George, that's exactly right. Derating for the cable run and derating for terminations are separate factors. Copper is a good heat sink, but it's not going to transfer much heat over 10' of length. 10' allows enough space that cabling approaching 90C in design conditions in the raceway will not overheat the terminals past 60C. You just have to make sure that the cable you installed meets all derating factors for both sections.
1) Calculate the minimum ampacity for the terminals and first 10' using the 60C table and all appropriate derating factors.
2) Calculate the minimum ampacity for the rest of the run using the 90C table (if 90C cable) and all appropriate derating factors.
3) The larger cable of the two is the one you must use.
Greg: 310.15(B)(2) Exception 3 says you can ignore derating factors for "nipples" less than 24" in length. I'd say this applies to running through foamed studs and other short pieces of raceway.
