That is true, a number 10 can probably handle 40 amps as mentioned for short periods before the copper starts turning blue and baking the insulation a tad. My thought on that short spurt duration clause is .. is it really worth pushing a conductor to its molecular ability.
Last comment regarding what you want to do and what the engineer wants you to do, If its your license and your reputation, stick to your plan or let him put someone else's name on it, I don't do what others tell me to do especially if I know or think its wrong .. your call.
Lawyers claim we know more than engineers, and will try to hang us if we do what the engineer says, especially if we have an inkling that it might not be right. Been there, done that.
For the HVAC situation would need to be one with a motor/compressor load before you will be allowed to go over 30 amp breaker. The reason is to allow starting that motor or compressor, yet running current will remain below the conductor ampacity, and the motor overload will shut it down if it were to exceed motor rating anyway. This true for any size conductor for motors.
Still need to know what it is supplying, if a HVAC compressor, nameplate might even allow more than 40 amp breaker but still need at least 10 AWG conductor. MCA 29 MOCP 45 would be somewhat normal and would be allowed on 10 AWG NM cable.
334.80 Will restrict #10 romex to 60C in all situations, the insulation is not THHN, unless its marked.
And for art 430/440 applications you still could need a conductor ampacity of up to 30 amps but also have OCPD of more than 30.
Hmmm good point
Regardless of what your nameplate indicates this motor must have conductor ampacity of 38.5.
I agree.
