Motor wire size and temperature deration

Twophase

Member
Location
USA
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

Member
Location
USA
#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

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
#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

Member
Location
USA
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.
 
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