The way I see it: an appliance with a 5-20p (rare as they are) on it can draw 16A continuous or right at 20A non-continuous. Both of those numbers are higher than the allowable maximum ampacity of 14ga wire and/or a 15A breaker. Now, why there is the exception for a simplex 20A to be on a 15A individual branch circuit, I dunno, the same parameters are still in play, tho there is no chance of any other load on that circuit. I suppose the CMP decided that 16A continuous on a 15A breaker/#14 is ok in that instance, much the same as using #14 on a 20A breaker to a motor load is one of the few times you can exceed 15A on #14. Welders are the only other thing that comes to mind.
In a nutshell, there are only a few exceptions to installing receptacles on circuits where the receptacle rating exceeds the OCPD's ampacity. 20A duplexes on a 15A circuit is not one of those exceptions. and tho JNOE's install is a code violation, in reality a HO will never ever find a 20A plug in appliance to use, and tossing spec/hospital grade receptacles for cheap ones is arguable a less safe install, but that's neither here nor there....
These 'exceptional' installs are largely academic anyway because nearly every appliance that uses 15A/12A continuous will have mfg instructions to use a 20A circuit/#12 wire.
Electricians do it until it Hertz!
So I put a 20 amp receptacle on a 15 amp circuit, there is still 15 amp overcurrent protection, where is the safety hazard? If supplying life safety equipment, maybe we have something to be concerned with, but for general use equipment the 15 amp overcurrent does it's job of limiting current on that circuit at the 15 amp level even if there were a 50 amp receptacle connected to it.
90.1 - This code is not intended as a design specification or an instruction manual for untrained persons.
To me, that means the code is not intended to be used as a specification guide, NOR is it to be used as an instruction manual for untrained persons. (Two different things)
Bear in mind that the wording is of the intent of the code, there is nothing that prohibits the use of the code as a spec guide or as an instruction manual, which is probably why it is done so much.
Cheers and Stay Safe,
Marky the Sparky
The word 'specification is not in 90.1, regardless, what purpose is the NEC intended for if it cannot be used as a guide for creating a design? Are we supposed to blindly install stuff then hope that it complies with the code?
90.1 effectively says the intent of the NEC is not to make a person 'not be untrained'. Following only the NEC will likely not result in a safe installation. The NEC is a 'what must be done' not a 'how to do it' standard.
Just because you can, doesn't mean you should.
(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.regardless, what purpose is the NEC intended for if it cannot be used as a guide for creating a design?
You install stuff based upon your training and experience and do it to suit the situation. The code is consulted to assure that the bare minimum legal standards have been met.Are we supposed to blindly install stuff then hope that it complies with the code?
(C) Continued - Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.
Following only the NEC will certainly provide for a safe installation, but that's all. See (C) continued.90.1 effectively says the intent of the NEC is not to make a person 'not be untrained'. Following only the NEC will likely not result in a safe installation. The NEC is a 'what must be done' not a 'how to do it' standard.
As I stated earlier, nothing says that using the NEC as a spec guide or a training manual is prohibited. If a person wants to use a book that explicitly states the book's intent is NOT to be used as a spec guide or training manual is up to the user. Sticking strictly to the NEC will provide an installation that is, basically, the worst allowed by law.
Cheers and Stay Safe,
Marky the Sparky
A 20 amp outlet on a 15 amp circuit and breaker is considered deceiving. If the home owner is out shopping for treadmills. They may look at the male plug (with one prong turned side ways) and say oh we have those.