Charging at home at 16 amps instead of 32

[Eka A]

So.. Don't know how much of the thread you've read through, but the Tesla Mobile Connector (TMC) maxes out at 32A. This may or may not be Tesla cheaping out; turns out that the NEC (as you've discovered) allows for a 40A breaker, 40A wire, and a 50A socket, a NEMA14-50, as you've discovered. The apparent reason for this is that 50A wire is expensive, electricians run into 40A loads from time to time that could use 40A wire, but there's no NEMA connectors in existence that are rated for 40A on the nose. Since a TMC can't tell if it's got a 50A circuit or a 40A circuit, in the interests of preventing House Fires (not joking) it's thought they assume the smaller current circuit; the max draw (jiggered into the TMC) is then 80% of 40A. And that's 32A.

As far as the heating.. The general Tesla rule is No Extension Cords. With 120 VAC circuits, this is pretty obvious: The "Heavy Duty" extension cords that one finds at Home Depot are only "heavy" in that they use a lot of plastic, but as little copper as they can get away with. Plastic being cheap and copper being expensive.

In general, when one uses copper, copper has resistance, and the resistance is greater the thinner the wire is. Let's say the resistance of the extension cord is R.

The voltage drop across the wire is going to be 2*R * Amps (There's a "2" in there since there's one drop from the socket in the wall to the other end of the extension wire on one hot, and the same drop from the end of the extension cord on the neutral/other hot back to the socket.). A Tesla will happily measure the voltage when first plugged in with no current flowing (and no drop), and remeasure under load. If there's 'way too much drop, it'll simply not charge; if there's some drop, it'll reduce the current draw, in the interests of Not Catching Things On fire.

On Fire? Yeah, well, to a first-order approximation, the power dissipation in the extension cord is P = 2*(R * Current). Put your hands on the cord for a running vacuum cleaner and, yup, it's warm, and That's Why. If one gets the wire in said extension cord hot enough from this, one can cause the insulation to either burn (bad news, but not typical) or simply degrade. "Degrade" means the plastic insulation is breaking down and, when it finally goes, blowing breakers/house fires R Us. None of which is good for one's health, especially if one is charging overnight and is asleep.

Along those lines: Suppose the insulation finally decides that This Is The Evening That It's Going To Die. It gets hot enough to trip the TMC, which is nice, and the TMC goes off, which is also nice. Fine: The TMC is off, but the insulation is leaking current. And you've got a 40A breaker on this thing. Suppose 8A at 240VAC is what's leaking: That 1.92 kW right into the copper wire. We're going to get flames. Suppose this thing is lying up against the wall. The wall catches fire. Without a single breaker being popped. Um. This is a hazard to life and property. I'm saying this seriously, now: It's one thing if you risk your own life in the dead of night by not having a decent extension cord, but what about your loved ones?

Seriously, given that you're playing literally with fire, I would strongly suggest ditching that extension cord and finding one that is Truly Duly Rated for 40A for the length of wire. And not just intermittent use by a welder.

Or, even better, getting an electrician in there to move your 40A socket from wherever you've got it close enough to the car that you can use the TMC sans issues. Yeah, it might be a couple hundred bucks. But how much is your life worth?

Yep, I know the TMC can only do 32 Amps. I read it's manual before I purchased the car.

Yes, I have a 50 Amp outlet on a 40 Amp circuit breaker, and the wires are not large enough for any more than a 40 Amp circuit. That is all the welder needed when it was installed, and I didn't install it. The electrician the previous owner hired is the one who did. I just repurposed the circuit and wires for the EVSEs. I now have a Tesla Wall Connector EVSE configured to 40 Amps so it will only supply a max of 32 Amp charge current. This ain't rocket science. It's just the application of simple, often logical, rules to what needs to be done. One just needs to learn the language and how it applies to the world. Similar to what I did when I helped my ex craft treaty text in Napoleonic Code legalese. OK, so those rules may have not been as concrete.

I've likely forgotten more about AC electrical wiring than most residential electricians ever know. I also know how to look up what I'm not sure of in a NEC code book. I'm more experienced on the DC side and could design about any part of the electronics and write the software for the control systems for a battery EV. It wouldn't have the refinement one sees in a Tesla, but it would be functional and safe. Would I try to, no. I get a bunch of engineers who know their areas, and give them the rough parameters to design too. Then be the conductor to get the best symphony out of them.

That cable was fine except for one improperly crimped crimp that corroded, and thus it heated up. It was still providing proper pinch pressure on the blade which is what I was worried about. It wasn't crimped hard enough to weld the contact to the copper wire and exclude oxygen and water. Thus corrosion got in and made the contact area smaller so it heated up more. It wasn't heating up enough yet to be an issue with setting the insulation on fire, just getting it hotter than designed for. It has 6 AWG copper wires, and the cable has all the right markings for when it was made back in the 1990s. It spent most of it's life hanging on the machine shed wall so it's copper and insulation flex life is still ok. I think the EVSE was the highest current it ever carried. The welder only draws 30 Amps when maxed out making a spark. I'm not sure I could buy a better extension cord without making my own.