Off the top of my head, I think that 10 gauge wire carrying 40 amps is about the limit. If you want to be really safe and have a few amps to spare, you can run service cable instead of 10 gauge. A service cable capable of carrying 60 amps isn't too awfully much more expensive than 10 gauge, is better insulated, and leaves you room for expansion in the future (when you start plugging in your electric car to charge the batteries). Plus it will give you the ability to run 240 volts safely if need be, in case you want to use an arc welder or something else heavy duty. It's a little more expensive, but not a lot (probably less than $20 more, if that much {this is off the top of my head and I'm not consulting texts or price lists, which is something that you should do}).
