Ah. As they say, "That's your problem".
First, and most importantly, what you're doing is
definitely not safe, unless you're using a ganged breaker.
First, obligatory Wikipedia graphic:
View attachment 1008687
So, a NEMA14-30 is supposed to have one hot on one blade (on the above graphic, let's say the left blade), and another hot on the right blade.
As in normal in a North American house, each hot, with respect to neutral, is 120 VAC.
Now, the tricky bit: North American houses typically get a split-phase feed. There's three wires coming down from the pole: A neutral (which, in the breaker box, gets connected to a ground stake where the feed enters the house), a hot that is 120/_0 degrees, and another hot that is 120/_180 degrees.
The voltage from hot to hot is 240 VAC. If you were to look at these two hots on a dual-channel oscilloscope, both of them would be sine waves: But, when one wave is going up, the other wave is going
down, resulting in double the voltage.
The two hots, after going through the meter, get connected to
two bus bars. A typical breaker box has clip-on breakers. The first breaker, at the top, gets connected to the first hot; the next breaker down gets connected to the second hot; the next breaker after that gets connected to the first hot again, and so on.
When one wants 120 VAC, one takes the wire coming out of one of these breakers, pairs it with a neutral wire (also on a bus), and runs it off somewhere, since both hots are 120 VAC with respect to neutral.
If one want 240 VAC, one puts in a
duplex breaker that takes up
two slots and is
ganged, so if either of the two breakers has an overcurrent,
both breakers trip. The two wires coming out of the two breakers have 240 VAC across them. That NEMA14-30 is supposed to have a duplex 30A breaker pair, two wires coming out of the breaker and, along with those wires, a neutral wire, and a safety ground wire.
Funny rules come into play with a sub-panel. There's a copper-staked ground, 6 feet long, that's been pounded into the earth next to where the electrical entry is located. Said wire goes to the ground bus in the very first breaker box in the house; and the neutral wire from the electrical pole gets connected to that same ground bus. So,
at that breaker box and nowhere else, neutral and ground are connected together.
Now, consider a 120 VAC load, like a lightbulb or microwave oven. There is
very definitely current flowing on the hot
and the neutral going to that load; the safety ground, with malice aforethought, will not have any current flowing on it. And that's true of all the safety grounds in the house - no current. When there
is current on the safety ground, that's an emergency, because it means some piece of grounded metal or other (say, on the skin of a refrigerator) has a connection to something it ought not to, and you, a human, touching that energized piece of metal might find yourself slightly dead. Or a fire might start. This is what GFCIs are all about - in places where there's sinks, possibility of water intrusion, and all that, a GFCI is there to detect current on that ground wire and Open Up Neutral And Ground, thereby saving the day.
On a sub-panel, like, one located across the basement or in a detached garage somewhere, there's the issue that, due to voltage drops on the neutral going to the sub-panel, the voltage between neutral and ground would Not Be Zero; so, on the sub-panel, there may very well be a connection between the safety ground bus in that panel and a ground stake somewhere, but there will be an isolated-from-ground neutral bus bar, making a sub-panel Different than a main panel.
Now, back to you and your funky experimentation. First, nomenclature: Let's call, for the purposes of this discussion, the two hot phases A and B. If you wire up that NEMA14-30 to have A on both hots, or B on both hots, then the voltage between the two hots will be Zero, not 240 VAC.
Second: That NEMA14-30
must have wire capable of carrying 30A. This is not a maybe, this is not a suggestion, and this is because
wire gets warm when current runs through it. Standard 120 VAC wall sockets in a house are rigged to be (mostly) on 15A circuits, sometimes (with the blade at right angles) on a 20A circuit. If one runs something higher than the rated current through that wire, Our Friends at the National Electric Code have Done Studies. Heat is dissipated in that wire. The heat has to go somewhere - through the insulation, through the insulation of the house, and air flow inside the walls. If Too Much Heat For The Wire Is Present, Bad, Life-Threatening Things Are Going To Happen. The insulation can get hotter than its rating and degrade, to the point of becoming
conductive. It would be nice if it went to a dead short, but suppose, on a 15A circuit, it goes to a short that uses up 7A. That's not enough to pop the breaker, but it could be hot enough to set the wood timbers of the house On Fire.
You might be willing to live with the idea of getting a better charging rate on your car. But are you willing to have your nearest and dearest, also living in that house, to be woken up with flames in their bedroom door? Assuming that they just don't die of smoke inhalation without even waking up. Don't Screw With This - either do it right or hire somebody who knows what they're doing (there's a reason that these people are named
licensed electricians).
Looking at that NEMA chart, above, there's very few 30A 120 VAC sockets typically found in normal households. So, if you're playing mix-and-match with 15A or 20A circuits to build yourself a 240 VAC 30A circuit - I beg you, DON'T DO THAT THING. Unless you (a) live alone and (b) got a serious death wish.
Doing it right: Make sure your breaker box can take an additional 30A load. (On the web, search for Load Analysis.) Pull some four wire-cable (two hots, a neutral, and a safety ground) from the location of your breaker box to where your socket is going to be). Get a 30A, duplex, GFCI breaker (the GFCI is if your car is in a garage or outside), then wire everything up properly. And, if you're sane, go file an electrical permit application with your local municipal authority.
Why the electrical permit? Around here, they don't require a rough-in inspection and all that for simply adding a single outlet. But what the inspector
will do is look over your work. If the inspector is happy, they'll sign it off without a word. If they're not happy, then, good: Another pair of expert eyes has just kept you from immolating yourself. Believe me, worth the $40 or so for the permit, right?
Finally: All that stuff above about what one phase is with respect to another phase, where neutral is in this, and that the two phases have equal (but opposite sine wave) voltages? Both the car and the Tesla Mobile Connector
test for all this. If I understand the cognoscenti on this forum correctly, they even do a little checking to make sure that ground isn't inadvertently connected to anything that it ought not to be. (Which drives some GFCIs nuts, but that's life.) So, if you get the wiring wrong, the TMC isn't likely to work well. The TMC/car is also tuned to catch unexpected voltage drops - like what happens when one uses an undersized gauge of wire, which you appear to be doing.
And, yeah, the car/TMC is expecting that those two hots are 240 VAC from each other because
that's what it's going to use when charging the battery. Not 120VAC to neutral and another 120VAC to neutral, independent.
