Breaker boxes

Question

I have 12-2 wire to my garage what size box can I use

Answer

12/2 is good for a single 20amp circuit. If by "box" you mean junction box then the standard size nail on plastic will work or a regular depth 4" square metal with a raised device cover. If "box" means breaker panel your out of luck since the wore can only handle 120v 20a or 240v single phase three wire(no neutral). If you need more power in the garage you will need a new feed installed. If you want a sub panel in the garage for multiple circuits you will need a four conductor cable sized as follows:

1. 12 = 20a 240v = 4800 watts
2. 10 = 30a 240v = 7200 watts
3. 8 = 40a 240v = 9600 watts
4. 6 = 60a 240v = 14400 watts

Figure your connected load and size 25% larger as a bare minimum. Hope this helps. Good luck. Rick