This question is, I think, different from the others I found here about "using bare ground wire for neutral." In those, I think people were asking about sharing the ground and neutral function on the same wire, or connecting them somewhere. I wouldn't even begin to go there.

My situation is: I need to add a second light to a light that is on a three-way switch. (The two lights are to go on or off together at all times.) Due to circumstances that are long, elaborate, and I think unrelated to the real question, I find myself with an option for a properly routed hot wire, but without a corresponding neutral one.

There is however, a physical piece of romex that runs between the old and new lights, for which I could fully disconnect the ground wire and have it isolated to just the run between the two lights. (I have a solid ground opportunity for the new light separately). Then I could theoretically utilize that isolated length of ground wire inside the romex sheeting for my neutral.

I would tape the bare ground as it emerged from the romex sheathing at each end carefully with white tape and label it as neutral. Electrically, the circuit design would be completely proper. The only difference in the physical connections in the circuit would be that the new neutral wire wouldn't have its own sheathing inside the romex as the black, white, and red wires do.

The concerns I would have, and the best mitigations I could propose, are:

1) Issue: If there were sufficient load on that leg of the circuit, the bare-wire-inside-romex neutral might create heat issues that a true neutral, with it's own white sheathing, wouldn't have. Mitigation: The installation calls for just the single 75-watt lighting fixture to utilize this neutral, which might not be a worry, though I'd check into that too.

2) Issue: Future owners or electricians might be mislead or confused by nonstandard use of the ground wire. Mitigation: I would include folded documentation of the wiring kluge in the light fixture for future owners who might get the idea they want to add a row of halogen track lights at that end of the circuit.

With all that said, clearly the right thing to do is to rip out some sheetrock and run a new proper wire. But if the lack of insulation inside the romex isn't an issue for a neutral, I'd still consider the kluge.

But I'm looking for experienced eyes to say "Yeah, you should do it the right way, but theoretically there's no reason not to do what you propose" vs. "No, it flat out won't work or is dangerous for _____ reason that you haven't thought of."

Thoughts? Thanks, Allen