20 Amp Dedicated line: 10 gauge vs 12 gauge wire

From all that I've read, it's about voltage drop over the distance of the circuit run. At shorter distances from your breaker box (perhaps under 30') go with 12 gauge, at longer distances (50' or more) definitely use 10 gauge. I've seen some charts showing the voltage drop over distance, and it can be significant, but I can't find them now.
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No problem using 10 gauge instead of 12, however if all your outlets are rated at no more than 20amps, you should use a 20amp breaker.

Neither wire will heat up noticably, because you will only draw a couple of amps no matter what the wire, outlets, and breaker are rated for.

Like Eldartford says, heat will not be a factor unless you have some very unusual system, giant Krell amps always on along with an older massive projector system, and a small stove, etc.
However, imo the 10 guage is the superior choice for the reason stated by Rushton.
Some will say the difference is not significant; but, as Keith Johnson of Spectral Audio has stated- its the sum of many small improvements that can result in a significant advantage. The 10 guage of the same quality wire will provide reduced impedance to the current flow, and that is desirable for amplifiers in particular. I would suggest the 'quality' of wire is just as, or more, important as the difference in guage. There is much higher quality wire used in robotics for example, with very significantly better shielding/grounding. I would examine that option as opposed to simply larger guage. Good luck & happy listening. Pete

Well it's been a few years, but I know my 20A lines are 10 guage wire. I think it's code here in Michigan?

Dave