Dedicated circuits

@immatthewj A 15A receptacle on a 20A breaker is not an issue as any 15A appliance will never exceed the wiring in the circut. The reverse of a 20A outlet on a 15A circut is where the potential of fire comes into play. We assume the breaker will trip and protect us from harm, but what it it doesn’t? That’s why we don’t push our luck. Cheers.

@bigtwin , thank you for bearing with me on this; I know that I have a tendency to be kind of dense on this stuff. What I have is essentially three circuits that consist of 20A outlets connected to the box by 12-2 Romex and I put 15A breakers in those slots as I felt the 15A breaker was actually added protection. I can easily rectify this by putting 20A breakers in it. I just felt that as long as the circuit breaker’s amperage capacity did not exceed the capacity of the rest of the circuit, I was good. I guess I felt that by taking the breaker down to 15A I was being extra good.

You can plug numerous items into a 15A receptacle that exceed 15 amps, portable heaters come to mind. My Skilsaw. The outlet is undersized for the circuit, and the potential load(s).

@builder3 wouldn’t one be better off if the outlet was oversized for the circuit and the potential load(s)?

@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.