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Thread: Wiring 220v Circuit for Central Air Conditioner

  1. #1

    Default Wiring 220v Circuit for Central Air Conditioner

    I just purchased a 3 1/2 ton Central Air Condition unit. I was told that I need to run an 8 gage 40 amp circuit. I will have a Electrician come over and do the final hook up, what I wanted to do is mount the disconnect box and run the wire. That way I can take my time drilling the holes for the box, and line set ensuring it comes out nice and neat. This is a brand new house and I don't trust anyone anymore.

    My question is... what type of wire should I buy, the run will be about 30 - 40 feet. Should it be 8-AWG 2 wire plus a ground our 3 wire plus a ground.

    The disconnect box has 4 connections along top ( 2-line and 2 load) and right below a bus with 2 screws.

    Like I said I just want to run the wire, mount the disconnect box, pull the wire into the disconnect box, and leave several feet at the circuit breaker box.

    I did open the panel tonight and saw what I expected. For example the 50 amp range has 3 wire and a ground. The black and red are connected to the breaker, the white and copper are connected to the bus. Is that the way this 40amp circuit should be hooked up, if so which wires would be connected to what in the disconnect box.

    Sorry for the long and unclear question.

    Thanks in advance.


  2. #2
    Join Date
    Sep 2002


    In all likelyhood it will be 2 wire plus ground.

    Don't know who told you 8AWG and 40A or where they got their info but look at the nameplate on the unit. It will tell you 2 things:

    Minimum Circuit Ampacity - This is used to size the conductors

    Maximum Fuse or Circuit Breaker (OCPD)- This is the largest overcurrent protective device allowed. If it only says fuse, then fuses must be used (not likely)

    Make sure there is 3' of unobstructed clearance in front of the disconnect.

  3. #3

    Default More information

    I did check the compressor unit of the A/C- the Minimum Circuit Ampacity is 25 and the Maximum Circuit breaker is 40.

    With this information what wire gage should I use and what amp circuit breaker should I use.

    Thanks for your help!


  4. #4
    Join Date
    Sep 2002


    A #12AWG would be permissible but the cost difference of #10 at that length would be negligible so I would recommend 10/2w/ground NM and the 40A breaker.

  5. #5


    Spakz, 440.32 and 44.33 of hte NEC requires the branch circuit conductor ampacity to be increased 25% more than the branch circuit ampacity rating. 12 awg is only good for 25 amps. 10 awg copper is only good for 30 amps. If you calculate the minimum circuit ampacity rating on the name plate by multiplying by 125% then the ampacity required woud be 31 amps calling for a minimum of an 8 awg copper branch circuit conductor minium size. Am I missing something?



  6. #6

    Default 8 Gage wire question

    I went to my local Home Depot and the 8 Gage wire they had was stranded and NOT solid. Is this ok? Is stranded better?

    They only have 8- 3 with a ground, so it looks like I will get that and if my electrician doesn't use the 3rd wire he can just cut it off.

    Sound ok???

    Thanks once again.


  7. #7


    YOu will find under normal circumstances any wire larger than 10 awg wire will be stranded.

    YOu may run 8/3wGrnd if you like, that being a red, black, white, and bare. However the white wire will not be used because a a/c unit only uses 220 volts and no neutral is required.

    8/2wGrnd is fine being a black white and bare with the white reidentified as a hot by a ring of black tape anywhere the white wire is able to be seen. This tells anyone the white wire is now being used as a hot wire. This is also normal.

    Hope this helps


  8. #8
    Join Date
    Sep 2002



    The nameplate Minimum Circuit Ampacity already has the 125% factored in by the manufacturer.

  9. #9


    Spakz, you are correct. See the copied section of the NEC handbook commentary found below to help support your statement.



    Branch circuits for listed air-conditioning and refrigeration equipment that have a nameplate marked with the branch-circuit conductor size and branch-circuit short-circuit protective device size are not required to have the branch-circuit conductors sized in accordance with 440.33. The testing laboratory standard includes the 25 percent increase for the largest motor or compressor in the group plus the other nonmotor or noncompressor load; therefore, the actual nameplate full-load amperes for the complete assembly can be used to size the branch-circuit conductors.

    Good Catch


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