Kentucky, USA; 2017 NEC / NFPA 70
I live in a house built in the 50s that was originally wired with ungrounded outlets. Many parts of the house have been rewired, but a room that I'd like to use as a home office still has only ungrounded receptacles. I'd like to convert at least one of these to a grounded receptacle.
The receptacle in question is on the 1st floor, on an exterior wall; is fed by cloth sheathed conductors (not in conduit); and is contained within a metal outlet box. In the basement just below, there is a grounding electrode conductor (GEC) of bare copper, probably 6 or 8 gauge, running within a few feet of the outlet. I think it will be possible to fish a cable from the GEC to the location of the outlet, following the same path as the existing conductors.
What I'd like to do is keep using the existing conductors for hot and neutral, and run a separate equipment grounding conductor between the GEC and the outlet, as allowed by NEC 300.3(B)(2) and 250.130(C)(2). I'm planning to use 12-AWG solid green copper THHN wire in accordance with NEC 250.122A (size), and 310.6(C) and 250.119 (insulation/covering and color). I haven't found any code requirements as to stranded vs solid or to insulated vs covered vs bare. I decided on solid and insulated because I thought it would better withstand the fishing process and being nearby the existing conductors. I'm planning to replace the old box with a PVC old work box.
How do I connect my new ground to the GEC?
I think the answer is probably a split bolt connector, sized based on the larger wire, not insulated or wrapped, and not in a junction box. But I haven't had much luck finding code citations that support that choice.