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Home Charging Issue w/Gen 2 UMC


Frugal But Classy!
Jan 7, 2017
Just curious if anyone else is having the same issue that we're experiencing with our 2nd Gen UMC. We took possession of our M3 almost a month ago and haven't had to charge at home until very recently. When we plug the 2nd Gen UMC into a traditional 120V outlet (GFI), it trips the circuit almost immediately.

We thought that there was an issue with the provided charger, but we had another Tesla as a loaner while ours was being looked at and it also tripped the GFI. Since we didn't have any issues charging our S at the same GFI's, I borrowed a 1st Gen charger from a friend. No issues whatsoever charging the M3 with it.

Again, just curious if anyone else is experiencing this..?
The Gen 1 UMC seems to work fine for me. I reported the issue to my local Tesla service center, but they couldn't replicate the problem when they had it. Oddly enough, the Gen 2 UMC trips the GFI of a friend of mine when he tried it also...
Doubt it is a UMC issue. Far more likely: the electrical contractor involved in first building the house took short cuts or cheated, as he figured no one would use the plug at high amperage until years after he was paid. There is a tendency to "get er' done" with contractors.

Have a well-reputed electrician come check the line. Let him/her know your UMC will draw at 12 amps, continuous, and so the line needs to be correct. He or she will most likely fix it with 1 hour's labor. That being said, there are greater issues with install of electricity than is immediately apparent to those of use who aren't electricians. Its actually easier for an electrician to balance the load if he/she installs a 240V line. Depending on your systems capacity, you are best off to have them install a 240x 50 amp plug, or a 240x 30 if that's the best you can get. That can take a lot longer than an hour; it just depends on your system. And before the work is done, find out which plug is planned, and make sure it is one that Tesla supports WITHOUT an external adaptor. Only use the adaptor which is part of the UMC - Model S/X/3 Gen 2 NEMA Adapters
Yes I'm having the same problem with gen 1 vs gen 2 UMC, but in my case it's with a NEMA 14-50 outlet on a 50-amp GFCI breaker. Never had a problem drawing 40 amps on either of two different gen 1 connectors charging a Model S. But now with a Model 3 using the gen 2 connector, it has randomly tripped the breaker several times, despite being limited to 32 amps.

Installed the entire circuit myself, so I know no corners were cut. I know some people have said "you don't need GFCI on the outlet because the UMC has it" but GFCI on the UMC doesn't protect against problems in the outlet or the wiring leading up to it. Very disappointing that the new UMC seems to be a step backward, both with this GFCI issue and with the lower current limit....
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To the original posters comments:

I would try replacing the GFCI outlet or breaker that is tripping. They have a massive failure rate and it may just be more sensitive than it should be. The UMC Gen 2 may leak a little bit of current perhaps when it tests its own built in GFCI? Obviously this does not fail on *every* GFCI protected circuit out there since if that were the case the forums would be constantly lit up about this (I have seen it mentioned a number of times, but it still must be somewhat of an edge case). Nearly all the 120v outlets I would ever think of plugging my UMC Gen 2 into would already be GFCI protected (in garages or outdoors).

The NEMA 14-50 though on a GFCI is not very common. I am not sure I have ever run across one wired that way (don't get me wrong, it is a GOOD safety idea per the reasons NickM mentions - especially in a wet location) - but I am curious how it handles the neutral. On a 120v circuit the GFCI calculation is EASY - current on "hot" must equal current on "neutral". On a 14-50 it is a lot more complex. Current on one hot and the other hot can equal each other, with neutral current being zero and that is ok. Current on one hot being 5a, current on the other being 5a, and current on neutral being 5a is OK. But any time you basically have current return some way other than a hot or the neutral then you need to trip.

Question: Do the 240v 50a capable GFCI's they sell even have the ability to calculate the neutral return current? Or are they intended only for 240v motors (i.e. for pools and spas and pumps) that have no neutral?

Though now that I think about it, I think the UMC does not even have a connection to the neutral pin at all... So that can't be the issue.