I need help with the wiring of an AC unit. 4 different Electrical contractors / electricians are suggesting 4 different Breaker, Disconnect and wire sizes.

Name Plate info: Min. 16.7 circuit amps Max fuse 25 amps Max. circuit Breaker 25 amps Short circuit current: 5kA rms. symmetrical, 230V

  • Welcome Tim. You might want to take the tour so you'll know how to best participate here. Apr 10, 2020 at 15:13
  • Are you in North America down as far as Colombia, or US territories, or the Philippines, or Japan, or somewhere other than any of those? Apr 10, 2020 at 23:33

3 Answers 3


Assuming you're in the US, the minimum you would need based on the nameplate info would be 12/2+ground (12 gauge wire with 2 conductors and a ground wire). You could go with larger wire (10 gauge), but not smaller (14 gauge).

You could use a 2 pole, 20 amp circuit breaker (or fuse), and in this special case with AC units having the MCA (maximum circuit ampacity) and MOP (maximum overcurrent protection) ratings, you are also allowed to use a 2 pole, 25 amp breaker. That breaker would normally be too large for 12 gauge wire, it's only allowed in a case like this where the A/C unit is rated as such. If you go smaller then 20 amp, you will experience nuisance tripping, and you are not allowed to go larger than 25 amps.

If you have the circuit breaker at the panel, you could use a non-fused disconnect. Most of them are rated at 30 or 60 amps, which is more than what you need, and would be fine.

Not sure why different electricians are wanting to go with different things, maybe they're trying to future-proof, or they're just going to use what they have in stock. You should ask the contractors about that.

  • A suggestion: A 2P, 20A breaker will also work for this AC, and might be easier to source Apr 10, 2020 at 16:57
  • True, I will add that, Thank you! Apr 10, 2020 at 17:15

Edit: I see you have called a professional. My apologies for misreading your question.

It is not unheard of to have different contractors tell you different size wires- some will go for the minimum, some for future proofing. The cost difference between 10 gauge and 12 gauge is minimal, especially for short runs (other than being more difficult to bend). Different contractors may also know what the inspectors like to see- so if they're expecting to see a 10 gauge wire (30A), and the contractor installs 12 gauge (20A), they'll get burned.

With that said, absolute minimum is a 12gaug or 12/2 (so Hot-Hot Ground) However that assumes everything is 220V, and if the system has 110V anywhere then you might need a neutral. I don't know this particular unit, but a photo of the wiring diagram in the manual will help.

The run from my AC is just under 10'. It should be protected in conduit outside the house, run to a disconnect box (fused at 30A), and from there via flex into the compressor.

The cost of the wiring is negligible. The cost difference of the breakers should be 10$, maybe 20$ with markup. The labor is the expensive part.

Assuming they are running a new lineset these costs should be the same. The manufacturer says the size of the copper lines, and copper is copper. If they say 20' of 3/8 you can look up the base price of that- and everyone should be scaled by the same amount (Do NOT think you're going to get copper lineset at store prices, not going to happen).

Overall a bit of future proofing should be useful, but without more details- distance, quotes, environment (do you need a new pad, etc), and what after-the-install service is, prices can vary.

There are websites you can go to to get an estimate as to what things cost in your community. They're distributions, obviously, but it may help.

You probably do not want the cheapest bid, especially if it's by a margin. Calculate the median, average, and std. deviation of the prices. Always a good metric...

You should call a professional.

No, really.

4 individual contactors? Typically there's a compressor and fan, and then an 24v AC that turns them on.

You need the manual which will tell you the minimum sizes, wires, and whatnot, and you'll also need to know local code for what they need to be run to and any external or internal disconnects you have to provide for service.

Do this wrong and you'll be lucky to end up in the hospital.

  • 1
    If you read the question carefully, he has already called 4 different professionals, and they are giving him different information (it says contRactors, not contactors). He's just asking what the correct setup would be so he can choose from the bids he's received. Apr 10, 2020 at 16:51
  • Mate, I totally missed that. I'm sorry. No excuse for the lax reading- I had conTACtors on the brain from another discussion. I'll update.
    – J.Hirsch
    Apr 10, 2020 at 22:14

Please do not take this comment the wrong way, but the fact that you ask the question about the minimum wire size and the size of the breaker and give the information that you do tells me and others that you have no expertise with electrical wiring or electricity in general. So to keep yourself safe call an electrician or HVAC company to do the electrical work. The wire size would be 12/2WG minimum (I would use #10 wire) and a 20 amp breaker. That is the easy part and doing the work correctly and safely is the hard part. So as @J.Hirsch wrote do it right and safely. my 2 cents

  • I agree d.george. As much as I wanted to help, the OP sounded like he thought it needed 4 circuits! Complete misunderstanding of the basics. Wish we could have helped but in this case, he needs a pro. Apr 10, 2020 at 16:39
  • 3
    If you read the question carefully, he has already called 4 different professionals, and they are giving him different information (it says contRactors, not contactors). He's just asking what the correct setup would be so he can choose from the bids he's received. Apr 10, 2020 at 16:51
  • Actually, the quoted information seems consistent with an air conditioning unit. Apr 10, 2020 at 23:32

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