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