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I need to attach a few 10,12,14-ga NM cables perpendicular to the floor joists in my unfinished basement. They were originally attached to the sill plate. I am doing some insulating&weatherizing around the sill plate, so I need to move them out of the way. They are already crowded on the 1.5" of the plate, plus I might want to add more wiring later.

I may not just staple these NM cables to the bottom of the joists per NEC 334.15 "Exposed work"

(C) In Unfinished Basements and Crawl Spaces. Where cable is run at angles with joists in unfinished basements and crawl spaces, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables shall be run either through bored holes in joists or on running boards...."

I do not want to drill holes in my already weak 2x8 joists. It is unlikely I will ever want to finish the ceiling in that part of the basement. Conduit is too much trouble.

The simplest solution would be NEC-permitted "running boards" but I cannot find any description of what it is. May I use strips of 1/2 or 3/4 plywood, i.e. attach plywood to the bottom of the joists, and staple the cables to the bottom of plywood? Or should I use something sturdier like 2x4's?

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2 Answers 2

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Running boards or rat runs many times are a simple 2x4. If you have a bunch of cables 1/2” plywood is legal and meets code standards for support of cables #10 and smaller.

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  • Can you just staple the cable under the board? It doesn't have to run on top of the board like a wooden cable trough right? And, if you are just going from one joist bay to the next can you simply pass underneath? It would seem pointless to add a "running board" only 1.5" long.
    – jay613
    Oct 12, 2021 at 20:12
  • @jay613 code requires a running board when perpendicular to the joists for smaller sizes see NEC 334.15.c , so pointless only if you ignore the code requirement the larger sized cables have conductors not smaller than 6 awg with 2 conductors or more and the other is 8 awg with 3 conductors or more so in reality cables larger than 8 as there will be 3 conductors # 8 at a minimum for residential wiring. This is for exposed, when it comes to a ceilings ceiling that will be covered a running board to the sides is required and the NM needs to be an 1-1/4 away from the boards see NEC 300.4 .D
    – Ed Beal
    Oct 13, 2021 at 2:30
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Even 1x2 or 1x3 furring strips should be sufficient to service as running boards. As far as I can find, there's no specification as to what "counts" as a running board for construction or for the National Electrical Code.

The only thing that is mentioned is, when running along the side of a joist, you need to keep NM cable at least 1-1/4" away from the bottom or top edge. If that did apply to running boards (which is debatable), that would mean the minimum width would need to be the half inch or so that the NM cable will likely be plus 2-1/2", or 3" total.

The point of the requirement for a running board is so that, if you run into the wiring (either with yourself or with something you're holding), or hang something from it, the electrical cable isn't the thing providing the support; something else has to be supporting it (e.g. a board that the cable is simply attached to).

Use 8d or larger nails or an equivalent size screw (e.g. GRK R4 screw (a #9 structural screw) or even #8 deck screws, e.g. from Deck Mate) to fasten the running board to the underside of the joists.

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