Using 15A breaker for 12 guage cable

I had to re-run some new circuits in my house due to a wall removed. The cable that was cut out was 12 guage, so I ran a new 12 guage cable from the box to the light fixture it first goes to. It then feeds out to other lights and outlets. When I took apart the light fixture to connect this new cable, I noticed there was 14Guage wire that continued to the rest of the circuit. So I had to use a 15A breaker instead of 20A. So basically I have a yellow romex wire in my elec panel on a 15A breaker. Is there any code that says I can't do this? Should the cable be marked "15 A breaker only' so no one gets confused in the future and puts a 20A breaker on.
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It's perfectly OK to do, and yes, I would mark it just as you describe

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its fine I ran a 10 gauge line to my shed to minimize voltage drops but put it on a 15 amp breaker since it doesnt need 20 amps....
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Mikepier wrote:

M:
Although nothing in the code forbids you from doing this, I prefer to have #14 coming out of the panel if there is #14 anywhere on the circuit, to prevent the confusion you mentioned. If I use a larger gauge than required in running a new circuit, to avoid voltage drop or just because that's what I have on hand, I use that throughout the circuit. However, if somebody sees a 15A breaker protecting #12 wire, that should tell him to look and make sure all that circuit is #12 before uprating the breaker. Not everybody will do this. A good electrician or conscientious DIYer will, but I'm afraid not all of them are good or conscientious. You might even forget some day that the #14 is there, so it's not entirely a matter of protecting or not protecting the careless (which you just can't do no matter how hard you try). Unless the #12 was there for voltage drop (how long is it?) I might be inclined to replace it with #14 and save the piece of #12 for later use.
As it is, though, you seem legal.
G P
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I needed to extend a circuit that had a single 15a outlet on it; so I used #14 wire. Then I found out it had a 20a breaker on it, contrary to code. (it was #12 wire, but if it is a single outlet on a 20a circuit, it must be a 20a outlet) So I had to put in a smaller breaker.
Doing things right does avoid problems for people in the future.
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indelible marker, and label the wire in the box, and maybe also write a note on the service panel faceplate next to the breaker. I leave notes inside hardware all the time for the next poor SOB, or for me five years later. I'd have to get real motivated to rip'n' replace a perfectly good run just because the wire was too big.
aem sends...
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