I would say make sure your running at least 12awg romex for a 20 amp circuit breaker. 10awg romex is better, but 12awg will be okay. If you're looking for a 30/40 amp breaker, 10awg minimum for sure. Maybe ever 8 awg recommended.
By the way, I'm running a 40 Amp 220 breaker for some woodworking power tools (5 HP tablesaw). I have an extension cord that 10awg for that and it has been fine. The circuit breaker hasn't tripped on that at all.