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I am looking at DIY-MULTI3-27HP230C Electrical is described around page 42 https://mrcool.com/wp-content/dox_repo/mc-diy-4-mz-im-um-en-01.pdf

I've read MRCool 24k BTU breaker size, wire gauge and disconnect box and still have some questions.

The main points are: MCA 26 , MOP 40 AWG min 10, AWG preferred 8

The manual also states: The circuit, including any switches, should have a capacity 1.5 times the maximum unit current (amps).

So based on previous answer MCA26 would fit into 30AMP breaker, however 26*1.5=39 which puts me into a 40AMP requirement?

The manual also doesn't specify 8/2 vs 8/3, however on page 41(42pdf) there is a diagram for DIY-MULTI3-27HP230C, which shows L1 and L2 plus a ground, so that implies 8/2 (10/2)?

I already have a 6 foot whip which is 8/2 (+1 10).

So should I use 40amp breaker? Can I use 10/2 wire to disconnect, and 8/2 from disconnect to unit? Or stick with one size everywhere? Thank you!

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  • You appear to have attempted to edit your question. This is 100% acceptable, however, your edit shouldn't have gone to the review queue. You need to log back in using this user name, then you'll be able to edit your own question without needing any approval.
    – FreeMan
    Oct 24, 2023 at 15:35
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    All wires need to be the minimum size for the breaker. You must use 8 gauge on a 40 amp breaker, no smaller. If the disconnect is fused at 30 amps then you could use 10 gauge from the disconnect to the unit.
    – crip659
    Oct 24, 2023 at 15:35

2 Answers 2

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If you use 10/2, you are limited to a 30A breaker. So if you want to use a 40A breaker, you need to go to 8/2 (or aluminum 6/2, or aluminum 8/2 in a construction rated 75°C or higher - conduit or some types of cable, but not NM/B)

I suspect the oversizing is a hold-over for non-inverter drive units, so if it's an inverter-drive (variable speed) 30A and 10 AWG copper or 8 AWG aluminum are likely plenty. An inverter drive does not have a huge starting surge like an all-or-nothing single speed compressor.

If it is not an inverter drive, the 40A breaker and suitable wiring would be a safer bet. But it states that it is a DC inverter drive, so as with the other question, I'd pay attention to the MCA more than the MOP when installing the wiring.

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  • It is indeed weird that they recommend the larger size wire. Personally I have been less than impressed with MrCool units and don't suggest them to my friends. For saving a couple hundred bucks to have an HVAC tech come out and vacuum the system down (if you don't have the tools) you seem to end up with a much worse unit overall.
    – KMJ
    Oct 24, 2023 at 16:38
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    The problem is not in having an HVAC come in, the problem is in finding an hvac who wants to do the job. The "more than one truck" companies quadruple the price, smaller guys are willing to do install but are nowhere found for service/warranty.And the issue with non-DIY companies is that they will not talk to you if you dont have a license. The small guy doesnt want to sit on the phone and replace things part by part. You basically have no warranty if you have a non-diy product and the installer is gone.
    – David
    Oct 24, 2023 at 18:21
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That manual is confusing.

It appears to say that you could run a 40 amp circuit with 8 gauge wire, a 35 amp circuit with 8 gauge wire, or a 30 amp circuit with 10 gauge wire. You can't run 10 gauge wire anywhere on a circuit larger than 30 amps, so if you use one of the larger breakers use the larger wire.

You don't need a neutral.

Personally given that vague set of instructions I would run a 40 amp circuit with 8 gauge.

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