I know there was a recent question here on the subject. My question is a) whether there are certain specific instances when it is permissible to mix wire sizes (according to the NEC, anyhow), and b) what some of those specific instances are.
I'm talking, of course, about downgrading the "last inch" of wiring, say a short run to a device like a switch or light fixture. For example, running a short leg of 14 ga. wire on a 20 A circuit. I'm pretty sure such exemptions exist to the otherwise ironclad rule of sizing conductors according to the "ampacity" of the circuit.
If you think about it, *all* our circuits are actually wired that way, or at least connected that way. In fact, it's way worse than that: a typical 20 A outlet circuit has lots of devices plugged into it with, say #18 cord sets (maybe even smaller). (Yes, I fully understand why you don't want to have any #18 wire *inside* your walls, thank you very much.)
I will say that in practice, I never do this (run smaller wire to part of a circuit), since I'm not sure of these exemptions; even if I do learn of specific cases, I may still never do so.