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Electrical Wiring in the Home/40 Amp Cicuit with #10 Gauge Wire for AC

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Question
Having trouble verifying which expert is correct. We are purchasing a brand new home and our inspector said that the condensor hooked up to a 40 amp circuit breaker with a #10 wire is not compatible. The home has passed local inspections and the electricians say they have met the requirements. Everyone refers me to the NEC 440 but i don't have a copy or the expertise. Is there a way to check what my unit needs? From what I can tell if I take the rated amps * volts it will give me the watts. I can then look up the wire size to see if has that capacity. Is this correct?

Answer
Here are the facts so you can make your own determination:

NEC 440 is dealing with A/C and Refrigeration motors. What it is saying is you must have a "minimum" wire size of #10 for a 24 amp 240 volt inductive load. (24amp x 1.25 = 30 amps and #10 can carry 30 amps)

Then it is saying you must have a "PROTECTION DEVICE" ie breaker rated high enough to handle the starting amps. Which in this case would be 40 amps (24amps x 1.75 = 42amp and you downsize to nearest and that is 40amp breaker).

So NEC 440 concludes that for a 24 amp inductive load that has a large starting amp draw (ie A/C or Refrigerator) you can have #10 wire protected by a 40 amp breaker.

Here is the rub, the NEC also states that #10 conductors need to be protected by a 30 amp breaker. That is what they are rated for as far as carrying amps.

This sounds like what your inspector is going by in his decision. And he is correct in almost all cases, but not this one.

This is a special case intended just for your A/C.

To me I would have ran #8 and called it a day. But that is not necessary in this particular case.

What you have to decide is what are your running amps (stated on the nameplate or you can use a amp meter) and then multiple that by 1.25 to figure out your wire size. Then, take that same number of amps and multiple it by 1.75 to get your protection (breaker) size.

This is why NEC 440 makes an exception in this case:
1. Your running amps are lower than your starting amps.
2. You need a breaker that can handle the starting amps.
3. You need a wire size to handle your running amps, not starting amps.

Again, this is only used in the A/C and Refrigeration circuits and not to any other circuits.

If you have a length of run over 100 feet you have to derate the wire size, meaning go to the next larger size.


I hope this helps and if you have more questions or concerns please write back to me as I am happy to help.
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Greg Hughes

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