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In my house, I have 2 breaker boxes. In the basement there is a 200-amp main box, with the ground and neutral bars bonded. Feeding the main floor, there is a 100-amp sub-panel (branched off the main box) that has a separate ground and neutral bar.

I am currently finishing the basement, and am installing the lighting. For the stairway going downstairs, I have an existing 3-way switch that controls the stairway lights. This circuit is fed from the 100-amp sub-panel upstairs (area in dashed box in picture). I now want to add a lighting circuit for the main room in the basement. This involves switching out the 1-gang box at the bottom of the stairs for a 2-gang, adding power in from the existing 200-amp service in the basement, and running my wire off to the lights.

My question is: since I am mixing not only circuits, but one circuit from the main panel (the downstairs lights), with a circuit from the sub-panel (the stairway lighting), what do I do with the ground wires? Do I tie them together, or isolate them? Unfortunately, the box at the top of the stairs is part of a 2-gang box that feeds upstairs lighting as well, so even if I switch the stairway light circuit to be fed from the 200-amp panel at the bottom of the stairs, I have the same problem in the upstairs 2-gang box. (For code purposes/passing my inspection, the state I am in has adopted the 2014 NEC)

diagram

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  • Your sub panel should have a ground coming from your main so they should be at the same potential. Make sure not to tie any common (white wires) together from the separate panels this would form a path for objectionable current. (A very bad thing to do.)
    – Ed Beal
    Commented Feb 18, 2016 at 23:13
  • 2
    Due to recent Code changes which require Actual Neutral to be supplied in switch loops, won't you need 4 wires + ground between the light and downstairs switch? 2 messengers, the switched hot (which you have as gray there) and real neutral. I am told they do make 14/4 or 14/2/2 which will do the job.
    – Harper - Reinstate Monica
    Commented Feb 18, 2016 at 23:27
  • Is the bottom-of-stairs box metal or plastic btw?
    – ThreePhaseEel
    Commented Feb 18, 2016 at 23:35
  • Part of the reason I am asking- in my main panel, the ground and neutral bus bars are bonded. However, in the sub panel, it is required that the ground bar and neutral bar are isolated (see an old question of mine here: diy.stackexchange.com/questions/1706/…). My concern is that if I now connect the ground from the main panel (which is bonded to the neutral from the main panel) to the ground of the sub panel, haven't I now essentially created a new ground "loop" from the main panel to the sub panel?
    – MarkD
    Commented Feb 19, 2016 at 0:59
  • @ThreePhaseEel: it is plastic.
    – MarkD
    Commented Feb 19, 2016 at 17:58

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Good question, but pause to think: What if all your wiring was in EMT metal conduit? (In which the conduit is the ground). There'd be no choice to bond the grounds, and thus, it is OK.

This question is even easier since you are working out of a sub-panel, which already has its ground bonded to the main panel; that is, one ground rod for the whole house. I have a facility where the 120/240v is served out of two main panels, each with its own supply transformer and its own ground. Circuits from both main panels go into a spiderweb of EMT, and while the neutrals never cross, the EMT certainly does, which means the ground does. What's more, the EMT is attached to an all steel building via girder clamps that are entirely made of metal. It would be impractical to separate the grounds. All this to say: If I am allowed to mix grounds, you certainly are.

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  • Thanks for the feedback Wolf. Re: if all my wiring was in metal conduit, I suppose the code might say that you would have to isolate the two runs (i.e.- I couldn't use a 2-gang box there, due to the mixing of grounds from the sub to the main panel). But it sounds like this is not the case.
    – MarkD
    Commented Feb 19, 2016 at 17:56
  • also they would not want a situation where touching the cover screws on both boxes could result in a shock because of some voltage difference between the grounds,
    – Harper - Reinstate Monica
    Commented Feb 19, 2016 at 18:24
  • Makes sense. Glad to know the two circuits in one box won't be an issue (obviously not tying the neutral leads together ;) ). Hopefully I didn't draw my wiring diagram right on the 3-way switch, so I don't have to dive into more drywall work.. ugh.
    – MarkD
    Commented Feb 19, 2016 at 18:37

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