 and 440.22(A)
The Short circuit and ground fault protection can be up to 225% of the load current. At 225% your 21.25 load = 69 amps so you could put a 60 amp breaker and still use #12s
The overloads in the unit protect the branch circuit conductors from overload.
[ March 11, 2003, 10:56 AM: Message edited by: iwire ]
Re: a/c wire size
Thanks iwire & bph this question was brought up to the local ahj he agreed that I could have used a 12. At that time I was taking a class and we were going over 430. My instructor mentioned that on a certain motor with all the formulas figured in that a 14 wire could be fused at 70 amps and be perfectly legal. Thanks again its great to get professional advice.
Thanks iwire & bph this question was brought up to the local ahj he agreed that I could have used a 12. At that time I was taking a class and we were going over 430. My instructor mentioned that on a certain motor with all the formulas figured in that a 14 wire could be fused at 70 amps and be perfectly legal. Thanks again its great to get professional advice.
abode electric
Member
- Location
- New jersey
Re: a/c wire size
Butch,
I had a unit just like this with #12 nmc feeding it and a 30 amp breaker and the inspector failed it not for the breaker but the #12 he said was only good for 20 amps. I tried to fight him but I couldn't hold the job up so I changed it. I did contact the codes and standards department of the state of NJ and got a letter saying that the #12 is all the code requires. That letter was also sent to the inspector. I did it so the next guy who wires an ac that complies with the code doesn't have to go through the same thing I did with that inspector. If you want a copy of the letter it's in the old forum. If you can't find it I can mail you a copy.
Butch,
I had a unit just like this with #12 nmc feeding it and a 30 amp breaker and the inspector failed it not for the breaker but the #12 he said was only good for 20 amps. I tried to fight him but I couldn't hold the job up so I changed it. I did contact the codes and standards department of the state of NJ and got a letter saying that the #12 is all the code requires. That letter was also sent to the inspector. I did it so the next guy who wires an ac that complies with the code doesn't have to go through the same thing I did with that inspector. If you want a copy of the letter it's in the old forum. If you can't find it I can mail you a copy.
abode electric
Member
- Location
- New jersey
Re: a/c wire size
This is the letter I received from the state from the codes and standards department.
The AC states 22amp min circuit. max 30 amp ocpd. I installed a 12-2 nmc on 30amp breaker and the inspector said it did not comply.
Dear Mr Sperry:
This is in response to your letter dated January 23, 2003 concerning the selection of branch circuit conductor size feeding an air-conditioning unit. Based on the information submitted by you, I have the following comments:
The air-conditioning unit in question has a minimum circuit ampacity 22 amperes marked on the unit. This value is the required minimum ampacity of the branch circuit conductors selected from table 310-16 of the electrical sub code (NEC 1999). The table shows that a #12 awg typeTW, TWN, THWN copper conductors can safely carry 25 amperes continuously where operated in an ambient temperature not exceeding 86 degree F. Type NM cable size #12 AWG is in compliance with the electrical subcode as it can carry 25 amperes at 60 degree C. The footnotes that limit the overcurrent protection AWG sizes 14, 12, 10, do not apply in the case of motor circuits. The conductors in type NM cable are rated for 90 degree c. If no derating for ambient temperature is required, the electrical subcode does permit the installation of #12 AWG copper type NM cable feeding the motor compressor to have an overcurrent protection device rated at 30 amperes, the maximum permitted by the manufacturer of the unit.
The 30-ampere fuse or circuit breaker at the source of the circuit will protect the conductors against overcurrent due to short circuits and ground fault as stated in section 440-21 of the electrical subcode. The provisions of part C of article 440 are in addition to or amendatory of the provisions of article 240. The motor winding and circuit conductors are protected against overload relay device usually contained in the motor controller, which is set at 115% or 125% of the running current. The combination of above-mentioned two protection elements, in my opinion, provides the protection necessary for the safety of the hermetic motor compressor circuit.
Sincerely Yours,
Ashok K. Mehta
This is the letter I received from the state from the codes and standards department.
The AC states 22amp min circuit. max 30 amp ocpd. I installed a 12-2 nmc on 30amp breaker and the inspector said it did not comply.
Dear Mr Sperry:
This is in response to your letter dated January 23, 2003 concerning the selection of branch circuit conductor size feeding an air-conditioning unit. Based on the information submitted by you, I have the following comments:
The air-conditioning unit in question has a minimum circuit ampacity 22 amperes marked on the unit. This value is the required minimum ampacity of the branch circuit conductors selected from table 310-16 of the electrical sub code (NEC 1999). The table shows that a #12 awg typeTW, TWN, THWN copper conductors can safely carry 25 amperes continuously where operated in an ambient temperature not exceeding 86 degree F. Type NM cable size #12 AWG is in compliance with the electrical subcode as it can carry 25 amperes at 60 degree C. The footnotes that limit the overcurrent protection AWG sizes 14, 12, 10, do not apply in the case of motor circuits. The conductors in type NM cable are rated for 90 degree c. If no derating for ambient temperature is required, the electrical subcode does permit the installation of #12 AWG copper type NM cable feeding the motor compressor to have an overcurrent protection device rated at 30 amperes, the maximum permitted by the manufacturer of the unit.
The 30-ampere fuse or circuit breaker at the source of the circuit will protect the conductors against overcurrent due to short circuits and ground fault as stated in section 440-21 of the electrical subcode. The provisions of part C of article 440 are in addition to or amendatory of the provisions of article 240. The motor winding and circuit conductors are protected against overload relay device usually contained in the motor controller, which is set at 115% or 125% of the running current. The combination of above-mentioned two protection elements, in my opinion, provides the protection necessary for the safety of the hermetic motor compressor circuit.
Sincerely Yours,
Ashok K. Mehta
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