66wv,
Well, for #14 through #10, see the asterisk at the bottom of table 310.16.

But, to answer your question, derate the CONDUCTOR.
In other words, if you have a 60 amp load with 2 THHN conductors no derating is required. Use the 75c column(per 110-14(C)(1)) which shows a #6 as good for 65 amps.
If, however you had 4 current carrying conductors and needed to derate to 80% as required by table 310.15(B)(2)(a), you may use the 90c column which shows 75 amps for THHN. 75 x .8 =60 amps. You are still good with #6. If, however you used the 75c column to derate, you would have needed to go to a larger conductor because .8 x 65 =52 amps.
I hope this helps (and I hope I'm correct) [Linked Image]