My very generous employer (looking over my shoulder) is installing a NEMA 14-50 outlet for me to use with the UMC.
It turns out we have the 440/277 setup. Is this not recommended?
We do have a 110/208 side, but there is a perfect storm scenario where it could see full load and blow the breaker for the entire office.
You should not under any circumstances wire a NEMA 14-50 to 277v. It is utterly not code compliant and could be very dangerous. ;-)
Some other folks here have under very controlled environments done some neat experiments, but I could never recommend what they have done for a general use setup. Not only is the receptacle not rated for it (and you would have to wire the receptacle with neutral connected to a hot and hot connected to the other hot which would make it extremely dangerous if you like plugged in an RV), but the UMC is also not rated for it (we really don't know if the UMC is just not rated for it, or if the UMC does not actually have sufficient isolation for the higher voltage - it does seem to work though).
If you want to use 277V, you have to use a hard-wired HPWC. There is no other safe and code compliant way. Connecting 277V to NEMA 14-50 or 6-50 is just asking for trouble. These sockets are 240/208V line to line, while 277V service is line to neutral. That means the line to ground voltage is more than double.
^--- What he said.
Also, so while you could use a Wall Connector, it technically is no longer in the manual that it supports 277v (though it used to be in the manual and Tesla Charging Support has confirmed that it works and they did not indicate any safety issues with it). There are circumstances where I would do this for the "coolness" factor, but it is kind of hard for me to advocate for in the "general" use case since Tesla is not pushing this deployment model right now and there are some existing Tesla's out there that might not charge reliably on it (Model 3's are fine).
You would need to do a load calculation to be sure, but the worst case scenario for loads on that step down transformer you mention likely would never happen, so perhaps connecting to that at 208v is an option. You could also do a lower amperage circuit (14-30 receptacle for example) if needed (something is better than nothing!). Or even a NEMA 6-20...
Note that code now requires GFCI protection on EV charging receptacles and I am not sure how difficult it would be to get a single pole GFCI breaker for a 277v feed if you went that route with just a receptacle. Though actually, now that I think of it, the GFCI requirement does not apply to 277v receptacles so nevermind. ;-)
One other option could be to install a NEMA 7-50R receptacle on the 277v (which is rated for it), and then use a custom adapter to NEMA 14-50 for the car. This still would not be in spec for the UMC, but at least the hard wired portion would be code compliant. I can't necessarily advocate for this, but it would be vastly better than wiring 277v to a 14-50 permanent receptacle on the wrong pins.
