Here is a discussion of how this works and why this is allowed:
For a
single receptacle on a branch circuit NEC 2017 210.21(B)(1) says:
"a single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit."
A 50 amp receptacle is, indeed, not less than the rating of
@dguitarnut's 30 amp circuit so the installation at his new house complies with 210.21(B)(1)!
The case of the Tesla Mobile Connector is not completely compatible with NEMA's design because the MC will set the car's charging rate (up to 32 amps) depending on the type of NEMA adapter that is used with the MC. It appears the Tesla engineers who designed the MC did not take into account the possibility of a NEMA 14-50 receptacle being fed with a 30 amp circuit, and were not aware that 210.21(B)(1) might allow this to happen.
I suspect the builder or electrician that installed this circuit is aware that a lot of EVSE's are equipped with a NEMA 14-50 plug, and can be configured for whatever amp draw the circuit is capable of, and in a effort to "check the box" of a potential home buyer's requirement for EV charging capability they installed what they thought would work (and comply with the NEC) at the lowest possible price. They might think they have done a good job and be surprised to see someone is not happy with what they have done and I think
@dguitarnut could have a hard time arguing with the builder or electrician that they should consider this a warranty claim because the circuit does not violate the NEC, but of course the builder or electrician might want to install a NEMA 14-30 receptacle out of the goodness of their hearts since it is such a cheap way to fix the problem. In any case, the minimal amount of effort to make this circuit to work properly with the MC is a very inexpensive thing to do: Just change the 14-50 to a 14-30 will do that.
I doubt the builder or electrician is obligated to do even that, but if they did it would resolve the problem.
I still say the best resolution of this would be to install a hard wired Tesla Wall Connector properly configured for the ampacity of whatever circuit is already installed. Even if it is a 30 amp circuit, charging at 80% (24 amps) it will very likely be satisfactory unless you run your car down to less than 20% every day or can't charge it overnight.
(Another way to fix this as a DIY project without involving the builder or electrician would be to purchase a non-Tesla EVSE that is equipped with a NEMA 14-50 plug and configure it for 24 amp charging. You would then have to use the J1772 to Tesla adapter that comes with the Model Y unless you can find a non-Tesla EVSE with a Tesla connector on the end of the cable. The You Tube channel "State of Charge" has lots of videos on just about every EVSE made. I suspect with Ford's upcoming decision to use the Tesla connector starting in 2025 there will be lots of EVSE's built with Tesla connectors, but I am unaware of any that are made today. Anyone know of any?)
A little on why this is allowed:
Sometimes it is necessary to install a receptacle that has a higher rating than the circuit because certain size receptacles are not made. For example, a NEMA 14-50 is often installed on a 40 amp circuit. Usually this is done for a range, which is semi-permanently plugged in, usually until the kitchen is renovated, and the receptacle is behind the range. Most people go through life and never realize this receptacle is even there. Note that for houses built in the olden days the receptacle will probably be a NEMA 10-50
Just for a complete understanding, and to underscore the difference, for
two or more receptacles on a branch circuit NEC 2017 210.21(B)(3) says:
210.21(B)(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or, where rated higher than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.
View attachment 943290
Note in the case of when there are 2 or more receptacles on a branch circuit, it would
not be permissible to install a NEMA 14-50 amp receptacle on a 30 amp circuit.
But a 15 amp receptacle can be installed on a 20 amp circuit. The reason this is allowed is because the feed through part of a 5-15 receptacle that feeds a downstream receptacle is designed to carry 20 amps, and the cords used on appliances that are equipped with a NEMA 5-15 plug are not used on appliances which draw more than 12 amps or 1500 watts (at 125 volts), and the rare 120 volt appliance that draws more than 12 amps is equipped with a NEMA 5-20 plug (the plug with one blade turned 90 degrees from normal). Tesla sells a NEMA adapter with a 5-20 plug for 16 amp 120 volt charging for use with the Tesla Mobile Connector.
A lot of thought has gone into the NEMA standards for receptacles and plugs!
I hope this is interesting, and I am sure I will be informed I have made any mistakes.