[MrForbes]

For short runs (under a foot long) 12 would be fine, but for longer runs you should consider using 10 gauge. The motor wires are 14 gauge....you can also cut them shorter and splice 10 gauge to them, if you're really concerned about voltage drop.

We usually use 12, but this year we had a bit longer runs from the PDB to the speed controllers, so we went with 10. I doubt it would make a noticeable difference on most robots, though.