Dedicated circuits

@immatthewj   In therory, there is nothing wrong with your argument.  However, the building code clearly says you can't do it.  There is a reason why a 20A receptacle will accept both a 20A plug and a 15A plug, but the reverse is not true.  My guess would be something like this.  Your 12 gauge wire can handle 20A, but not the 15A breaker.  What happens if the breaker tries to trip but fails to operate properly.  Could it start to arc at the main breaker box?  Installing a breaker that you know can't support the potential current draw is simply unwise.  I have told my story and have nothing more to say. 🤣

I think these "dedicated circuit" threads always set the standard for misinformation.  and much of the information that is correct is cited for the wrong reason.  People don't know the difference between physics and code requiremets.  Many people want to just tell others that they are wrong, no matter what the answer is.  the most common error is trying to equate watts to wire size.  If you just use ampacity of the wire, a 250W amplifier is good on 26 awg wire which is good for 2.2 amps on common ampacity charts.   Starting to see that it isn't that simple?  

So if you're a newbie trying to figure it out, this isn't the place.  Good luck.  Jerry

immathew, that was my point. Or more properly, that the outlet should be sized correctly at 20A rating, not undersized at 15A.

In your case, with 15A breakers on 12 gauge wire, you haven't done anything unsafe, but you have placed an unnecessary restriction on your circuit.

Thank you, @bigtwin and @builder3 Okay, the 20A breakers are going back in.

I believe that one of the misconceptions I had was that the 20A outlet was actually a heavier duty outlet built to safely carry more current than a 15A outlet . . . that isn’t so, is it? The difference is only in the ’T’ slot, and that difference is to protect a 15A circuit from having a 20A appliance connected to it?

Okay, so bear with me for one more question, and I assure you that this is strictly a hypothetical question. If 12-2 Romex was in between a 15A breaker and a 15A outlet, that would be legit, wouldn’t it? The 12-2  in between the breaker and the outlet would be overkill, but there would be nothing technically wrong with that because the outlet matches the breaker’s rating?

You see, that’s what I thought I was doing with the 15A breaker protecting what I felt was, in all other aspects, a 20A circuit. But I am starting to see the error in my thinking, and I do want my homeowners to cover me in event of catastrophe, so seeing as how changing breakers is easier than changing outlets, the 20A breakers are going back in tomorrow.

@immatthewj  There is nothing wrong with using the 12G wire between 15A breaker and 15A plugs.  You would use 10G or 8G if you wanted.  The wire has nothing to do with the flow of current.  That is held in check by the breaker.  The amount of "draw" on the circut is determained by the appliance (s) connected to it.  There is an ongoing argument as to any benefit to using the heavier wire.  I can't hurt.