cgw
In my experience the 90 degree table is misunderstood. Most electricians, even EE, don't know how to derate. I've seen it time and again.
There is no 90 degree equipment made. There are only 90 degree conductors, lugs, etc. Various references to requiring 90 degree conductors will be found on all kinds of stuff, but they are still not themselves rated 90 degrees.
The 75 degree rule may be a legacy from older insulators, or it may have crossed over from underground/ exterior ratings. Remember that THHN is typically double rated as THWN and that means wet at 75 degrees.
Having seen enough I just don't use EMT as a grounding path. In the real world: just too many untightened set screws/ KO reducers.
A couple of runs of THHN green are not going to kill me. Lacking same might.
