XHHW in a wet location has a maximum operating temperature of 75c according to the UL White Book, 310.15(B)(16) and 310.104. So, ampacity correction begins with the 75c ampacity due to the wet location. Unfortunately, the "HH" does not always mean 90c.Yep, well you should know, I guess.
I always use XHHW-2 so this never came up. Interesting.
Digging into this calculation further, situation with all the following variables:
ambient temp correction
Larger of these two:
Ampacity of XHHW from 75C column x 1.25 then pick wire size from 75C
Temp correction / ampacity rating from 90C column then pick wire size from 90C
Choose the larger wire size for calculation.
This is how I would do this. Is this correct?