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Charging with 30 Amps

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Right from the Wikipedia page you linked from:

The 10-series receptacle provides 2 hots and a neutral conductor. Grounding is accomplished through the neutral. HOWEVER, if all you have is a ground conductor, you cannot attach a 10-series receptacle to it. Here's why - the ground conductor (see NEC 250.122) may be smaller than what is required to carry the current (even though it's sufficient for fault current) *or* it may be a bare wire conductor. I recognize that for a 30-amp circuit, EGC is the same size (#10); however, if it's a bare wire, then you can't use it on a 10-series receptacle because the grounded conductor (neutral) cannot be bare.

Indeed, 3-wire appliances on a 10-series receptacle *may* ground through the neutral blade, which is why Tesla can sell you a 10-series adapter. However, it is illegal to install a 10-series receptacle onto a cable that has only 2 hots and a ground conductor. You *may* take a cable that formerly supplied a 10-series receptacle, remark the white as green (provided it's a cable and not wire-in-raceway), and use it as a 6-series -- but not the other way around. I recognize the UMC only needs 2 line conductors, but that doesn't make the installation of a 10-30 receptacle legal.

And to your final question, unless the original water heater circuit had a 10-series plug on it, then no, it does not fall under "existing installation"; the water heater was installed with "permanent wiring methods"; installing a receptacle on that is changing it. The same applies to formerly hard-wired cooktops/ranges that required only 3-wire hot-hot-neutral before the early 1990's -- it is illegal to install a 10-series receptacle on that wire for a new range that plugs in -- you must run a new 4-wire circuit to place a receptacle on it.

See my FAQ for more information. Just because it works does not mean it is legal nor safe.
 
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Right about now, I think the OP is sorry he asked :)

Actually this is all good info to hear. I would not mind upgrading to a 50 A circuit but I don't think I have the capacity in my panel. And for the record I do not have a $100,000 car. See my signature.

I am pretty sure my panel is maxed out because I have a tankless hot water heater now that replaced the tanked one. It takes 3 60A circuits and I believe that I just passed code because it was not considered a constant draw.

I have no spare circuits space available other than the one 30 A that is no longer in use.
 
I have no spare circuits space available other than the one 30 A that is no longer in use.

There are several load calculators that are available. Plug in all your data and it will tell you the room you have. As mentioned up thread a bit, I would consider using the 30A circuit's breaker space (just cap off the old wires with wire nuts in the box and label them for future use) and running a new cable, #6 if you can.

I suspect you'll have room for the charging load but it may become iffy if you have long periods where the on-demand heater is running full-bore.
 
Or he could install a NEMA 6-30 receptacle, and then use a pigtail adapter to go from the 6-30 to a 10-30 and use Tesla's 10-30 adapter with the supplied UMC.

As far as I know, the HPWC dip switches can only be set for a minimum 40A breaker, so the 30A breaker wouldn't work.

But yes, the best thing would be the to rewire and put in a 40A breaker and wire and install a HPWC.
 
Not to mention what liability they might incur to their residence with less than premium (Electrical code) sized wiring.
Would really hate trying to explain the wiring situation to their insurance agent after an untoward event.

Now you're just making stuff up to be cynical. It is already properly wired as a 30A circuit. Using it as a 30A circuit is not doing anything against electrical code, as you falsely suggest. Wiring in a Clipper Creek 30A unit or a 6-30 type of outlet is completely proper and conforming with electrical code, poses no danger, and is cost effective.