When derating the wire ampacity for a conductor you use the appropriate ampacities of the conductor from Article 310 (e.g. table 310.16 for 3 conductors in a conduit). If you're using #10 THHN, for example, you use the 90 degree C rating (40 amps), regardless of the temperature rating of the equipment , circuit breaker, etc. (usually 75 degrees). My question is, do you use the same logic for Minimum Circuit Ampacity (MCA) determination? I have an AC unit with an MCA of 36.2 amps. I say I can use 3 #10 THHN's for this; others I have discussed this with say I have to use the 75 degree rating (which would require #8 THHN).