One bedroom of the house had a short because of the insulation flaking off. I'm rewiring the 15 amp circuit that runs the lights/outlets upstairs. I ran #12 to 1 bedroom's outlets/lights and would like to tap into the old wiring from the rest of that circuit; that wiring is #14. Is there any problem having that room wired with #12 at the end of a run of old #14 wire? The alternative is to wait until I've completed the whole circuit including the main wire to the panel, all with #12. Thanks! This site has given me a lot of info

There is no problem with different wire size as long as u use a breaker for the smaller sized wire. #14 wire for 15amp breaker.

Chuck

Assuming you are planning to rewire the whole circuit, and follow Chuck's instructions about using the 15A breaker/fuse until you're finished - should be OK. If you're not going to finish rewiring, make sure it is well documented that the remainder of the circuit is undersized, so an unwary follower (subesquent homebuyer or electrician/handyman) doesn't burn the house down bu installing a 20A breaker/fuse in the future.