Install 14-50 receptacle on new circuit max 30A. Is this legal? Or should they be installing 14-30?
No, a 14-50 needs to go with a 50A or 40A breaker. The 40A is allowed because there's no such thing as a 14-40 outlet. On a 30A breaker you'd use a 14-30 outlet (if you have a neutral) or a 6-30 outlet (if you have no neutral)
Ugh. I hate having to give this answer, but I have followed that one down through code, and I believe it is actually allowed. However, it's kind of weird and dumb to do it, so just don't when you have real options of matching them, like a 30 to a 30.
There is a chart in NEC that shows what ratings of outlets are allowed to be put on what ratings of circuits, and that's where people are getting that, but the chart says it is for when there are MULTIPLE outlets on one circuit. That is not the case here, where EV charging circuits must be dedicated, with only one outlet. So the specification for that is just that the breaker must not be higher rated than the outlet. But it can be lower. So it does seem to be allowed to put on a 50A outlet type but have it on a 20 or 30 amp circuit with a 20 or 30 amp circuit breaker.
There are some pieces of equipment, like air compressors and things that are built for smaller circuits, like 20A, but come with 6-50 or 14-50 plugs. So this I think is meant for wider compatibility within a shop where maybe they do have a few things that need to be plugged in and out, so they may put in a higher capacity 14-50 on a 50A circuit, and everything can plug into that. But if you just have the one smaller thing, they seem to allow this 50 on a 20 so people don't have to cut off and change the plug on their air compressor if that's the only thing the outlet is for.
So it seems kind of weird but makes some sense in a way that it's on the safer side of things. You don't allow the breaker higher, because it would allow too high current for the wire and be a heat or fire risk. If you allow the breaker to be smaller, that's hyper protecting, and then if someone tries to use it for higher level things, it will keep tripping and people will realize they need to redo it as a higher level circuit.