Derating THHN for a motor - understanding 75 degC vs 90 degC

[Twophase]

I have a 20HP motor that will be used in a 575V, 3 phase system. The FLA per 430.250 (2017 edition) is 22 amps, which yields 27.5A when applying the 25% factor required.

This will be operated at a 40 deg. C ambient environment. I am sizing the wire for this motor. I have been burned (figuratively) by wire sizing in the past (see post history, ripping out conduit is not fun), so I am trying to make sure this is done right.

From what I have learned, motors have 75 deg terminals (NEMA design). This being the case, do I start with the 75 degree column of 310.15(B)(16) and derate from there? If so, then I get #8 AWG, but this doesn't make sense to me since I am using THHN (I mean, that must have some relevance right?)

Do I look at the 75 deg column, and compare the values to 27.5A?

If I do the this, I see that 27.5A would correspond to a #10 AWG conductor. I then take #10 THHN, which has an ampacity of 40A, and then adjust it for 40 deg C using the formula in 310.15(B)(2) --> (0.912871 * 40) = 36.52, which is greater than 27.5A, so this would be acceptable.

Is this the correct process for sizing wires when terminal ratings do not match the wire rating?

 

[infinity]

I then take #10 THHN, which has an ampacity of 40A, and then adjust it for 40 deg C using the formula in 310.15(B)(2) --> (0.912871 * 40) = 36.52, which is greater than 27.5A, so this would be acceptable.

Is this the correct process for sizing wires when terminal ratings do not match the wire rating?

Yes, since you're using a 90° C conductor (THHN) you can use the 90° C ampacity as the starting point for your calcuation.

 

[petersonra]

You can start with the 90 deg C column and derate from there.

 

[Twophase]

Yes, since you're using a 90° C conductor (THHN) you can use the 90° C ampacity as the starting point for your calcuation.

So where does the 75 deg C limitation come in here? In a previous project, I used 90 deg C conductors, but was told that I can't use those ampacities due to the 75 deg C terminals, so I effectively ended up using the 75 deg C column for size (but we installed THHN)

 

[AC\DC]

Your limited to your weakest link. If your terminals are rated for 75 deg then you can’t go past that for how many amps that wire is used for in the 75 degree column, regardless if your wire actually falls in the 90 degrees column. 90 degrees can be used for ampacity correction if your wire falls in that column, like temperate correction

 

[augie47]

Just so you don't get "burned" again. Keep in mind ALL motors don't have a 75° termination rating. Take a look at 110.14