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I think this panel wiring is pretty messed up but I want to confirm I'm looking at it correctly.

My house is from 1960 and has mostly the original ungrounded wiring (2-wire cloth NM) for branch circuits. The panel inside the house, where all of the branch circuits originate, is actually a subpanel. It's fed by a 100A breaker on the main/meter panel, located outside. The house subpanel is a retrofit circa 1985.

It looks to me like the 4-wire subpanel feed was wired with the ground going to the left bus, and the neutral on the right. I don't see any strap connecting the two busses, and no bonding screw on the neutral side. So the ground and neutral bars do seem to be isolated in this subpanel. But it looks different to photos of other panels I've seen, where the two busses on either side of the breakers are tied to neutral, with a separate ground bar off to the side.

In my case the branch circuit neutrals are wired to either side without any regard except for which was closer. Only one circuit has an EGC going back to the panel (a 240v circuit for the AC condenser) and that EGC is landed to the neutral side. I understand you might mix ground and neutral in a main panel where N and G are bonded, but not in a sub.

Mainly I want to see if more experienced people agree with my assessment of what's going on with the left and right busses.

EDIT: Adding a photo of the wiring diagram on the panel. On the diagram it looks like there is a strap that connects the two busses, but I cannot see it. If it exists it is behind the plastic frame which seems odd.

Also I do not see the neutral bonding screw in the location shown on the diagram.

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Here's a shot of the text at the top of the panel diagram:

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  • Can you post photos of the labeling on the inside of the panel's door please? Also, can you post a photo clearly showing what's going on at the bottom of the panel's interior for that matter? Commented Sep 5, 2022 at 1:06
  • Can you add a photo that shows the text cut off at the top of your existing photo of the panel's labeling please? Commented Sep 5, 2022 at 14:20
  • So, you need some accessory ground bar kits, or at least one, made for your panel. It's perfectly normal to not see the neutral strap, especially with the breakers in place. And a sub-panel should not have a bonding screw in place (nor should it have grounds on the neutral busses)
    – Ecnerwal
    Commented Sep 5, 2022 at 14:30

2 Answers 2

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The original installers weren't on the same page

While it looks like the actual panel may have been installed more or less correctly for a split-neutral setup (removing the tie strap and moving any bonding screw present to the left-side bar) with a four-wire feeder, whoever came back in and installed the branch circuits screwed up by slobbering their neutrals all over the place. Worse yet, you have too many neutrals (14) for a split-bar setup to even be an option in your situation, and Siemens doesn't sell replacements for the tie bar the original installers removed and promptly threw away.

So, you'll need to do some work to retrofit this panel. First off, you'll need to use a length of 4AWG white copper wire to jumper between the two neutral bars, as that's the best we can do under the circumstances unless the original tie bar is somehow hiding under the bottom breakers. Second, we'll need to make sure that there are no bonding screws connecting the left-hand bar to the panel cabinet. Finally, you'll need to get an ECGB10 and install it in the upper left, moving the incoming grounding wire and the ground wire for the air conditioner to that bar.

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It looks like you have neutral bars on left and right, neutral going to a neutral lug connected on the right, and the ground wire incorrectly connected to a neutral screw on the left.

If you want to make 100% sure this is the case, disconnect the feed neutral and ground wires and any additional ground wires and then:

  • Check for continuity between left and right neutral bars. It should show 0 Ohm. If not, see ThreePhaseEel's answer for how to fix this.
  • Check for continuity between the neutral bars and the cabinet. It should show no connection. If it does show a connection, look for a screw connecting the neutral to the panel (bonding screw).

If all is well, then:

  • Install an accessory ground bar (ECGB10 - Thank you ThreePhaseEel for the specific part number.)
  • Move the main ground wire and any additional ground wires to the ground bar.
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    Thank you all. I am inclined to agree with ThreePhaseEel, but I need to verify that the two busses are not connected as Mannassehkatz suggests.
    – Robert
    Commented Sep 5, 2022 at 16:16
  • While folks are looking at this, I just realized I may have an issue with the two tandem breakers occupying slots 5 and 7. They do not seat fully compared to the other breakers. I now read that an ITC class load center will physically prevent tandem breakers from being installed except in certain locations, and some panels may not allow any tandem breakers. With 20 full size slots and a 125A max rating, mine could be the latter. Can you tell from the label/diagram?
    – Robert
    Commented Sep 5, 2022 at 16:30
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    Based on the diagrams I have seen elsewhere, it would appear that your panel can't take any tandems. Plus I suspect in the part # G2020ML1125 that 2020 = 20 spaces, 20 circuits (i.e., no tandems allowed), 1 = 1 phase, 125 = 125A max. Commented Sep 5, 2022 at 17:05
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    That sounds right to me. So they must have hacked at the breaker or something to get it to sort of fit. The can of worms gets deeper...sigh. All I wanted to do was add a ground so I could extend a circuit.
    – Robert
    Commented Sep 5, 2022 at 17:14

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