No prohibition on using the #12 as you describe.

bus_driver, I was under the impression one must maintain consistent wire gauge throughout a given NM branch such that a over-current event would always occur at an accessible junction point or at the breaker (not along a length of wire). Granted going up a step as johns08 describes would be very unlikely to fall into this category, but is it the "right" way to do it?

Dan

Why would a #12 conductor, which is permitted to be protected from overcurrent at 20 amperes for ordinary residential circuits, pose an additional hazard when used on circuits protected at 15 amperes?

"I was under the impression one must maintain consistent wire gauge throughout a given NM branch such that a over-current event would always occur at an accessible junction point or at the breaker (not along a length of wire)."

What?

Any current flows in the entire circuit.

Using a larger conductor is acceptable, though it can lead to confusion later on if someone tries to increase the circuit ampacity without checking every section of the circuit.

The OP said nothing about changing conductors outside a JB.

My point is that IF you have an over-current condition, the spot with the most resistance would heat faster (i.e. why you don't put in a section of 14g wire in a 20A circuit). The comment was hypothetical and I wasn't suggesting the OP's method was in any way unsafe, but that it might, as you say, "lead to confusion later".

Dan

"My point is that IF you have an over-current condition, the spot with the most resistance would heat faster"

The only heat that matters is the breaker or fuse.

That is the device that is going to open the circuit.