I know nothing about electricity. I finally decided to pay for installation of a quick-charging outlet for my universal charger (for the last 3 months I've been using a normal wall outlet). The electrician installed everything today, but when I started charging tonight, the breaker tripped 3 times in a row. I looked at the panel and saw that he had assigned the charger to a 30-amp dual breaker. I pulled up installation instructions on the Web which say that the NEMA 14-50 adapter should be used with a 50-amp breaker, and I'm assuming that's the issue (?). I called him and he pointed out that on the adapter itself it says "30a", which is why he used a 30-amp breaker. I guess my first question is, which is correct? My second question (anticipating the first question he's going to ask me tomorrow morning when he shows back up at my house) is, why would the adapter say "30a" when it requires a 50-amp breaker?
I'm sure these are very basic questions for someone with an electrical background, but I want to be prepared for when he tries to charge me twice for having to redo the install. Thanks.
The NEC defines all electric vehicle charging loads as "Continuous Loads". This means you must size the branch circuit (including the breaker) for 125% of the actual load. Any electrician should be aware of this continuous load calculation and I would think by now that any electrician worth their title would have heard that EV charging is ALWAYS considered a continuous load. This gives me pretty serious concerns about your electrician.
Note that if you want to charge tonight, just set the max limit to 24 amps and reset the breaker. This should be safe on a 30a circuit (assuming nothing else was done wrong).
I should also call out why your breaker is tripping: So a 30a breaker is only rated for 30a under "standard test conditions". This is with the breaker installed in a panel with no surrounding breakers. Yours presumably have other breakers around it which increase heat buildup and will cause the breaker in question to trip easier. Breakers are not precise devices - they may trip slightly above or below their rating as well. Also, your Tesla is drawing 32a (more on that below).
It seems to work as expected: you are pulling 30-amp so the 30-amp breaker is doing its job every time, to avoid overload.
So you need a margin: 40-amp breaker for 32-amp operation, 50-amp breaker for 40-amp operation, or 125% overcurrent protection.
The "electrician" should read the documentation, something like:
https://www.tesla.com/sites/default/files/downloads/US/universalmobileconnector_nema_14-50.pdf
And the wires between the panel and the outlet has to support the current too. 14-50 is up to 40-amp so it should be 50-amp breaker according to code.
A real electrician should be able to confirm...
Yeah, a good call out on the wire between the panel and the receptacle. Technically it would be totally code compliant to do this with either a 40a or a 50a circuit (with the corresponding rated wire) since the UMC Gen 2 is a 32a device. I *strongly* recommend though only doing this on a 50a circuit since in the future it is likely you or a future homeowner may connect a 40a EVSE that needs the full 50a circuit headroom. Tesla suggests this as well in the doc linked above.
Actually, the doc you linked to above is technically for the UMC Gen 1, and you have the UMC Gen 2. Effectively though the wiring you would do for a UMC Gen 1 would be more than sufficient for a UMC Gen 2 since the Gen 1 is 40a and the Gen 2 is 32a. Here is the updated doc:
https://www.tesla.com/sites/default.../NEMA_14-50_installation_guide_NA_US_2017.pdf
I was going to make fun of your electrician, but the 14-50 adapter for the Gen 2 UMC says, "30A." I happen to also have the Gen 1 UMC, and its 14-50 adapter says "50A." What the.....? Is it a typo?
Edit: the NEMA 14-50 plug should always go with a 50 amp breaker. Your electrician needs to change that.
Yeah, so you have stumbled onto something that drove me crazy early on. Different end adapters for the UMC Gen 2 that went into 6-50 or 14-50 receptacles were either stamped with 32a or 30a (same part # but a different rev). They all actually function at 32a though. I went round and round with Tesla thinking this was a mistake. Here is a thread I started on it (see my last post in the thread):
Rating on 6-50 UMC Gen 2 Adapter is only 30a rated - why?
So basically, I think Tesla is taking advantage in some fudge in the rules. My supposition is that if you label something 32a, then it actually has to draw within plus or minus of 10% of that. So the "device" (in this case the Tesla) could in theory draw up to 35.2 (which it never would since it is digitally controlled). But that means your adapter needs to be lab tested to 35.2 I think. So I am guessing Tesla said "fine, we will rate our adapter to 30a, but our car will always draw 32a (which is within the + or - 10% range), and then we only have to test the adapter to 33a (going to the 35.2a might have required them to upsize the wire or something).
So to the comment on the 14-50 receptacle: I think technically on a 14-50 that is the only receptacle on a circuit you are basically allowed any size breaker as long as it is 50a or less (not exceeding the receptacle rating). But from a practical standpoint, you should never install a 14-50 on anything but a 40a or 50a breaker (with appropriate wire).
Please do check what kind of wire your electrician ends up using. 8awg is the minimum size for a 40a circuit. For a 50a circuit you most commonly need 6awg, though by NEC, 8 awg in conduit is allowed assuming no other derating (like for ambient temp is needed). Tesla always recommends 6 awg so I would probably always go with that.
Thanks for the replies.
Renambot - I have a follow-up basic question: is it always standard practice to exceed the rated amperage by 125%, or is that something unique to this charger? For example, would the same thing need to be done for an electric clothes dryer? Thanks.
A great question! It is the standard for "continuous loads". Continuous loads are things that the maximum current is expected to be drawn for over three hours. A residential dryer would generally not fall into that category, but an EV absolutely would. Also, for the avoidance of doubt, the NEC just flat out says all EV's are continuous loads just to put that question to bed. ;-)
P.S. An electric dryer is a great example here btw since it is typically on a 30a circuit, but it is not a continuous load.
