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I have a problem with my 100 amp sub panel in my garage. I'm using only one 40 amp branch circuit to power a service unit to charge my Nissan Leaf. The service unit is connected to the 40 amp breaker through a NEMA 10-50 socket over 8 gauge wire. I charge the car every night for about three to four hours. Occasionally the main, 100 amp, circuit breaker in the sub panel will trip during the charge period, it happens once every five to seven days, but the 40 amp branch breaker never trips. There are no other circuits in the sub panel. There's a couple extra circuit breakers installed in the panel but there's nothing connected to them and they are switched off. What's the mostly likely reason for this to be happening and what are some steps to resolve it? Thanks.

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    Is it a simple circuit breaker or GFCI/AFCI(one with test button)? Picture of the sub panel will help also.
    – crip659
    Mar 23 at 14:32
  • Is the breaker warm when it trips? Can you plug the car in during the day and get this to happen with you around when the breaker trips? Mar 24 at 2:07
  • it's a simple breaker, not a GFCI, photo is available here, photos.app.goo.gl/PsRRVBkpf4Xkaif4A. I just noticed that after about 30 minutes of charging, the 100 amp breaker is very hot. too hot to touch for more than an instant. The 40 amp breaker is warm but easily bearable.
    – Ed R.
    Mar 24 at 5:15
  • I'd say that if you're charging an EV, then the 40A breaker being warm is probably to be expected and likely normal. If the 40A charger is truly the only thing going through that 100A breaker, then if everything is good, it probably shouldn't get even slightly warm to the touch. Do you have the ability to take the cover off your breaker panel so we can see the inside wiring? That might help to see what's going on and offer more advice. I'm not familiar with the Leaf and what it can report - do you have any way to see how much power it says it's consuming while charging?
    – Milwrdfan
    Mar 24 at 14:02

2 Answers 2

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Bad connection to the 100A breaker

The position of the 100A breaker in the panel, and the fact it's tripping on thermal overload while the 40A breaker isn't budging, is telling me that one of the connections to the 100A breaker has gone bad. Hopefully, this can be fixed fairly simply by undoing the faulty connection, cutting off the heat-damaged section of wire, and redoing the connection properly with a torque wrench to set the lug screw to its specification torque.

However, if this has been happening for a while, then the breaker lug is most likely shot, which means that the breaker itself needs replacing. Luckily, QOM100VH breakers are readily available, presuming your subpanel is 100A or 125A, which a 12-space Homeline panel is likely to be.

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  • I think that was it. I pulled the cover off the panel and checked the connections between the 100 amp breaker and the two bus bars. Both were not even snugged down, much less torqued-down. I removed the breaker and there was some heavy burn marks on one of the two connections. I then scrubbed everything clean with some fine sandpaper and reinstalled it all. Now, the car has been charging for over an hour and the 100 amp breaker is as cool as a cucumber and the 40 amp is so lightly warm it's hardly noticeable. I think all is good now. Thanks!
    – Ed R.
    Mar 25 at 5:33
  • @EdR. where precisely were the burn marks? the sandpaper job probably damaged the plating on the lug, so while it's holding together for now, I'd get a replacement main breaker for the subpanel on order Mar 25 at 11:41
  • Thanks for the tip. The burn marks were primarily on the lug of the breaker that attaches to the bus bar. They were minimal on the bus bar itself.
    – Ed R.
    Mar 25 at 18:43
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Concluding from the comment, that the big breaker is very hot, hotter then the smaller breaker, there are 2 possibilities left:

  1. The breaker itself is defect, i.e. the switching contacts have a too-high resistance.
  2. The connection resistance of the wires of the big breaker are too high. F.e., the bolds/screws need a re-tightening, and/or some oxyde layers have been developed in the garage (outdoor) conditions (which may have also affected the switching contacts of the breaker).

In both cases, the thermal system in the breaker (bimetal) gets above the limit after some time, since heat will easily move in copper or aluminum wires.

In order to have an exact evidence by values, some measurements could be made.

Here the voltage at the downstream side of the big 100A and small 40A breaker is very interesting:

  • case A if the car is charged, test point downstream the 100A breaker
  • case B if the car is charged, test point downstream the 40A breaker
  • case C if no power is consumed via the 100A breaker, test point anywhere downstream the 100A or 40A breaker, or upstream the 100A breaker.

The measurements case A and B should be done after the car has been loaded for a while.

Those 3 values will show if the small breaker does also need some maintenance.

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