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)
Best Answer
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.