Dedicated circuits

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.   

@immatthewj   Yes.  The 20A outlet may be a bit more sturdy, but the 15A outlet is missing the "t" slot specifically to keep 20A plugs from geing inserted.  

@immatthewj

Myth is a breaker will trip when the current passing through the breaker reaches its handle rating.

NEMA requirements says a breaker must trip in less that one hour if overloaded 135%. It also says a breaker must trip within 2 minutes if overloaded 200%.

15A breaker X 135% = 20.25A. Can take up to an hour to trip.

15A breaker X 200% = 30A. Can take up to two minutes to trip.

Both calculations above are for constant, continuous, load current.

In most cases the branch circuit wiring is 14 gauge with an ampacity rating of 15 amps for a 15 amp circuit, breaker. (The breaker amperage rating determines the size of the branch circuit. (Not the branch circuit wiring.)

(Old white paper but still holds true for a regular type breaker.)

https://goodsonengineering.com/wp-content/uploads/2017/08/CircuitBreakerMyths_web.pdf

FWIW, in most cases spec grade or better 5-15R (15A receptacles) use the same internal contacts as a 5-20R (20A receptacle). The only difference is the face plate.

For cord and plug connected appliances and equipment NEMA has requirements that manufacturers must follow. To be able to use a 15A plug the FLA of the appliance or equipment can not exceed a continuous load current of 12A. (Continuous is defined as three hours or more.) For most consumer made products manufactures make sure the FLA is 12 amps or less. Why? Because the majority of wall duplex receptacles outlets in residential dwellings are 15 amp... Even though the kitchen, laundry, dinning room, bathroom, and garage, branch circuits are 20 amp, 15 amp receptacle outlets can be used. NEC, two or more on a 20 amp branch circuit. (A duplex receptacle is two.)

If the FLA is greater than 12A continuous the plug must be a 20A. NEMA Says the FLA can not exceed 16A FLA Continuous for a 20A plug .

The face plate on the 15A outlet makes the outlet idiot proof. A 20 amp plug will not plug into it... It prevents a possible 16 amp load from being connected to 14 gauge copper wiring in the wall. A continuous 16 amp load probably would never trip a 15 amp circuit breaker.

FWIW, what you did is electrically safe,... It just doesn’t meet code.