UMC 5-20 trips GFCI, 5-15 works great

darco

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#1
Hello everyone,

I've been charging my Model 3 at home using the Tesla 2nd Gen UMC with the 5-15 adapter. It has been (slowly) working great so far. The outlet and circuit are 20-amp with a 20-amp GFCI, so I bought a 5-20 adapter to get a (slightly) faster charge. This same GFCI outlet works great with my 5-15 adapter, but when I use the 5-20 adapter it quickly trips due to a ground fault. I tried the adapter on other 20-amp GFCI outlets, and they all seem to trip.

I've done a continuity test on the 5-20 adapter, and it seems to match that of the 5-15... So there aren't any crossed wires as far as I can tell.

I tried preemptively lowering the charge current to 12 amps, but this seemed to have no effect. The GFCI would trip as soon as the contractor in the UMC engages (which is before the car starts using the power to charge the car).

At this point I'm quite baffled. Anyone have ideas on what might be going wrong here? Remember, I can confirm that this outlet is wired correctly, and the same outlet when used with the 5-15 adapter works great.

I'm tempted to build a breakout box so that I can get a multimeter reading on the ground line to see just how much juice (if any) is feeding back.
 

MelindaV

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#2
At this point I'm quite baffled. Anyone have ideas on what might be going wrong here? Remember, I can confirm that this outlet is wired correctly, and the same outlet when used with the 5-15 adapter works great.
the UMC does not behave well with GFCI
here's a search (with 6 pages of results) here for GFCI
 

darco

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#3
But the 5-15 works so well for me! Seems strange that the 5-20 would fail but the 5-15 wouldn’t.
 

MelindaV

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#4
But the 5-15 works so well for me! Seems strange that the 5-20 would fail but the 5-15 wouldn’t.
does that 5-20 receptacle have GFCI across both the 5-15 slots and the 5-20? or does it only monitor the extra 5-20 slot? I would think to dummy proof it, it would be for either config, but IDK. (GFCI looks for current on the ground line though, right? so should be the same for either using it as a 5-15 or 5-20?)
 
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13004

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#5
Yes, unfortunately, the problem I encountered was nearly 6 months ago. At my age, i can barely remember what I ate for breakfast, so I don’t remember but so much. In general, I mostly stay away from GFIs and GFCIs and I did not have one on the 20A 120V branch branch circuit that i was using to charge on. I have good stiff voltage with very little wiggle at my house, so that ruled out that potential issue. My Gen I UMC charges flawlessly on that very same 5-20 receptacle. I changed out my Gen II MC for a new one and that still did not rectify my issue. I wonder if the fault is with the Gen II 5-20 adapter. I went ahead and converted the 5-20 to 6-20 and never bothered to determine the root cause of my 5-20 120v charging problem. Since reading your post, I have since switched back to charging on a 5-20 to see if can duplicate the problem I had 6 months ago and will report back with any additional info.

Good luck!
 

MelindaV

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#6
GFCI looks for current on the ground line though, right? so should be the same for either using it as a 5-15 or 5-20?
the gfci is on the neutral (not ground). still not sure how this would be different between using it as a 5-15 or 5-20 though, unless the internals of the receptacle has the 5-15 bi-passing the gcfi
 

Frank99

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#7
On a 120V outlet, current flows from the hot wire to the neutral wire; any current on the ground wire is a fault. The GFCI compares the current on the hot wire to the current on the neutral wire; if there's more than about 5 milliamps of difference, the GFCI trips. This would be the case if there was current leaking into the ground wire through a short (or through your body if you happened to grab onto the hot and ground wires), or if there was current leaking from the circuit to earth ground through some other fault (for example, you grabbed the hot wire while standing barefoot in a puddle) in which case there'd be a fault but no current through the ground wire.
It's a problem that Tesla has to solve. Give them a call.