What is the best and/or code-compliant way to run NM cables on the underside of floor joists in a crawl space?
Background. My wiring, done as a retrofit in the 1970s, has a set of runs (five #12/2's and one #14/3) perpendicular to the floor joists, held in place with plumber's tape strapping roughly every 4' (3 joists.) Not ideal for sure.
Interestingly enough, the pre-purchase home inspector didn't mention this as an issue; they only flagged one run that had been added and not strapped up at all, making earth contact. (Yes, I know, home inspectors aren't electricians, but still...)
My understanding is that for this situation, code (NEC 334.15 Exposed Work) requires that I do one or more of the following for cables smaller than 2x #6AWG or 3x #8AWG:
- drill the joists and run the cables through them
- put the cables in conduit
- attach running boards to the joist underside and staple the cables to them
Note that this is a crawlspace, not a basement, so the issue of hanging clothes from exposed cable or covering in drywall isn't a problem. Critters gnawing on the wires? Might be, though I've seen no evidence of this.
I was thinking to replace the plumber's tape with plastic strapping, and add straps to every joist so that there is no sag. I think that would hold them well enough without risking abrading the sheaths. Would this be acceptable, or should I do the running boards + staples approach? (Not gonna do conduit or drill joists.)