I have an old main panel that went in in 1971. (Square D.)
All of the circuits going out are run in conduit, and I believe grounded through it to the chassis of the panel.
I want to replace/alter many of the circuits in this panel. I also will be adding a 40A Subpanel run. Should I run the grounds from any new NM wiring into the same bus as the neutrals, or should I add a ground bus bar?
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Also, my subpanel that I will be installing is a 125A Main Lug panel. It did not come w/ a ground bus installed. Is that typical? Where can I get one? Any big box store, or do I need to go to an electrical supply place? (I bought the panel from a big box store. It's a Cutler-Hammer)
In a main panel, the ground and neutral are the same bus electrically and this should be bonded (connected) to the metal box. So you can use the neutral bus for the grounds as well. Or if not enough connections, add an additional bus with a large wire connecting it to the existing neutral bar. Or you can add an additional ground/neutral bar just for a neat looking wiring job or whatever. I would probably add a bar lower down to keep the bare ground wires away from the hot wires.
Many panels come with a "bonding screw". This connects the neutral/ground bar to the metal cabinet. You would install this screw if it were a main panel, but not if it was used for a subpanel.
A subpanel has a separate ground bar from the neutral bar. The neutral bar can't be bonded to the metal box. Separate ground and neutral wires must be run back to the main panel (where they would both connect to the same neutral bar). This is done for safety in case the neutral between the main and subpanel accidentally becomes disconnected. (Not an unusual situation!)
Many panels do not come with a separate ground bar. Just buy a separate ground bar (two if large panel), drill screw holes in the cabinet, and use self tapping screws to install.
So far as grounds and conduit, I like to run a ground wire in the conduit and not use the conduit itself as a ground. I have many times seen conduits hanging down with the connection between sections of the conduit broken (and thus the ground connection was lost).
When doing this type of work, you may be required to bring your main electrical panel/ground/service up to code if it is not currently.
For example if you just have a cold water pipe ground, you may be required to add two ground rods with a separate ground wire. So before doing anything, go ask your electrical inspector what will be required (around here they have certain office hours for questions).
Take plenty of pictures of panels, your main service where electric company wires enter building, where main panel is grounded, where you to plan to install subpanel, etc. Then tell them what you plan to do exactly. Then they can tell you in ADVANCE if something will not be allowed or additional work will be needed.