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Worked fine for us. The concern is if you use an adapter that is only rated for 40A, it doesn't tell the car that, it just passes through what the Tesla charger is rated for.

 

[Dark-Fx]

To your point what Amperage do you really need in this device. There is also an 80AMP version. This would be what you need to support the Lyriq at full speed from a Tesla charger that can deliver the performance for 19.2KW. Amazon.com

I have an 80A one, it worked on some of the Tesla chargers, but not ones that were on commercial 277V. Tesla will charge off that but I don't know of any J1772 spec cars that will.

 

[Dark-Fx]

I wouldn't be afraid of charging off a generator unless it's one of those exceptionally cheap ones. This is strictly CYA legalese.