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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.)

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  • Usually if below 8 feet from the floor, the cables need some type of protection. Either running boards, conduit, drywall, so the cables are not used as clothes lines, people hanging from them. I am not sure if this means in crawl spaces, so your inspector might be right.. Tape does not seem to be meant for a long term use.
    – crip659
    Commented May 11, 2023 at 21:05
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    I would not consider a crawl space to be "exposed work"., unless the crawl space is inhabited (by people). Same thing I think would apply to NM runs in an attic.
    – SteveSh
    Commented May 11, 2023 at 21:22
  • I think the rationale for a crawlspace is that personnel who are unlucky enough to have to do work in that tight quarter (raises hand) should not bump into the hanging cables, so the cables need to be tied up and out of the way. A running board would do that, but I argue that straps on every joist the wires cross over could also do that.
    – hacktastical
    Commented May 11, 2023 at 23:41
  • Agreed about the need to fasten cables. Mine are just fastened to the underside of the joists in some places, while other run through holes in the joists. No running boards.
    – SteveSh
    Commented May 12, 2023 at 2:03

2 Answers 2

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Given what code says, for full compliance I would get some 1x lumber and do the running board approach with staples. It will be simple and relatively inexpensive, plus won't require you to rerun anything.

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  • In practice, unless there is crawl space hockey or some activity like that we're not aware of, this exercise is more likely to damage the cables than whatever it is meant to protect them from
    – jay613
    Commented May 11, 2023 at 21:57
  • For sure. That's why I say 'for full compliance'.
    – KMJ
    Commented May 12, 2023 at 4:46
  • And I ended up doing exactly that, and replacing all the plumbers tape with plastic strap to reduce scuffing.
    – hacktastical
    Commented Apr 16 at 22:48
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I would use these. enter image description here

You screw them to the joist with the cable out of harm's way. After that they are adjustable and reusable.

Doing a lot of nailing and stapling around the cables in a space that will see little to no future activity seems unproductive.

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  • Those look like something you'd use for Cat-5 or Cat-6, not NM cable. The material I'm considering is plastic plumber's tape held by screws.
    – hacktastical
    Commented May 11, 2023 at 22:14
  • Do you mean plastic pipe strap? That would work, but seems fussier. Plumbers tape to me is Teflon thread tape :)
    – jay613
    Commented May 11, 2023 at 22:35
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    If I'm already in the crawlspace with my screw gun, seems like the same effort either way.
    – hacktastical
    Commented May 11, 2023 at 23:43
  • One other issue, that also applies to plastic pipe hanger tape: thermal. Plastic tape is usually used with DWV pipe, but may not be rated for what wire can handle (e.g., 85 deg. C.) So that would favor doing running boards and staples - then there's no question.
    – hacktastical
    Commented May 12, 2023 at 0:33
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    If your cable is heating a plastic cable tie to 85C through its outer insulation in an open space you have bigger problems than choice of support method.
    – jay613
    Commented May 12, 2023 at 0:50

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