Dedicated circuits

@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.

jea48, thanks for all you do here on these threads, it’s appreciated.

As an aside, we had a job 10 years ago with in-floor heat, continuous cable laid under stone tile. The largest zone was apparently a bit too big, on a 20A breaker, and pulling a consistent 17-18 amps. The breaker would hold it for hours, but then finally trip.

Thank you, @jea48 .

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

I am putting the 20A breakers back in tomorrow.  In the event of a catastrophe, even an unrelated catastrophe, I'd rather meet code and not be trying to argue with the insurance code about it.

@bigtwin 

Wow!! That's a 30 amp plug alright.   I'm confused by your comment. 

I've never seen an audio amplifier with a 30 amp plug on it before. 

. . . you see, my misunderstanding was that I was considering the outlet part of the Romex. In other words, since is legit to run heavier Romex in a circuit than what the breaker is rated for, I was thinking that the same would apply to the outlet. In other words, it would NOT be legit to have a 20A breaker for a circuit ran with 14 gauge Romex, but it would be legit to have 12-2 Romex with a 15A breaker, and therefore, I was thinking that as long as the other components of the circuit (Romex and outlets) were rated for more than the breaker, that would be safe and legit.

Thank you for clarifying for me what was wrong with that logic--safe, but not legit.