I am looking to install a: MrCool E Star DIY 4th Gen 24k BTU Ductless Mini-Split Heat Pump Complete System 208-230V/60Hz

I reached out to MRCool support and they confirmed what I found through google and forums: that it requires a 30amp breaker with 10-2 wire.

I’m trying to understand their manual. Under a table titled: Minimum wire gauge for power cables, It has listed for the 24k unit: MCA 22 amps, MOP 35 amps. AWG: 8 min 6 Pref.

When I asked the customer service person they said: the metric system that the manual was pulled from makes it a 8 or 6 gauge wire, which you can run from the breaker to the disconnect. However, from the disconnect to the condenser 10-2 will be needed because the screw will not hold the 6 or 8 gauge on those units.

I already have a 240, 30amp breaker and 10-2 wire run in my garage, My understanding is I can put a sub panel to power my air compressor and then run outside to a disconnect for the mini split.

So my question is will a 30amp disconnect outside work for the mini split or do I put a 35amp fuse somehow in the disconnect? The product brochure says: maximum fuse (outside) 35 amps and again lists the 22 MCA. If I put a 35 amp fuse outside do I then need to have 8 gauge wire and a 40 amp breaker?

Thank you

1 Answer 1


MCA of 22A means it will work on a 25A breaker. Or a 30A breaker. MOP means "not bigger than" 35A breaker (or fuse.) You don't NEED anything more than 25A as the first available size bigger than or equal to 22A.

The Mr. Cool install manual wire sizes are, frankly, ludicrous. Doubly ludicrous for an inverter-drive unit (no huge starting surge.) 10AWG copper should be plenty.

Technically, you are supposed to follow manufacturer instructions. In this case, I think you have a very reasonable argument that you are following the manufacturer wire size instructions you got by telephone or internet rather than the ones that appear to be a mis-print or mis-translation in the printed documents. 10AWG copper is the correct size for 25 or 30A circuits under NEC.

The disconnect does not need to be fused at all. It can be any size equal to or greater than the circuit breaker feeding it. (I have 60A non-fused disconnects on 15A heat pump circuits - because they were the least costly ones in stock; and I see no particular value to having fuses in series with a breaker unless the breaker is itself a faulty fire hazard that should be replaced ala Federal Pacific / Zinsco...)

  • Are there any issues putting the max 35Amp Breaker with the Min. recommended wire 8awg Cu? I'm installing the same Mini Split and already bought the more expense 8/2 romex wire (Cu), and prefer to give the circuit as much capacity as it is able to handle for those extreme weather days with prolonged usage. Just to make sure, unlike what could be implied from the their ludicrous installation manual since 8awg (Cu) is ideal for a 40amp breaker, a 40amp breaker is overrated for this 24K btu 240v Mini Split, given the nameplate specification of "35amps" in the Max Fuse Amps?
    – user289394
    May 29, 2023 at 22:52
  • 1
    8 AWG copper at 60°C rating is good for 40A, so it's good for 35A. A 40A breaker is indeed more than the manufacturer (and thus UL and thus NEC) allows, unless it's feeding a 35A fused disconnect, and even that might get failed given that 35A dual-pole breakers are available.. Frankly, 30A is already 136% of the required power, so I think you're chasing something that really won't change a thing, operationally, by supplying 159% but you are allowed to if it makes you happy.
    – Ecnerwal
    May 30, 2023 at 1:00

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