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The original wiring (1994) is along the best route where wires are stapled at the top of the interior wall plate above the finished drywall. I want to run 4 new circuits, but there is no room to staple new wire, and I don't wish to drill all the floor joists. Can I staple new wire to the bottom of a running board mounted perpendicular across the floor joists and about 6 inches away and parallel to the original run?

This is finished basement with an unfinished ceiling and the run area is where the plumbing, air etc. also run across the joists and hang well below the proposed running board. I consider to be not subject to damage. While unlikely that the ceiling will ever be finished in this area, any ceiling would have to be installed at least 6 inches inches below any wiring to clear the existing services.

If so what would be the required staple spacing on the running board? As an alternative, can I secure the new wire to the bottom of the running board using a plastic MultiCable Staple (NEC 300-4-(d) compliant) which snaps closed, or are these only for horizontal or vertical cable support, you know, not cable supported hanging down?

If I decide to drill the joists, can you give me a guide for the max number of cables (12ga NM) through one hole, what size hole for x number of cables gives good clearance, and avoids "bundling" concerns, and are any grommets and what kind are required though timber joists? I know about drilling center and so on.

Also, the original wiring occasionally put two wires under a single staple where room was scarce. Are max 2 wires under a staple still permitted when necessary, and would a preferred method be to stagger which 2 wires are under one staple, i.e. not run the same 2 wires continuously under one staple to avoid bundling over more then 24"? I live in Western MA.

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According to NEC 334.15(C) you can run NM cable on running boards exposed in a basement.

304.4(D) refers to running parallel to framing members but the stackable cable clamps could be consider support in this case. There is nothing in the code that I know of regarding the number of cables under one staple. You would have to refer to the manufacturer's instructions on the staples and their capacity.

The "bundling" you are referring to I believe is here:

310.15(B)(3) (3) Adjustment Factors. (a) More Than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a).

The question here is the intepretation of the word "spacing". I would argue having them on the same board next to each other or in one of those stackable clamps is properly spaced. The clamps are sold and apparently approved for just that propose.

And here:

334.80 Ampacity Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) and the provisions of 310.15(A)(2), Exception, shall not apply.

Where more than two NM cables containing two or more current-carrying conductors are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a).

Notice this is only when run through an opening to be caulked or insulated or in contact with thermal insulation. Most jurisdictions don't consider it bundling to run multiple cables through bored holes or on a running board together. However, others want only two cables or less through the same hole. Check with your inspector to be safe.

Good luck!

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