The 80% rule is a requirement in the National Electric Code (NEC) mandating that any circuit that is used for a continuous load should be de-rated to 80% of its breaker rating. A charging EV falls into that continuous use category, so you should only charge an EV at 80% of the breaker rating.
In practice, if you use consumer EV charging products, they will obey this rule. So an EVSE that plugs into a normal 15A household circuit will only draw 12A. The biggest receptacle in common use is a 50A NEMA 14-50 or NEMA 6-50, and EVSEs that can plug into those receptacles will limit their charging to 40A.
Incidentally, there is a quirk in the electric code that allows you to install a 50A rated receptacle with only a 40A breaker. That's the reason why the Tesla Mobile Connector, for example, only charges at 32A with a 50A rated adapter plug. Since many consumers won't realize they need to check their breaker for a random 50A receptacle, the Tesla Mobile Connector errs on the side of caution and maxes out at 32A (of course, this makes the product cheaper, lighter and smaller too).
Copyright 2023 CarCharging.us