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I have a fairly new QO main svc panel (QO142M200PCAFVP) which is being used as a service panel (bonded, not a sub-panel). The green screw (top right) bonds the panel appropriately. The main 3 connectors are identified (120v/120v/neutral). There is a smaller lug next to the neutral lug which I don't see being utilized anywhere, marked in green "???" in my picture. It can take a 4GA or maybe slightly bigger wire.

What is the official role of that lug, and can I throw my outside ground wire (attached to an 8ft ground rod) on it?

Thank you very much for the explanation below, "ThreePhaseEel"!

The resulting service panel wiring is in the second picture. enter image description here

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    It would be good to edit your question to include a pic of the labeling on the panel, or at least list the model number of the panel. – FreeMan Aug 2 '20 at 18:51
  • You already have separate ground bars. Any reason not to put your grounding electrode on the ground bars? If you do, then your neutral-ground bond (note stranded wire top left) will be functional, and then, you'll be able to put a clamp ammeter around it. Useful diagnostic tool! – Harper - Reinstate Monica Aug 2 '20 at 19:02
  • Uh, you don't need to move the existing bonding jumpers.... – ThreePhaseEel Aug 2 '20 at 20:51
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That lug is intended to be used as a grounding electrode conductor termination, but can be used as an extra neutral instead

The Square-D neutral design basically intends for the grounding electrode conductor ("outside-ground-rod wire" or water pipe bond wire) to be landed on the N-G strap at the lug you point out, with the larger lug next to it used for the service neutral, and the bond to the panel cabinet provided by the green screw through the neutral intertie crossbar; this keeps all the busbar holes provided by the panel free for branch neutrals and grounds.

However, since you have removed the bonding screw, and are using a wire as your bonding jumper instead, NEC 250.24(D) permits you to do as Harper describes and use the left-hand equipment grounding bar to land your grounding electrode conductor instead. As a result of this, you can now use the lug you point out as a spare branch neutral lug to make up for losing a branch neutral hole to the bonding jumper. Doing as you and the manufacturer intend and using the small lug on the neutral strap for the grounding electrode conductor is fine still, though.

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  • Want to confirm that in the 2nd picture, added in after the original posting, that the 2 wires under the screw the OP labeled "ground" are OK. I know that there are a lot of concerns about 2 wires under one lug... – FreeMan Aug 3 '20 at 12:13
  • I would second that inquiry for confirmation, with the only note that the lug can probably fit two or more of the larger stranded wire going in there (so it's not a space issue), and that I would be interested in seeing any rationale behind putting the second "water pipe bonding ground wire" onto a separate ground lug on the side ground bars. – sil80 Aug 3 '20 at 14:55
  • I made a separate question here: diy.stackexchange.com/questions/200195 – sil80 Aug 3 '20 at 17:45

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