This is a purely hypothetical case that I'm posing. Let's say you have an existing 20 amp branch circuit which you, for whatever reason, tap into somewhere with 14 gauge wire. So you downsize the fuse or circuit breaker in the panel to 15A to protect the 14 gauge wire. Here is my question. Is there anything in the code that says you have to leave a note on the inside cover of the panel to explain why a 12 gauge wire in the panel has only 15A overcurrent protection? It seems to me that common sense indicates that you should. I'm just wondering if anyone knows offhand if the code has anything to say about this.
It couldn't hurt labeling it in the panel. I don't know of any code rules requiring it though. It probably is not considered much of a concern as the ratings are close and if you look at Table 310-16 14awg wire (at 60C) has an allowable ampacity of 20 amps anyway - you just aren't supposed to use it as such.
I'm involved in a renovation , all the BX and K&T is being weeded out. But this is being done room by room. I am replacing all accessable with 12-2, but this still has some of the old circuitry connected until the renovation pushes ahead . So i have put all 12-2, 12-3 home runs on 15A. I have had to catalog this job as you have described.