good to know. I never used it to derate what about continuous load factor ? could I use 90 degree then?
Continuous load factor is figured BEFORE the application of any adjustment or correction factors (see 215.2(A)(1)). Using a higher temperature rated conductor is permitted to be used for ampacity adjustment, correction or both (110.14(C)). So the continuous load factor should be figured at the ampacity based on the termination temperature rating (or the lowest device/termination rating in the circuit.)
For example, if you had a 200A load that was continuous, per 215.2(A)(1) you would need a minimum conductor size that had an ampacity (before any adjustment/correction) of 200 * 1.25 = 250A. A 90deg 4/0 or a 75deg 250mcm has sufficient ampacity, but if the terminations are rated 75 deg you would have to use the 250mcm as the minimum size.
Now lets consider if the ampacity had to be corrected for 120degF ambient (0.75 for 75deg conductors or 0.82 for 90deg conductors.) If you used a 75 deg conductor for the feeder (say THWN) then the corrected ampacity would be 255*0.75=191A. This ampacity is too small for the 200A load, so you would need to increase the conductor size
OR you could consider using a 90deg rated conductor. Using 250mcm THWN-2, the corrected ampacity would be 290*0.82=238A. This ampacity is large enough for the 200A load, so it would be acceptable.