adding 12 gauge to 15a circuit
 

i have a 14-2 15a circuit in my basement. we are moving some things around, and consequently the florescent lighting is no longer over my work bench. the run is 14-2, with a 15a breaker on the box. i need to move the light at most 4 feet over. this is the last junction on the run as well.

i have some 12-2 from an existing project. can i use this? i plan on marking the sheathing to indicate a 15a circuit.

Yeah you would be fine, not the best work practice but your install will be correct and safe.

deleted

(not exactly on topic) In this case it would be overkill but is perfectly acceptable. While mixing of wire size within a circuit is usually discouraged, if you had a long run (like over 75 feet) and will be loading the circuit to near maximum, then it may be necessary to use thicker wire to avoid excessive voltage drop.

Unlike plumbing, it is not mandatory to run heavier wire for the entire length of a long run, but rather every additional foot of run for which the wire was heavier helps. The performance depends on exactly how much wire of each size was used and how many amperes are being drawn.

Sometimes 12 gauge wire for a long run extending an existing 14 gauge circuit is insufficient but 10 gauge wire for the extension will do the trick. That is, if it is too much trouble to run a new 12 gauge (15 amp) circuit.