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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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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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