Can I run the ground wire without conduit to sub panel in the same building

Question

First, thanks for all the excellent advice on this site.

While wiring an auxiliary heat strip to the 125A sub panel in my basement, I noticed that the sub panel does not have a grounding wire from the main panel. The main panel is 25 feet away in the basement. I would like to add the ground wire. The neutral bar on the sub panel is not bonded; I will add the grounding bar. I am in North Carolina. Here are my questions:

1. Can I run the grounding wire on it's own zip tied to the whip carrying the hot and neutral wires (I cannot pull any wires through the whip as it is pre-made)?
2. I'm planning on using #6 wire. Is that sufficient for the 125A sub panel?

Answer 1

In turn:

1. Yes, you can run the grounding wire on its own as per NEC 250.130(C) point 5. Also, since you are using a #6 wire, it can be run unprotected as per 250.120(C), although Bare Armored Ground cable may be a better bet to keep dimwits with wire cutters at bay. However, zip-tying the new grounding conductor to the existing feeder cable is a violation of 300.11(C). You're better off simply fishing the wire where it needs to go, or having it take its own route to the panel.

2. Yes, 6 AWG copper wire is an adequate grounding conductor for a 125A subpanel -- in fact, 6 AWG copper is good all the way up to 200A. However, if you had aluminum or copper-clad wire instead, you'd need a 4AWG wire for the job.