It will not use 100% capacity. It will automatically use only 80% per code for continuous loads. There is no danger what so ever using the portable EVSE on a standard 15amp residential circuit.
My apologies for being blunt, but you are wrong on both counts.
An EVSE is by definition a continuous load according to NEC. For continuous loads you must calculate them as if they are 125% of their actual draw. So the Tesla will take 12 amps on a 15 amp circuit. 12 times 1.25 is 15 amps. Hence according to NEC load calculations that circuit is 100% subscribed (assuming it is 15a and not 20a). Above 80% load there is a chance your breaker will eventually blow as well during charging since they are thermal devices and are sensitive to heat. It is a long story, but standard test conditions for breakers don’t take into account that’s they are installed with adjacent breakers which are also getting hot and can impact the breaker in question.
On the comment about “no danger whatsoever” that is extremely strong language to use. There are several issues:
1. We don’t know what else is actually on that circuit. If there are other loads on that circuit then you are relying on a circuit breaker to trip when an overload happens instead of starting a fire. There are defective circuit breakers out there.
2. You are relying on the installation quality of all the components and connections in the chain. If any one of them is bad it could result in heat/arcing/fire.
3. You are also trusting that damage has not been done over the years (plug tension is low due to repeated use, etc...)
I agree that in general on modern wiring with a quality installation it *should* be safe, but I would never claim there is no danger whatsoever.
It is important for people to realize and understand that due to the massive amount of energy that EV’s need, their default behavior is to suck as much juice as possible from whatever circuit they are on so that they can charge as fast as possible. My M3 LR can suck down 11.5kw given access to a high enough amperage circuit. That is nearly triple what the next highest load in my house is and if I was using the UMC it would be the vastly highest load that was not hard wired as well.
My callout here is just that caution should be exercised. EV’s are a fantastic way to find “latent defects” in electrical wiring systems that may have existed for 20 years and nobody realized it. I would venture to guess that over time, loads on random 15a convenience circuits have gone down recently as lighting has moved nearly exclusively to LED and CFL. So EV’s are probably the only thing (except maybe a space heater) that will really push 15/20a convenience circuits hard (and heaters generally cycle on and off periodically giving the wire a chance to cool a bit).
Tesla’s do have fantastic features to attempt to detect dangerous conditions (by monitoring voltage drop during charging), but this is not foolproof.
Ironically, I feel like charging off random 15a and 20a 120v convenience receptacle circuits is more dangerous than 240v circuits since there are so many additional points of failure and because they have to charge for so many hours constantly to gain any reasonable amount of range. This also pushes them into charging overnight when the danger of a house fire is greatest since folks are sleeping. While I would not dictate my life schedule around this, my M3 charges in a little over an hour when I get home. Generally during dinner. (eventually I will change this once there is a financial incentive for me to move it off peak)
