Min.wire size for 40Amp breaker with MCA 21.6

Are you asking what size wire to use to a 40 amp breaker serving a piece of equipment with a min. circuit ampacity of 21.6 amps? If so, I have questions before I can answer.
1. If you only need 21.6 amps, why are you using a 40 amp breaker? A 30 should be more then sufficient.
2. How long & what kind of wire (copper or aluminum)?