I have no idea if it has T-Slots, I will have to ask him tomorrow.
This is confusing to me though. What I mean is, the GFCI senses current leakage and if there is any leakage then it will trip. So basically, whatever amount of current being drawn on the hot side of the line, must be returned to the neutral side. If the two sides of the line do not match, then it shuts itself down. I was under the impression that although the GFCI outlet is capable of handling 20 amps, the 15 amp circuit breaker will trip if more than 15 amps are being drawn.
In any case, I will ask about the T-Slots and if it has them I will tell him that he will have to get a 15 amp GFCI for his bathroom and not use this one.
You are correct how a GFCI works. Around 5ma difference between the black and white, and the GFCI trips.
This is a violation of the NEC that I fully dont understand. If it was a dedicated 15 amp circuit and it was a single 20 amp receptacle, then it would be ok. The single receptacle has to be rated for at least the size of the circuit.
But per the NEC, A duplex receptacle is two receptacles. Now the rules change. When you have two or more receptacles on a 15 amp circuit, the rating of the receptacles can not be larger than 15 amps. You will find this in 210.21(B)(3) which refers you to table 210.21(B)(3) of the NEC.
I cant see how it would be a circuit overloading issue, as far as plugging a 20 amp load into it and taking the breaker up to and past its limit, because you can easily do that with a single receptacle. So maybe with multiple 20 amp receptacles on a 15 amp circuit, you could really overload it. And depending on the tripping curve of the breaker, it could take that extra current for a bit until tripping.
Your not going to see a whole lot of equipment in a home with a 120 volt 20 amp plug on it, especially in a bathroom. But the NEC doesnt limit the location of the receptacle(s) for this rule. It can be anywhere
So I guess what Im trying to say, It is a violation of the NEC to put that 20 amp duplex receptacle on that 15 amp circuit, but will something bad happen with this particular installation, probably not.