Is it wrong to transition a circuit to thinner wire somewhere along its path even if the breaker is correct for the thinner wire? Specifically, if I have a circuit that currently is 20A and all #12 wire, and I tee off it in a junction box with some #14 wire and replace the breaker with a 15A one, is that ok?
Not part of the question, but to answer "why":
I have an old house, with one 15A breaker feeding lights and outlets in several bedrooms. That circuit is observed to run pretty close to 15A some of the time. I have another circuit, that I installed, a 20A breaker feeding just one outlet in one bedroom and used for just a TV and a phone charger. Never even hits 1A. I cabled it that way because I needed an outlet in that location and it happened to be very easy to do that way. Now, years later, I'm renovating an adjacent room, going back to studs and I have the opportunity to split the big upstairs circuit into two halves from a junction box there. So I figure I can keep one half on its existing 15A breaker, and connect the other half to where my new outlet is, if I replace the 20A breaker with a 15A one. Just uncomfortable with the idea of a #12 cable exiting the panel and transitioning to #14 in the walls.
