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I am re-doing the wiring in my home and I have encountered a situation where I need to run 4 sets of 12-2 NM-B (Romex) wire from my attic down to the same fixture. Per 334.30, I am allowed to leave cable free-hanging where it runs through a finished wall per section 334.30 B-1 of the NEC:

334.30 Securing and Supporting. Nonmetallic-sheathed cable shall be supported and secured by staples, cable ties, straps, hangers, or similar fittings designed and installed so as not to damage the cable, at intervals not exceeding 1.4 m (4 1⁄2 ft) and within 300 mm (12 in.) of every outlet box. junction box, cabinet, or fitting. Flat cables shall not be stapled on edge. Sections of cable protected from physical damage by raceway shall not be required to be secured within the raceway.

(A) Horizontal Runs Through Holes and Notches. In other than vertical runs, cables installed in accordance with 300.4 shall be considered to be supported and secured where such support does not exceed 1.4-m (4 1⁄2-ft) intervals and the nonmetallic- sheathed cable is securely fastened in place by an approved means within 300 mm (12 in.) of each box, cabinet, conduit body, or other nonmetallicsheathed cable termination.

FPN: See 314.17(C) for support where nonmetallic boxes are used

(B) Unsupported Cables. Nonmetallic-sheathed cable shall be permitted to be unsupported where the cable:

(1) Is fished between access points through concealed spaces in finished buildings or structures and supporting is impracticable.

(2) Is not more than 1.4 m (4 1 ⁄2 ft) from the last point of cable support to the point of connection to a luminaire or other piece of electrical equipment and the cable and point of connection are within an accessible ceiling

However, this also leaves me with some concern about code compliance regarding "bundling" or "not maintaining proper spacing" of the multiple wires within the wall cavity without having to derate the ampacity of the cables. Therefore, I am looking for a code compliant way of running the wires without being required to derate the ampacity of the cables.

The (vague) definition of bundled from section 520.2 of the NEC and an article that speaks to "not maintaining" spacing for NM-type cabling.

520.2 Definitions.

Bundled. Cables or conductors that are tied, wrapped, taped, or otherwise periodically bound together.

...

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15.

...

310.15 Ampacities for Conductors Rated 0–2000 Volts. ...

(B) Tables

...

(3) Adjustment Factors.

(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. 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). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

If absolutely necessary, I could cut a small (1-gang or 2-gang sized) hole on the wall near the ceiling to secure the wiring, but this would be non-ideal and I cannot seem to find any solutions that would prevent my bundling problem for running along the framing member in any event. I have considered options like this multiple cable staple, but it seems to only aid with article 300.4 d (keeping multiple wires 1.25" away from the edge of frame members), but makes no claims about aiding maintaining proper spacing between the wires for runs greater than 24" as I cannot find anywhere that "proper spacing" is defined for NM-type cabling.

Of course, maybe I am worrying about this derating for business for naught. I am running the four cables to power lighting and one switch that I am using requires a direct connection to neutral. That said, I would still like to know if there is an NEC-approved (year 2008 or newer) way to run the four wires without having to derate them.

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  • You can throw out the bit from article 520, unless you're working in a theater, motion picture or television studio, or a similar location.
    – Tester101
    Aug 12, 2013 at 18:05
  • @Tester101 I pulled the definition of bundled from section 520 because the word bundled is referred to in other sections without being defined. I would hope the NEC would not use multiple definitions for the same word within the same document (particularly since I cannot find a definition earlier in the document).
    – user14416
    Aug 12, 2013 at 18:07
  • Explaining why this odd situation, and the size of the loads, would help.
    – Bryce
    Aug 12, 2013 at 18:44
  • The size of the loads should be relatively small. I am running wire for light switches. Ultimately, derating the wires should not be a problem. That said, since I do not plan to live here forever and having seen some crazy wiring additions by the previous home-owner, I want to make it as difficult as possible for someone to unintentionally burn the place down. I am certainly willing to be practical, but if there is a solution that is easily code-compliant (consistent with the intent of preventing the fire hazard the code was written for) that doesn't require me to derate, I am very interested.
    – user14416
    Aug 12, 2013 at 18:53

1 Answer 1

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The derating would only occur if there is a section 24 in. long or more, where the cables are "bundled" together (from my understanding).

"without maintaining spacing for a continuous length longer than 600 mm (24 in.).

If the cables are run through a joist cavity, they'll be "bundled" for 1 1/2" - 3" at the top plate, and 1 1/2" - 3" at the bottom of the wall. Each of which is far under the 24 in. limit. Where the cables dangle through the cavity, they are not wrapped, taped, or bound together. They are only "bundled" for a short distance while they enter/leave the cavity.

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  • While I like your interpretation of the code and can certainly argue its merits, I am not sure 100% of inspectors would agree based on their own interpretations. Free-hanging wires (assuming they are installed without a lot of slack) would still be in relatively close proximity for the 6(-ish) feet they are run in parallel within the wall cavity. I guess the definition of "maintaining spacing" (diy.stackexchange.com/questions/30595/…) is needed for a guaranteed code compliant answer with no room for additional interpretation.
    – user14416
    Aug 12, 2013 at 18:44
  • I am accepting this answer based on the lack of a formal definition of "maintaining spacing" in the NEC. This interpretation is as valid as any other. If worse comes to worse, for the loads I have on the wires, the amount of derating required should still be up to code regardless.
    – user14416
    Aug 13, 2013 at 13:08

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