GFCI breakers in a subpanel trip when a main panel circuit powering a Tesla wall charger is being used

Question

Four years ago we installed a 50A 240V circuit to power a hot tub. There is a standard 50A breaker in the main service panel (a 1990's vintage 200A Murray panel), with 4 wires (L1, L2, neutral and ground) to an Eaton subpanel (located about 50 ft from the main panel) that has 2 circuits - a 240V 30A circuit that powers the hot tub's heater that runs intermittently, and a 120V 20A circuit that powers the rest of the hot tub, including a 40W circulation pump that runs constantly. Both of these subpanel circuits are protected by GFCI breakers. This installation has worked perfectly with no issues, up until now.

Recently we installed a new 60A 240V circuit from the main panel to power a Tesla wall charger (or EVSE),located about 25 ft from the main panel. As required in the installation instructions, this circuit has only 3 wires (L1, L2 and ground) that are directly hardwired from the main panel to the charger. The charger is set to operate at its highest rating, which is 48A constant load.

Now, whenever we are charging the car, both GFCI breakers in the hot tub subpanel will trip. None of the breakers in the main panel trip, nor does the car charger diagnostics report any problems - the car continues to charge at full power. It is worth mentioning that the GFCI breakers do not trip right away, but several minutes to more than a half hour after the car starts charging. If I attempt to reset the breakers while the car is still charging, however, the breakers trip immediately. But once we stop charging the car, the GFCI breakers can be reset and those circuits function normally until we attempt to use the Tesla charger again.

Any clue as to what's triggering the GFCIs to trip after a time during the charging session and how to address it? I don't know of any possible connection between the two circuits but I have yet to open the main panel - I did try derating the Tesla charger to operate at 40A as a test to see if a lower power draw would solve the problem, but the hot tub GFCI breakers still will trip after a time.

Update 7/1: After further examination of the wiring did not reveal any obvious issues, as a test I turned off all breakers in the main panel except for the charger, spa, and a plug circuit that powers my wifi so I could control the car charging remotely. I also turned down the hot tub temp to prevent the spa heater from turning on during a charging session:

1. Charging the car at 40A trips the breakers within an hour.

2. When charging the car at 40A, and while the spa breakers were on, I tripped the breakers manually with the test button; the breakers then tripped immediately upon attempting to reset.

3. I then ran a series of tests similar to 2) but with the car charger drawing different amounts of current. I discovered that the 30A breaker cannot be reset if the car is drawing more than 18A, and the 20A breaker fails to reset if the car is drawing more than 22A. Below 19A I can manually trip and reset the breakers without issue while the car is charging, so I suspect that charging the car above 18A will eventually cause the tripping problem I'm observing at higher amperage.

Answer 1

Nothing suggests this is an overload problem, at least based on the loads you have described.

What GFCIs measure is basically 'is the same amount of return current coming back as I'm sending out?' For 120V that is comparing the load to the neutral. For 240V GFCI breakers, the comparison has to be made between both the hot lines and the neutral as a pair of balanced measurements. In both cases the current should always balance, i.e. the currents one direction and the other direction across the two hots and neutral have to match to within a fairly small amount of current. That's really a situation where nothing else should be able to do much to what the GFCI sees, unless there's something funky going on with the neutral. If one of the hot connections is loose somewhere and low voltage, the current coming out of it is still going to be balanced by the other leg and neutral. In the case of my hot tub, the neutral isn't even connected, which means the only remaining players are the ground (somehow), electrical noise, a defective GFCI, or an actual ground fault.

Go around and triple-check the torque of every connection including panel feed and ground rod connections, make sure every breaker is in place fully, and test the resistance of your ground system. It's not supposed to matter, except it helps to stabilize the neutral to some degree so it has a little bit of influence there. I suspect there's something slightly funky about a neutral somewhere feeding this panel or upstream.

If checking every connection doesn't do it, there's a few more things you can check: does it work with a different car? You might have to rent one to confirm that, unless you have a friend with one. You could exchange the Wall Connector with Tesla in case it's causing problems, they are pretty easy to get warranty service through. You could try the shotgun approach: replace the GFCI breakers. I don't think this will work but it's one of the cheaper items you could swap. And the last option would be to get a power quality monitor put on your system to see exactly what is happening on the line - this will probably be expensive as it's a very specialized tool.

That's the ideas I've got. I'm not highly confident in this answer. This is a super weird one. The fix is ultimately likely to be something simple that we're not even thinking of, or something totally off the wall.