I'm very late to this party, but I note that nobody has been invoking the NEC to help the OP to understand what's going on. So,
@hybrid>EV, this is for you!
First: What the Heck Is Split Phase?
Coming on down from the power pole, you got these three wires. One is Neutral; One is Hot #1, and the voltage between it and Neutral is 120 VAC. The last one is
also a Hot, let's call it Hot #2. The voltage between it and Neutral is also 120 VAC. But, if you had an oscilloscope and was looking at the sine waves coming down on Hots #1 and #2, while Hot #1 is going up, Hot #2 is going down, and vice versa. The voltage
between Hot #1 and Hot #2 is 240 VAC.
When these wires come into the house (ignoring the meter), Hot #1 goes to a bus bar in your breaker panel; Hot #2 goes to a second bus bar in your breaker panel; and Neutral gets connected to a ground/Neutral bus bar in the breaker panel. (The ground/neutral bus bar also goes to a Big Thick Green Wire that goes back outside the house, down to where the ground (as in, dirt) is located, and there's a 6-foot copper bar, pounded into the Earth, and the green wire gets bolted to that. They don't call the Ground wire a ground wire for nothing...)
Going back to the breaker panel: Each slot on the breaker panel is a place where a breaker can be clipped in. The top slot goes to Bus/Hot #1; the next slot down goes to Bus/Hot #2, then back to Bus/Hot #1 again, all the way down.
If one wants a 120 VAC circuit, then one clips in a (typically) 15A or 20A single-width breaker into one of the slots. A (typically black) wire is inserted into a hole in said breaker and clamped down; then, a wire from the neutral/ground bus bar (white colored) is bolted onto the ground/neutral bus bar, and the two of those go off to power lights/120 VAC circuits/what-have-you. For extra credit, a green wire is also attached to the ground/neutral bus bar (on the main breaker) and off it goes, too. The black wire goes to the narrow slot on a 120 VAC plug; the white one to the wide slot; and the green, safety ground goes to the ground pin.
If one wants a 240 VAC circuit, then a double-wide, physically ganged, duplex breaker is plugged into
two adjacent slots, thus picking up both Hot #1 and Hot #2. Each breaker has a wire coming off; the voltage between those two wires is 240 VAC. At a minimum a ground wire is paired with the two hots; often, there'll be four wires: The two hots, a neutral, and a ground. (See: NEMA14-50).
For your future edification, there's a large number of sockets, all at different current ratings and different number of wires.
See this link for nifty pictures.
Now, for the Safety Business. People have been dancing around about this on this thread, suggesting different things, but I want to give you the official, save-your-life-and-that-of-your-loved-ones line. And I am NOT kidding about any of what follows.
Wires are made out of copper, or, sometimes, aluminum. These are metals. These metals have resistance. When one runs current through them, they heat up. Power dissipation in a length of wire of resistance R goes as current*current*Resistance. A thicker wire has less resistance; a thinner wire has more. As an example, run a vacuum cleaner in your house for a bit, then grab the power cord. That cord will be
warm, and that's no accident.
Everything about current ratings in the breakers, the wires, the sockets, the plugs, is about controlling that heat so that none of the following happens:
- The wire heats up to the point of catching stuff on fire
- The wire heats up to the point of degrading the insulation around that wire. If that insulation gets degraded, it can break down, causing a short and thus causing a fire.
- The breakers get heated up unduly. The breakers are there for save-your-blinking-life purposes, i.e., shorts. Ideally, they should never trip unless it's a save-the-day situation. Why? Because making them trip, unnecessarily, wears them out. If one has a worn-out breaker then, if one is lucky, it gets stuck open. If one is not lucky, it gets stuck closed, in which case one's safety device is dead, dead, dead. And, as a result, in the dead of night, you get to be dead, dead, dead. Nuisance trips, as some above have mentioned, aren't just an annoyance. You might think it's worth the risk; but do you want to risk your loved ones?
People with more letters after their names than I've got have done Serious Studies about heat dissipation. Heat, when generated in a socket or wire, has to go
somewhere. So, Home Depot wire that gets placed into a wall has not just the heat conductivity of the insulation itself taken into account: It's also the heat conductivity of the insulation in the walls, the 2x4's in there, and what the ambient temperature outside is as well. No joke, test houses are built with thermocouples all over when the standards are figured out.
Towards this end, the National Electric Code (motto: Every rule on every page has a burnt-down house associated with it) has Rules. You violate those at risk of your life; electricians who violate them can lose their licenses or be criminally charged. If caught.
Here's the rules, so you know them:
- On a given circuit, with a breaker sized to amperage "A", the wire must be rated to that current and the socket, if any, must also be rated to that current. Hence, if one wants to use a NEMA14-20 (20A, 240 VAC) socket, then the wire has to be rated for that current and so must the breaker. No putting a 30A breaker on 20A wire/socket, or a 15A breaker on a 20A wire/socket. Period.
- When one has a continuous, heavy load, then the maximum current one can have on a circuit is 80% of the circuit rating. So: If one has a 120 VAC 15A circuit, with a NEMA5-15 socket, the maximum current is 80% of 15A, or 12A when one is charging a Tesla.
Again, just to be clear on this: Just because one has a 20A breaker, one does
not run 20A, steady, on that breaker. First, that 20A is the nominal point where the breaker will trip, which leads to nuisance trips. See above about Why Not. Second, that 80% derating is all about
continuous heat flow. One
really doesn't want to degrade the insulation in a house. A pulse of heat when, say, starting up a motor, is OK - but not continuous.
So, back to the fun. You've got a sub-panel in your garage with a 240VAC 30A circuit to it and a couple of 15A, 120VAC breakers in it, going to (probably) a $RANDOM outlet in the garage as well as your garage door openers. I take a wild guess: The general rule is that one when one has an appliance (stove, refrigerator, microwave, garbage disposal) each appliance gets its own circuit. You probably have one 115V socket; so, you probably have three (3) 15A 120VAC breakers in your sub-panel.
Now, this is where life gets a bit interesting. If one counts on one's fingers, three x 15 = 45, so, why isn't the circuit to the panel a 45A or 50A circuit?
Answer: The Official Assumption that not everything is on at the same time. And it is Official. The name for this rule is called, "National Electric Code Load Analysis". If you go to your Main Breaker Panel and add up all the breaker values you see, you'll note that the sum of all of those is a
lot more than the main breaker going to the outside wires. It's perfectly legit; the Load Analysis was done when they built your house, otherwise it wouldn't have gotten its electrical inspection certificate and certificate of occupancy.
And this is the reason that, before adding any breakers to your sub-panel, the electrician should look at said sub-panel, do a load analysis, and see how much spare amperage can be plugged into the panel. If it's 20A, then you can put in duplex, ganged, 20A 240 VAC breaker, wired to 20A-capable wire, and from their either to a hard-wired Wall Connector (or similar from some non-Tesla company) or something like a NEMA14-20. I rather doubt you'll get 30A when the sub-panel is 30A itself.
If the load analysis says 10A.. or 0A, then you're going to need a bigger circuit from the main panel. You have been warned. And, by the by: DON'T PLAY GAMES WITH THIS. I wasn't kidding when I said the NEC has a house-burnt-down-per-page deal on the Rules. Don't you be the owner of that house.
Now, finally: That bit about "80% of the circuit rating". Say, for this example, you actually do have a spare 20A @ 240 VAC available. Say you're using a Tesla Mobile Connector with a NEMA14-20 adapter and the electrician has installed a NEMA14-20 socket, wire to match, and a 20A breaker. You plug everything in, turn on the charging, and take a look-see at the screen in the car to see how it's doing.
You're not going to see 20A. You'll see 80% of that, 16A, and the actual power into your car will be 240VAC * 16A = 3840W. (A TMC knows perfectly well what adapter it's got plugged into it; between its communications with the car, and the car, the appropriate 80% factor gets factored in right off.)
With a standard Model Y that gets 280 W-hr/mile, that'll be a charge rate of 3840W/(280 W-hr/mile) = 13.7 miles of charge per hour. Say you've parked the car and gone to bed and it's been doing that for, say, 10 hours: You'll have 137 miles of additional charge on the car.
If you use a hardwired Tesla Wall Connector (or similar), a commissioning step is to
tell the TWC what amperage circuit it's plugged into. Again, it and the car communicate and set the current to the safe value.
Say you want to charge at the maximum rate possible, 48A for a LR or P Model Y. What size circuit would you need? (48A/0.8) = 60A circuit. Yep, out in my garage, I've got a duplex 60A 240 VAC breaker on a 200A panel, going to a TWC, and both the M3 and MY out there charge at 48A, which works out to be 45 or 46 miles of charge per hour.
Final note: I am very definitely not an electrician, but I truly am an electronics engineer and have designed fuses and breakers into all sorts of weird equipment. Admittedly mostly not AC-powered equipment, but, still. I also have a seriously healthy respect for Standards and don't believe in taking short cuts when it comes to stuff that can light a house on fire.
