How should this circuit be properly grounded?

Question

I am hiring an electrician to do the work so my question is more for my education and helping me ask the proper questions to the perspective contractors.

Purchased a condo recently and want to put in some recessed lights. The condo has a subpanel in the laundry room with the main service panel in a common area on the main floor. All the current wiring is run in metal conduit, I presume that it's THHN wiring. The subpanel has a neutral bar that's not bonded to the panel. The panel is grounded via a grounding wire that I would presume runs to the main service panel. There is no ground bus within the service panel.

So, my question. For the new recessed lights (6" LEDs), can one run nonmetallic sheathed cable (NM cable) not in conduit? Or must this be THHN in conduit. If one runs NM cable (again, not in conduit) would you install a ground bar in the sub panel to bond the romNM cable ground to it, or can the NM cable ground be bonded to the metal electrical gang box (ground clip?) that the switch will be in? In this case the switch for the new lights could go into an existing metal gang box (where there is currently a switch for a ceiling fan). This box has metal conduit back to the sub panel. So from the panel, NM cable to the existing switch box, from there NM cable to the recessed lights. I am wondering if this is possible, would one / should one install a ground bus in the panel or bond ground to the existing switch box.

I hope this makes sense and I just didn't completely confuse everyone.

Answer 1

I will do my best here. I take it the reason you are asking about using NM wire is to save money? Purchasing NM is much cheaper. You can still run NM inside the conduct if there is room left. So if your conduit is 3/4 inch. you maybe able to pull 14/2 wire through with Lube.

Your subpanel would need a separate ground bus since the conduct is considered the (Ground) By installing a Ground bus and using the NM 14/2 or 12/2 you must connect the Neutral and Ground separate as for the panel is not bonded. I believe the term is floating ground and neutral. Your Subpanel is connected to your Building since it is not Bonded. The question i have is why is your panel has only 1 bus bar? There should be 2 one for the Neutral and the other for the Ground. Your subpanel should have 1 Aluminum service line that contains 2 hots leads and 1 neutral and 1 ground. The other question i have is the service line coming from the Main Panel inside metal conduct as well? The gang boxes are grounded by the Conduct, but I would recommend you put a Pig Tail in the box.

Answer 2

The neutral bus must be ungrounded; it's a sub-panel. Neutral and ground are tied only one place: at the main panel. I won't go into why here.

There is no ground bus because there's no need for one: the metal conduit is the ground.

The conduit may be mandatory due to local electrical codes (this is not a single family house). If so you are stuck with it, except you might be able to use flexible cords for the last few feet to your lights per the luminaire exception in NEC 400.7.

I prefer THHN stranded in conduit. Conduit is expensive to lay... but once it's up, THHN is cheaper than NM and safer too. Do not unsheath NM and use the single wires in conduit, the insulation is not tough enough.

Another thing about pricing, the bump from 14 to 12 gauge is pricey in NM... but cheap in THHN. I don't even own any 14 THHN.

Don't waste your time pulling sheathed NM in metal conduit, it's hard, annoying and unnecessary, and will make it difficult to pull other wires later. To make a transition from conduit to NM, run conduit away from the service panel out to a quality steel box, run THHN (hot and neutral) to the box, then bring the NM into knockouts in the steel box with approved clamps. The steel box should have a bump with a hole tapped 10-32, for a ground screw. It's stylish to use little green screws. Come off that screw with a solid-core green or bare wire, and wirenut it to your ground wires. Here is a box with the bump, it is $1.