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Our new house is still under construction by a standard builder. After the home is finished, I will be installing a small mini split ductless AC system for a home office that will get much warmer than the rest of the house.

I received permission from the construction manager to run a cable from the breaker to an outdoor wall, so that I would not have to tear up the drywall or run conduit from the breaker to wherever the compressor will go. I will not be installing the wire into the breaker, or the outdoor outlet, until after we have closed on the home. It will essentially be a dead wire sitting in the wall until I add it do the breaker and then add the outlet.

That being said, I am not sure if the compressor will be a 110v or 220v configuration (I haven't decided on a unit). What would be the best cable to run to ensure that I can either install a 110v outlet or 220v outlet using the same wire? Is this possible?

Also, what should my outdoor outlet look like? Is just an outdoor junction box sufficient or should I use something similar to a non-fuse metallic disconnect?

Thanks for the help!

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    Sounds interesting! I'm more curious why your office will be getting so much hotter. Our home office is in the hottest room of the house (upstairs, most sun exposure with a NW-facing window), we are running four 24" LCD monitors, one 36" LCD tv, two high powered full tower PC's, a 500+ Watt audio receiver pushing two large bookshelf speakers, a 100W subwoofer, plus two adult bodies; it's a small room, and we live in a desert. Even in our situation, I can't imagine needing a dedicated air conditioner for the space, even during frequent stretches of 100+ degree weather. Either way, good luck! – Mike Sep 14 '17 at 20:00
  • Depends on the house. I have a separate AC (fully internal that vents to the window) for this room because it runs several degrees above the rest of the house. – Loren Pechtel Sep 15 '17 at 1:20
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    Practically speaking, the house is being built now, why not just ask the electrician and have them pick & run the wire & outlet and everything, all safe & to code & done? They're already working in the house, the extra cost for one more outlet should be minimal – Xen2050 Sep 15 '17 at 8:53
  • Our office also got very hot, because it has 4hrs of solid sun in the summer. I put up an awning and it solved the problem completely, thoroughly recommend it if the problem is just catching too much sun. – Jim W Sep 16 '17 at 0:33
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If you truly want to future proof, install conduit instead of cable. This will allow you to pull whatever wire you need, once you've decided what you want.

3/4" conduit should work fine, and would give you the option to install up to four 8 AWG condcutors (Schedule 80 PVC). If you think you'd ever upgrade to a whole house unit, this might be a good option. If you only ever plan to have a single mini-split unit, you'll probably be alright installing 1/2" conduit. Though upgrading to 3/4", would make pulling easier.

  • Could a person run three #8 plus ground in 1/2", should that be necessary? – isherwood Sep 14 '17 at 14:37
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    @isherwood You're right, but I'm not sure why you would run three conductors plus ground. However, even if Schedule 80 was used, you can only fit two 8 AWG conductors. I'll update my answer. I think in most residential situations, you'd only need two 10 AWG plus ground, but who knows. OP mentions only cooling a single room with a mini-split, so probably only looking at two 12-10 AWG plus ground. – Tester101 Sep 14 '17 at 14:44
  • @isherwood sure but he will need a subpanel on the far end, unless he has one device that will need 40A and also split-phase. – Harper Sep 15 '17 at 15:49
  • @Harper -- since we're dealing with hard-wired things (and all presumably in the same place), one could apply the branch circuit tap conductor stuff from 210.19(A)(4) Exception 1, point (c), perhaps? – ThreePhaseEel Sep 15 '17 at 21:16
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Installing a n-3r (raintite) junction box would be fine. I would install a three wire NM with ground. You didn't mention what size your unit would be a 10/3 would handle everything up to a 5 ton. Just make sure when you install the unit you check the MOCP (maximum overcurrent protection) and use the right breaker. Make sure you wirenut off the conductors in the junction box.

Good Luck

  • NM feeding an outside location? It is not rated for damp or wet locations. Perhaps you meant UF? – statueuphemism Sep 14 '17 at 14:49
  • Whatever gauge it is, use three wire so it can be a MWBC. +1 – Mazura Sep 14 '17 at 19:54
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Voltage is not an issue since any wire you're allowed to use is good for 300V.

Generally most whole-house air conditioners (for modest size houses) are served with a 30A circuit using 10AWG wire in 10/2 cable. This can be configured at 15, 20 or 30A, at 120V or 240V.

There is nothing wrong with wire that is too big. You can put a 20A receptacle on a 20A breaker on #6 wire. You'd have fitment problems getting the #6 to fit the terminals, but that is a trivial problem easily solved.

It's unlikely that you would need both 240V and neutral to run an air conditioner. But if you did, running /3 cable would cover that.

If you expect to have multiple pieces of equipment out there, a subpanel there may be necessary, and you might think about 8/3 or 6/3 cable. 6/3 is good to 60A, so it could power a whole cottage.

If you think you would need two circuits for equipment and wouldn't want a subpanel, then you could run two cables out there. However Cables cannot be paralleled so two 10/3's cannot substitute for a 6/3 for instance.

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Unless you're planning on having a lot of computing hardware or live somewhere really hot a 120V unit is almost certainly going to be sufficient. I'm cooling a 600 sqft apartment and ~1000W (actual 24/7 load, not PSU max output) of electronics with a 12000 BTU air conditioner on a standard 15A 120V plug.

I suspect I am running up against the upper limit of what it can do, but it lets me maintain an 75F internal temperature when the outside temp's as high as the mid 80s. It hasn't gotten that hot recently, but it was able to handle the ~700W I was running a few summers ago with a 90F outside temp.

  • 240v motors use the electricity more efficiently though, so that's definitely the way to go if they're not too much more expensive. – Perkins Sep 15 '17 at 22:07
  • @Perkins IN the US 240v models aren't readily available except in really large sizes (generally above what you can run on a 120V/15A outlet). Running a massively oversized unit is energy inefficient; so unless you're running a huge amount of hardware they lose on that account. – Dan Neely Sep 16 '17 at 13:55
  • @DanNeely -- not only is running an oversize unit energy inefficient, it can leave your house muggy and uncomfortable as a short-cycling AC won't dehumidify the air – ThreePhaseEel Sep 16 '17 at 14:50
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Why is everyone missing his question?

Wire is nice in this regard in that going over spec simply costs a bit of money, there's no need to actually match the wire to the task. (Mobile uses are another matter, thicker wire adds weight.)

110V vs 220V? Simply go with 220V--not that I think you actually have a choice here, I don't think they sell power wire not suitable for 220V.

Thickness? Look at the units you're considering, how much power do they draw? (Note that 110V units might draw more amps than 220V!) Pick the highest value you get, then round it up to the next breaker size. There are tables that tell you what wire thickness you need with what breaker amperage. End of problem.

(Note that the wire must be matched to the breaker, not to the air conditioner. No fixed wiring can be thinner than the rating of the breaker that guards it. It doesn't matter if you know that alarm circuit will never draw more than 1 amp, 20 amp breaker means #12 wire.)

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    Conduit is far more versatile long-run than whatever cable you whack in there... – ThreePhaseEel Sep 15 '17 at 1:40
  • +1 conduit. No one is ignoring or missing his question. He should use 4AWG mains wire... there covered, that's silly, Ok, use 14/2, we thats not going to cut it for the 240v (220)... then if he gets 10/3 12/3 he's paying more upfront and might not need that AWG. So people are asking relevant questions to recommend a reasonable size best bang for his buck. Thus the conduit conversation. Doesn't hurt, gives flexibility, doesn't even have to run it the length. – noybman Sep 15 '17 at 2:43
  • @noybman 14/2 works fine for 240. If you ever see me answer one of those "I want to run a post light at the end of my driveway 1000' away, do I really need 4/0 cable?" You'll see me put 240 on #14. – Harper Sep 15 '17 at 15:55
  • @Harper, Fair enough, I don't disagree it's allowable, I was just trying to emphasize that the questions everyone is asking all along and suggestions being made are attempts at helping op. Not that everyone was ignoring the question as this post suggested. – noybman Sep 15 '17 at 19:25
  • Go with what you'd need for 110 actually. The lower the voltage the higher the amperage to transfer the same amounts of watts, and amps are what drive cable size. That's why the 12v starter motor in your car has thumb-sized cables running to it while the same power of motor at 110v would run off of a regular 20A circuit (depending on how big your engine is anyway.) – Perkins Sep 15 '17 at 22:05

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