Motor wire size and temperature deration

[Twophase]

Recently went through NEC training (from the NFPA). For wire sizing, they give two options, one based on 125% of the full load, and one based on the conditions of use.

Is this correct, or are we supposed to use 125% and the temperature deration of conductors?

Example:
460V, 20HP motor, powered by a VFD. From the NEC table, this has an FLC of 27A, or 33.75A @125%
If using THHN, I would look at the 75 deg column since the VFD has 75 deg terminals. This yields #10AWG wire

Now, if I assume that this is in a 40degC environment, can I still use #10 THHN, or would I have to move to #8 THHN?

 

[Twophase]

#10 AWG THHN has an ampacity of 40A, or 35A if I use the 75 degree column. The deration factor for 40degC is 0.91, so that yields 36.4 (90deg column) and 31.85 (75 deg column)

 

[david luchini]

#10 AWG THHN has an ampacity of 40A, or 35A if I use the 75 degree column. The deration factor for 40degC is 0.91, so that yields 36.4 (90deg column) and 31.85 (75 deg column)

You could use #10 THHN in 40degC ambient. The ampacity (36.4) is larger than the minimum required ampacity (33.75).

#10 THWN in a 40degC ambient would have an ampacity of 30.8 (the correction factor is 0.88). This would not meet the minimum required.

 

[Twophase]

Thanks! I was freaking out yesterday (there are many similar examples that come to mind over the past year). The lightbulb (pun not intended) went off this morning, so that is a relief.