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What determines the onboard charger's ability to know the rating of the outlet that it is drawing power from. If I plug a Model S into a 20amp line, how does it know that it is limited by that rating?
Well, it won't know for 120v whether it's 12 or 16 (15 vs 20 circuit) and for 208/240 whether it's 24 or 40 (30 or 50) for example. You can set the amperage draw on the car itself. And if it's like the Roadster, it ties this to GPS location and remembers it so you don't have to set it every time (if you have special circumstances where you can't draw the max for the particular outlet configuration you're using).[/QUOT
Correct, but the plug adapter gives you the first 'safe' parameter to begin charging.
Correct, but the plug adapter gives you the first 'safe' parameter to begin charging.
You mean if I use the 120, it'll default to the max of 16? Or did you mean it'll default to the "safer" 12? (It's been my experience it defaults to 16 in the Roadster. I usually have to dial it down.) Less of an issue with the 208/240 outlets as the full current is more frequently available than it is with a US 120v outlet. Unless Tesla will require the use of a T-slotted 120v plug to draw 16a on the Model S. That would be nice.
Each of those plugs has it's own defined current limit:
NEMA 5-15 (standard socket) is 15A (12A when over 3 hours)
NEMA 5-20 (The T plug you mentioned) is 20A (16 when over 3 hours)
This is also true for the various 240V outlets, and I have been told that while the Tesla design will let you lower the charging current, you will not be able to override it upwards (i.e. you won't be able to pull 40A from a NEMA 14-30, rather a max of 24A).
Peter
By the way, here is the visible difference between a 5-15 and 5-20:
Tesla could have put a special plug-end on the adapter "keyed" for 5-20 use only like this:
But they picked the much more common vertical blade and leave it up to the user to turn down the current if they aren't plugging in to an actual 5-20 outlet.
I suspect most Roadster owners don't know about the two kinds of 110V plugs. The thing is, "continuous" really isn't much different from "instantaneous" when the electrical code defines the latter as several hours. So in reality you could probably get away with drawing the higher power from the 15A circuits.