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?
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.
(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.)