I'll pay you to convert to NEMA 14 (or a subpanel)
Not me personally. Home sellers are finding they get better offers when they say the garage is ready for level 2 EV charging. And yours is, but for the wrong socket. Install NEMA 14-50, write "EV Charging" on a Post-it note, and put it in the listing with a $1000-3000 increase in ask price. "That was easy"
But a subpanel will be much more versatile for you, so I suggest you hold out for that.
NEMA 14 is the way to go (or NEMA 6). I physically smash all NEMA 10 plugs and sockets. If you don't, they'll come back like the undead. They're insidious.*
#1 problem: Grounding to NEUTRAL is insane
Neutral is one of the three "hot" wires that normally handle current. If everything is going well, neutral is bonded a particular way and that makes it safer than the other two hot wires. Ha ha!
But I've had problems - twice! - that made neutral unsafe.
However when it goes wrong, "grounding" your equipment chassis to neutral will cause that chassis to become energized. This is the classic problem with NEMA 10 for ovens and ranges (permitted until NEC 1996), a simple break of the neutral wire assured the dryer chassis was energized.
However, a whole-house "lost neutral" can have the same effect. In this case, the house can't return neutral current via the neutral wire to the utility, so it tries to return it via the neutral-ground bond, the ground rods, and the dirt to everyone else's houses or the transformer. This works about as well as it sounds, which is to say "not very". I had one of those 2 years ago.
My other one was a lost neutral-ground bond in the panel (the green screw was there, but the thread was vaporized). An unrelated ground fault had pegged L1 to 0V and neutral to 120V. (we had our own transformer for that installation).
In your case your wall wiring has the 3rd pin on neutral (correct for NEMA 10) but your appliance has the chassis mis-wired to neutral (very wrong).
There is simply no excuse to use the wrong socket when the right one is $10.
And by the way, you could have a line of people around the block swearing they've never heard of anyone killed by a NEMA 10 receptacle. How would they know? It's not going to have a neon sign. NEMA 10 dryer fatalities are usually mis-reported as a miswired outlet, when they are correctly wired but experienced an ordinary wire break. THAT shouldn't make anything lethal, which is why NEMA 10 was banned in 1996. With welder accidents, the socket type is lost in details - example: here is a very detailed accident report that doesn't even mention socket type, although investigators had every opportunity to see the socket, and a faulty NEMA 10 circuit fits the facts.
Problem #2: 15-30A appliances are not certified to be protected by 50A breakers
NoSparksPlease does a great job of covering this, and so I won't.
But yeah, UL tests every appliance to make sure its failure modes will get an appropriate trip out of appropriately sized breaker. (It's also why UL allows appliances to use smaller, short wires inside a metal chassis).
That subpanel
NEC says that a feeder to a subpanel must be sufficient "for the load to be served". That means basically that there are no hard limits, as you only need to consider loads that will run simultaneously.
For each group of loads that will run together, you must provision the loads based on their nameplate data (not their circuit breaker). For instance a 240V motor might draw 3800VA (16A), nominally take a 20A breaker and #12 wire, yet UL may approve it for up to a 35A breaker to avoid nuisance trips from motor startup. That doesn't count for 35A, it counts for 16A.
Your ruling load will be a Tesla Model S charging at 44A (with 125% derate giving 55A, the thermal limit of your 6-3NM feeder).
As such, you may have ALL these breakers in the subpanel at the same time! Breaker spaces are laughably cheap, so really splurge on the number of spaces in your subpanel. We have a 24 space that is half full and we only have two 240V tools wired so far. 30 spaces is not excessive. When NEC 2020 is adopted in your state, 240 circuits will require GFCI protection, and that can only happen at the breaker, and they are only made in full-size, so the "circuits" number is useless.
Now, here's a spot of good news (especially if the NEC 2020 requirements hit you): 15A, 20A and 30A circuits can have any number of receptacles. (40-50A circuits cannot). And 15A plugs fit 20A receptacles.
So you only need 3 total circuits to cover all needs:
- A 20A circuit with NEMA 6-15 or 6-20 receptacles (or 14-20 if you had an application for that)
- A 30A circuit with any number of NEMA 6-30 and 14-30 receptacles
- A 50A circuit with one NEMA 14-50 or 6-50 receptacle (14-50 being the universal donor) Remember to mark that one "EV charger" - ka-ching!
Yes, you can put a 50A branch circuit off a 50A (well, 55A) feeder.
Since they don't make 55A breakers, you round up to the next available breaker size, 60A... just don't plan on using the extra 5A.
* They're insidious. NEMA 10 was adopted as a "private socket" by the welder community and some others. I assume when they coined this family standard, NEMA 10 dryer sockets were common and most people plugged into those as a habit. That was when nobody cared about grounding, but it stuck.
Today, even more are spread because when a home-buyer says "hey my dryer is 3-prong", the home builder or seller just changes to NEMA 10 socket because it's their house (and not their appliance). But when the appliance store sells them a new dryer and finds the socket is 3-prong, they change to NEMA 10 plug because it's their appliance (and not their house). We'll never get rid of the beast! But you should.
