Installed Tesla gen 3 home charger with 60amp breaker and car pulling 48 amps. The conduit from charger to power panel is approx 4 ft

Question

The conduit gets so hot you can't keep touching it. The certified electrician used #6 THHN. should #4 wire be used?

Answer 1

EV charging is the most demanding electrical load in your home. It is running the hardware at its absolute thermal red-line limit for a continuous load. The 6 AWG THHN is making full use of its 75°C (167°F) thermal limit in this application.

That is to say, the 60°C-rated "Romex" NM cable is not legal for 60A (at 6 AWG) because its insulation is not rated for a rise above 60°C (140°F or just into scalding). (Implied: this wire running around 60°C is to be expected). Did the electrician do anything WRONG? Well no, there's a gold-standard for right and wrong, NEC, and I see no defect here unless the wire is not actually 6 AWG copper.

In fact, NEC is alright with with that same conduit accommodating three such charging circuits: see 310.15(B)(3)(a).

The only exception I would see is if you have high ambient temperature conditions - the standard derates assume ambient temperature of 30°C. Phoenix attic, sunny side of dark painted house, etc. might warrant further thermal derates.

If you really, really, really want to "fix" it, then run 1/0 aluminum to a 60A breakered disconnect (reducing heating by 60% along that run) then then 4 AWG copper the last 2 feet to the Wall Connector (only 40% heat reduction, but only 2 feet). I spec #4 because it's the largest that will fit on a Tesla Wall Connector v3. I spec 1/0 aluminum because it's the largest that will fit on standard large-frame breakers, and you may have to use a 100A breaker to get there (hence the 60A disconnect).

Not your grandfather's golf cart charger

When they designed this generation of EVs, SAE and Tesla recognized that "needing to get an electrical service upgrade to charge at home" would be a huge barrier for adoption of EVs. So into the charging standard they designed tech to make service upgrades unnecessary: the car can draw any amount of current based on a capacity signal from the wall unit (which is a GFCI switch, not a charger).

The wall unit can be set to transmit one ampacity always, or vary it "on the fly" as other appliances turn on.

Home charging is rife with misconceptions. Like the "1982 golf cart" logic that if it's rated for 48A charging, it must be provisioned 48A (ignoring the abovementioned tech). Others greedily want the Fastest Charge Possible (tm).

The charging speed you need

Thermal heating is the square of amps. Double amps, quadruple heat and losses.

Technology Connections has a fantastic video on home EV charging, and let me cue it up to the "level 2 charging speed" conversation. Feel free to roll back for more stuff.

You only need to replace the miles you drove that day, + a 50% margin for recovery from unusual days.

Home charging can be like Uncle Ralph who owns a gas station, every night bringing over a 7 gallon can of gas and pouring as much as will fit into your tank. You leave full most days except after travel. And after travel, you'd be tempted to "game it" by arriving home empty and letting Ralph catch you back up over the next few days, wouldn't you?

But like the video says, "The fastest charger is expensive, and may come with more headaches than you bargained for". And here you are with one of the more minor headaches we see.

Cool your jets

Here we can borrow from Elon Musk via Wintergatan and their Magical Marble Machine (yes that thing works, not well) in less than 2 minutes of this video.

So I think the 48A was called out by a brilliant person, yet, I'm betting it's wild overkill for your home charging needs. So I would propose to simply turn it down to 24A (half). And then see where that puts you. That's still charging over 200 miles a night on a Tesla.

My rule of thumb is: if a reduced charge rate actually, in the real world, forces you to seek public charging during ordinary in-town activities (can't count travel) then the charge rate is too low. But if it does not, it is not too low.

That'll take care of the heating problem, and also, that heating isn't free, so you're saving some coin too.

And honestly for the vast majority of people, 16A would satisfy their needs. On the more efficient Teslas, possibly even 12A.

Now, how do you turn it down? Since your installation is actually safe and legal to 60A, you do not need to use the Wall Connector commissioning procedure to make this adjustment. You can simply use the Tesla's console inside the car.

(the setting in the Commissioning procedure is for when the cable has been downsized, e.g. #12 Romex for a 16A installation, and the EVSE must not exceed 16A. UL requires that setting be inaccessible to the end user.)

Answer 2

If I saw that then I'd suspect a ground fault issue.

The current flow on the wires from the breaker box to the EVSE should be balanced, and that means the net magnetic field around the wires is cancelled out. If there's a significant imbalance on the current flowing on the wires, as could happen if there's current flowing to ground somewhere outside the conduit, then there could be a magnetic field inducing a current on the steel conduit. Steel isn't a great conductor, it's good but not great, so it is more prone to heating with current flowing through it. This is steel, correct? Some kind of metal?

The ground fault may be simpler than some induced voltage, the ground fault is flowing back to the breaker box through the steel conduit and heating it up. Since this is heat that you feel and not an electrical shock then the voltage is close enough to ground potential that you feel only the heat.

If not a ground fault, and not undersized wiring (which a certified electrician is unlikely to do), then perhaps it is damaged wiring. An incomplete break in some wire could produce heat that is conducted (thermal conduction, not electrical) to the steel would be felt on the outside surface. This heating should be fairly local to that break, as in one spot on the conduit is hot but the rest is relatively cool. That is likely a sign of a bigger failure later. The wire could have been damaged as it was pulled through the conduit and left undetected in any testing done afterward.

One last thing in my "stream of consciousness" answer (I'm typing in possible problems as they pop in my head) is a very benign one. The sun is heating up the conduit. Is the conduit exposed to sunlight?

Whatever the cause I'd avoid touching it again until checked out. Do you have any kind of heat sensor that doesn't require touching it? My brother has an infrared thermometer that can check body temperature and such by just aiming it at the spot, it has a laser pointer or something to indicate where it is looking. The point of light isn't exactly where it is looking as the light would influence the result so account for that if using such a device to monitor the conduit heating. The electrician likely has one and will use it if there's a repeat visit to check on this.

I'd avoid touching the conduit again. Whether this is a safety hazard that requires turning off the circuit at the breaker is something of a judgement call in my opinion. And my opinion is worth precisely what you paid for it. That's the thoughts from a sometimes electrical engineer and amateur electrician.