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Have a small one bedroom cottage. I need to install a subpanel in it. I have a two hundred amp panel 30 feet away. The only things I will power in the cottage are two can lights, six outlets, small window unit and a small refrigerator. What gauge wire should I run to the subpanel? Also what size breaker as the main should I use in the sub?

  • Quick math: Lights not enough to matter, AC 4 amp, outlets 20 amp, minifridge 2 amp. – user3757614 Jul 20 at 19:49
  • ..I mean outlets 9 amp. That could be fit into one 20 amp circuit, but two 15 amp circuits might be better for futureproofing. – user3757614 Jul 20 at 20:00
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    Possibly more info needed , is there a cooking area? A bathroom or a laundry area, also your state / location may make a difference along with the square footage. – Ed Beal Jul 20 at 21:08
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    Is there cooking and/or laundry facilities in the unit? Also, how just how many square feet is it, how many BTU is the window unit, and how is this cottage heated? – ThreePhaseEel Jul 20 at 21:27
  • IMHO (not an electrician) 30A would be enough, but as a living area som jurisdictions may require more, like 60A or even 100A (kinda silly for a cottage). – Skaperen Jul 21 at 2:46
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If I were you, I'd go as follows (assuming US split 240v)

  • Run 8 gauge THHN from your main panel to your subpanel (in buried conduit) and use a 50A breaker in the main. Run 4 wires so you get 240v and ground. Should be more than sufficient for anything you can throw at it and you can always expand if needed. Be sure to ground your panel to a grounding rod while you're at it.
  • Run your lights on a single 15A breaker. Makes adding lights easier, it's cheaper (14 gauge wire and 15A switches, and don't forget to add neutrals to the switch boxes!) and you won't be in the dark if you blow a breaker in the subpanel
  • Run a 15A breaker to a single dedicated outlet for your AC. It's overkill, but I'm assuming you have a 5k-6k BTU window unit. That's 6-7 amps, which is a lot to share with other outlets, even on 20A (especially with 2-3A for a fridge). If you ever decide to upsize, you have no worries about rewiring it either.
  • Run the rest of your outlets on one 20A circuit (12 gauge wire). Don't skimp and run them on 15A because you have a lot of 14 gauge wire. You'll regret it the first time you pop that 15A breaker.
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    you can use 5-15 (these are the ones with just 2 vertical slots) style outlets on a 20A circuit. they are the same metal structure inside as the 5-20 ones. the code allows this case as long as there are at least 2 such outlets. – Skaperen Jul 21 at 3:13
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(Not an electrician; if one shows up, listen to them instead. Also, I'm assuming America, or at least a 120V country.)

As I noted in my comment, one 20 amp circuit could handle the load you specified. However, there's always something you've forgotten, so I'd recommend going bigger. (Consider the load of someone running a microwave (10 amp), a hotplate (10 amps), a hairdryer (10 amps), and other stuff (10 amps). You can easily hit 30 amps of usage just from kitchen equipment.) The difference in cost isn't that big, so I don't see a good reason to go below 40 amp, and you should go higher if you expect even a remote chance of people cooking in the cottage.

As for the wire sizes, a 40 amp breaker needs at least 8 gauge wire. (And remember to get outdoors-rated wire and protect it properly.)

https://www.totalhomesupply.com/wires-and-circuit-breakers

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    There are several issues here -- chiefly, NEC Art. 220 comes into play, and you haven't at all accounted for it – ThreePhaseEel Jul 20 at 21:47
  • America is really a 240V country (don't tell our electricians). we just put the ground in the middle. it's more balanced that way. my computer seems to like it. – Skaperen Jul 21 at 2:50

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