In central texas if that matters. I have a (not yet installed) Mr Cool 4 DIY Mini split. I'm finishing my garage, and that's going to be my AC in there. But it looks like this isn't so "DIY" after all. You need to hook it to your HVAC disconnect box. OK, I have one, but it doesn't look like there is room left for another unit (since I have central air already in the home). All of the videos on youtube of people installing this strategically gloss over this part - it's insane. Nobody wants to talk about it. I feel like maybe I'm missing something.

Attached are pics of the disconnect box I have outside. Am I correct in thinking that I need a new one that can accommodate 2 units? If so, is that a tough install? I only have experience with very minor electrical work, never something like this.enter image description hereenter image description here

  • The disconnect box needs to be close to the outside unit. I'm not sure the exact distance, which can be found in the NEC. But the purpose of the disconnect it to be able to remove power, locally, from the outside unit for servicing. It also needs to be close enough so that the tech can tell if the unit is disconnected or not.
    – SteveSh
    Jul 16, 2023 at 20:06
  • If memory serves the distance is the same as the allowed length for a whip, i.e. 6 feet of conduit.
    – KMJ
    Jul 16, 2023 at 20:23
  • NEC 440.14 requires location within sight and readily accessible. No distance or length of whip is specified. Jul 16, 2023 at 20:44
  • I might be thinking of a local amendment. Regardless if you're within six feet your LFNC-B doesn't need to be secured per 356.30 so your life is easier with a short enough conduit for it to be a whip.
    – KMJ
    Jul 16, 2023 at 21:07
  • 1
    OP said it's to air condition/heat his garage.
    – SteveSh
    Jul 16, 2023 at 21:46

3 Answers 3


That thing is a heat pump. It heats and cools. Don't install a big heater until you see how it performs.

You need to run a new, dedicated circuit from a panel (e.g. the subpanel in your garage) just for this unit. That is a standard requirement of all "installed" A/C and heat pumps. And for that matter, water heater, dryer etc. etc. If that's coming as a surprise to you, yeah, everything and everyone should have mentioned that.

You need to hook it to your HVAC disconnect box.

No, it doesn't work like that. There's not like "one disconnect to rule them all". Nope, nope.

Each unit must have its own circuit back to the panel, and its own disconnect (for capacity and circuit-sharing reasons). You will need to buy a disconnect. They're cheap, and you need to make a transition anyway from Romex (which can't go outside) to an MC cable, FMC conduit or wire whip for the last hop to the vibrating appliance. So you were going to mount a junction box anyway, now you spend $4 more and get a disconnect.

Each unit's disconnect must be near the unit and in plain line-of-sight to the unit. Why? So this doesn't happen: "why is it my heat/cool not working? Ah the disconnect is pulled, I'll just reconnect it now" OW OW OW WTH MAN? "Sorry, I did not see you working on the unit, since it was 50 feet away / around the corner of the building / behind a bush / etc. etc.")

Note that somewhere there is an A/C or heat pump, there must be an outdoor receptacle within 25' of it, because that's how long the cords are on vacuum pumps. DO NOT make this a "dedicated outlet to the heat pump" - place the outlet where YOU will most likely to get useful use out of it. It simply needs to be within the 25' cord radius. The reason is this plug will only get used for HVAC service every 10 years, and outdoor GFCIs don't last that long LOL, so it'll be dead everytime the guy goes to use it. Like any 120V outdoor outlet, it needs to be GFCI but can be protected from an outlet or breaker inside.

The Codebook is currently "highly in flux" as to whether GFCIs are needed on heat pumps (which include air conditioners; a traditional A/C simply lacks the $10 reversing valve needed to let it also heat). The problem is, some companies in the A/C industry aren't ready and aren't able to manufacture A/C units that don't trip GFCIs.

  • Every 10 years? Can you tell that to my air conditioner please? :)
    – jay613
    Jul 16, 2023 at 21:34

You can't put this mini split on the same circuit as your existing split system. It will need an entire new electrical circuit, including a disconnect near the unit. There is also a requirement for a regular outlet within 25 feet of the outdoor unit for servicing purposes. This almost always requires a permit and inspection for installation.

In addition, if you're on new enough NEC there might be a requirement for a GFCI. 210.8F in the 2020 code requires a GFCI for anything outdoors except for a few exceptions. In response, so far as I know most areas that have adopted the 2020 NEC have struck that section, meaning it likely does not apply. This is mostly mentioned as a curiosity, because the most likely scenario as of right now is that a GFCI is not required.

Beyond the electrical requirements, you may need a set of mounting brackets or concrete pad to mount the exterior unit.

  • NEC does not require GFCI for air conditioning. Jul 16, 2023 at 20:25
  • 1
    There's this whole weird argument going on with NEC 2020, so I mention it for completeness. In Texas where OP is, you're spot on, no GFCI required.
    – KMJ
    Jul 16, 2023 at 21:05

Assuming the feed to that disconnect is (or can be made) beefy enough to support both continuous loads and it is located near enough to the location of the new outside unit.

Then you can install a 4 space subpanel and have each 240V breaker be the disconnect for one of the units. A breaker in a electrical panel is allowed as a disconnect and at the same time it creates a new dedicated circuit for each heatpump.

If the location is a bit too far you can still put in the subpanel in that location and relocate the existing disconnect for the new unit.

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