I'm getting ready to install the Mr Cool DIY 24K Mini Split Heat Pump in my garage.


I'm having trouble determining a few things about the circuit. The outdoor unit and indoor unit both have an electrical specification sticker. The outdoor unit indicates its MCA is 15A and its Max Fuse is 25A. The indoor unit indicates its MCA is 3A and its Max Fuse is 15A. My confusion comes from the fact that the indoor unit plugs directly into the outdoor unit and only the outdoor unit receives power from the house.

First question; Does the sticker on the outdoor unit take this into account?

Some of my reading makes me think that since the sticker explicitly says Max Fuse that I must use a fuse instead of a breaker.

Second question; Is this true?

  • 3
    Remember that the circuit breaker in your home panel is present to protect the wiring to the unit, not the unit itself. The unit itself has fuses to protect its circuit boards and components. The wire size table on page 17 of the manual also adds confusion.
    – Tyson
    Apr 4, 2018 at 9:21

5 Answers 5


Use a 20A breaker and 12 AWG (or larger) wiring.

I read through the installation manual, which was surprisingly bereft of meaningful electrical information. Regardless, the MCA is the required Minimum Circuit Ampacity, meaning your circuit needs to be sized appropriately for at least that amperage. The 24kBTU unit has an MCA of 15, so you'd need a circuit capable of supplying at least 15A.

Because this is HVAC equipment, a 14 AWG circuit and 15A breaker is insufficiently sized. This is a continuous load, so you're only allowed to size to 80% of the rated ampacity, and the next larger size is 12 AWG and 20A.The Max Fuse simply states that the maximum size of the OCPD should not be larger than 25A.

HACR is likely to be listed on your breaker, but it's no longer required. Neither the NEC nor UL requires it to be labeled anymore, so don't stress over that.

You can do what others have suggested and use a larger ampacity circuit with an appropriately fused disconnect at the unit, but your circuit would be needlessly oversized.

To directly address your questions, the labels indicate electrical specifications for the unit, and if the instructions only show the indoor unit directly connected to the outdoor unit, then the outdoor unit should provide for any protection the indoor unit requires. The circuit only needs to supply enough current for the unit and not allow more current than the maximum rating.

And no, you don't need to use fuses over breakers.


The manual says 25 AMP and 10 AWG



As it happened my local building supply had an already open shutoff switch/fuse combo. Here is a pic. Note screw fuses.



The indoor unit is indeed fed from the outdoor unit. Fuses are definitely superior protection to breakers. The unit package should include a wiring cord that communicates between the units and provides power. The name plate sticker on the indoor unit is there for other reasons. It is not there to tell you how to provide power to it.

  • 1
    If you are going to use a breaker make sure it is labeled HARC. This marking indicates the circuit breaker is suitable for use with the group motor installations typically found in heating, air conditioning and refrigeration equipment. Apr 4, 2018 at 12:37
  • Could it be that the fuses in question are installed in the condensing unit? There would be a breaker in the electric panel in the hot wire(s) feeding the condensing unit. Alternatively, the cable from the panel would go to an external box on the wall near the condensing unit (which in my installation, 240-V 42 KBTU/h, contains a simple double pole switch) but could be a fuse box. Apr 4, 2018 at 13:21
  • 3
    This doesn't answer the question of "what size circuit do I need for this heat pump".
    – JPhi1618
    Apr 4, 2018 at 14:02

Use a 30A breaker to feed a fused disconnect with fuses (as your manual said 25A fuses), located next to the outdoor unit. The disconnect is required by code in the US (NEC 440.14)

  • At the risk of being another question, what do the fuses and fuse box even look like for an application like this? The only fuses I've seen are the Edison base fuses used in homes 60 years ago.
    – JPhi1618
    Apr 4, 2018 at 14:34
  • @JPhi1618 The fuses look like a little tube of chapstick. They’re a couple bucks. The disconnect is in a little enclosure, like 7x5” should be in the same aisle as fuses and breakers. Google fuses disconnect it’ll show many pictures. Or can be seen near almost any home heat pump or AC unit.
    – freshop
    Apr 4, 2018 at 15:32
  • I'm not an electrician, but all the heat pumps and AC units I've seen have a blade style external disconnect and a breaker inside. Am I violating any code or risking damage to my equipment if I use a breaker (an HARC breaker based on comments by others) instead of fuses? Apr 4, 2018 at 17:28

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