[Al Skierkiewicz]

The rules read...
<R82> You must use 12 AWG or larger diameter wire for all circuits protected by a 40A Circuit Breaker.
<R83> You must use 14 AWG or larger diameter wire for all circuits protected by a 30A Circuit Breaker.
<R84> You must use 18 AWG or larger diameter wire for all circuits protected by a 20A Circuit Breaker.

However, I recommend #10 feeding Chalupas with 40 amp breakers. You can cheat this a little by running #10 to the speed controller and placing it close enough to the motor that you can then use the motor leads to attach to the controller directly.