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Discussion Starter · #1 ·
I'm helping a friend convert a garage into a studio apartment. Part of this job is installing a DIY ductless HVAC unit. Because the space is small and well insulated, we have chosen a the 12kBTU unit, which requires a 20A 120V circuit (actually "115V, 15A disconnect, 20A breaker") .

I need to wire the 20A circuit to a box on the outside wall, where the disconnect will be installed. My first question is: What size box should I install? This question probably depends on the answer to the next question: What kind of disconnect should I use? Google shows me a large variety, many of them big and ugly. I'd like something fairly small and unobtrusive to be installed on the wall next to the condenser.
 

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Discussion Starter · #3 ·

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It's an HVAC not merely an A/C? It's almost certainly a "mini-split heat pump". They heat also. You must be very careful when sizing these units, so you also size them for heating.



So you know, older heat pumps don't work if it's too cold. So they have built in electric resistance "emergency heat" that they'd fall back onto - this took stupid amounts of power. A friend has 400A electric service because his heat pump has 140A of emergency heat lol.

Guess what happened in Texas. Electric loads in winter normally aren't that bad, but then millions of heat pumps failed over into emergency heat...

Your mini-split, there, does not have that problem. However, it is less efficient in heating than cooling.

Anyway, if you just size the unit for A/C and not for heating, the poor customer will have to fall back onto built-in electric resistance heating (e.g. Cadet baseboards) ... or if you don't install those, they'll use portable Holmes style plug-in heaters. That will blow up their electric bill, and with the plug-in jobs, potentially start a fire.
 

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Yes, you only need to disconnect hot(s). Feel free to disconnect neutral also as long as they disconnect together.

On weird/exotic stuff, call a couple of electrical supply houses. IME the big-box stores charge "gotcha prices" on less popular items, because they know 90% of their customers have no other option.
 

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Discussion Starter · #6 ·
It's an HVAC not merely an A/C? It's almost certainly a "mini-split heat pump". They heat also. You must be very careful when sizing these units, so you also size them for heating.
That is correct. It is a MrCool Ductless Mini-split.

So you know, older heat pumps don't work if it's too cold. So they have built in electric resistance "emergency heat" that they'd fall back onto - this took stupid amounts of power. A friend has 400A electric service because his heat pump has 140A of emergency heat lol.

Guess what happened in Texas. Electric loads in winter normally aren't that bad, but then millions of heat pumps failed over into emergency heat...
This is will probably not be a problem here in the South San Francisco Bay Area, where the winter highs are above 50° and lows are above 40°. The roof is foam, the walls have 1-2 layers of insulation, it is adjacent to the boiler room, and one wall is shared with the main house.
 

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mini splits have come a long way. Mine when I installed them wanted fused disconnects, 30 amp. was the size. Also want 10 wire more than 40 feet from the service, which they were.
 

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A pullout disconnect is around $10 at the big box. A 30 amp unit is fine.

No need to worry about wire sizing. The manufacturer has done all the math for you. Simpl6 install wiring that meets or exceeds the minimum circuit ampacity.
 

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You do not need an in-use cover for a switch.
648110
 

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A switch outside should have the weatherproof cover.

Is the switch motor rated for the load?
 

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Do or do not? I think he does need one.
He needs a weatherproof cover but it does not need to be an in-use cover.
 
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An in-use cover for a switch is kind of a contradiction in terms anyway. It's not like you're gonna have something plugged into a switch...

I might have guessed the unit. In pre-charged-line mini-splits, there's only one product line in that space. It is marketed under 2 different names, but are the same unit. Very popular obviously, the opportunity to git-r-dun without having to get on an HVAC guy's waiting list is quite the feature.
 
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