Here’s an update on the main panel neutral issues that I raised originally in another question. I relabeled the photo, and made a diagram to show the problem. This post restates the issue, I hope more clearly, and gives more info, but the basics are unchanged.
At the bottom right the neutral from the 100-amp subpanel (labeled SPN) is connected incorrectly to the panel enclosure, rather than the neutral / ground bus where it should be. As a result, current from any unbalanced load in the subpanel flows through the enclosure.
The neutral / ground buses are bonded to the enclosure in two ways I can find, and maybe others.
- The bonding jumper BJ at the top is a piece of bare copper, probably #10, which connects the left and right buses together and to a ground screw (BJS) in the enclosure .
- A recent solar installation includes the #6 solar ground SolG, which bonds the bushing for the incoming FMC from the solar controller to the neutral / ground bus on the left.
There are also screws in the mounting bar for the buses on each side – you can see the one on the left side in the photo – that look like they might be intended as bonding screws to the enclosure. I can't tell if they are, but both screws are loose and will not tighten.
The neutral / ground buses are connected to the incoming service neutral by a neutral jumper (NJ) on each side; after careful examination it appears there is no other connection. The NJ wires look to be #8. (You can see where they attach to the bus bars in the photo, but due to camera angle the connection to the service neutral is not visible.)
Current from the subpanel neutral will flow as shown in the diagram: through the enclosure, through the bonds between the enclosure and the neutral / ground bus, and finally through the neutral jumpers to the service neutral.
I confirmed this yesterday at a time when most loads in the house were off. I turned on a toaster oven on a 120V circuit in the subpanel. Current in the subpanel neutral was ~13A which is about right. There was ~4A in wire SolG , and ~4A in bonding jumper BJ in the center between the two buses (I couldn't get the clamp ammeter in to measure the current in that wire to the left of the left hand bus). As soon as I turned off the 100A subpanel breaker all of these went to 0. Obviously this current should not be flowing in the ground wires.
It seems to me the solution is to:
- Move the subpanel neutral to the bus bar using an appropriate neutral lug.
- Remove jumper BJ, which is not as near the service mains as it looks, but shouldn’t run across them in any case.
- Add correct bonding jumpers between the bus bars and the enclosure with #6 wire, one on each side.
- Remove the loose screws in the neutral / ground mounting bars.
Does that seem right?