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How many cables are you allowed to run through a single hole in a wooden floor joist?

We have a question and good answers for the NEC guidelines. Why would an inspection company flag this and note that there is an "overheating" chance?

Note that these wires are on the same circuit and oversized (just going to canned lights).

Is there any new guidelines I am missing here?

DMoore
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    Some inspectors look for what is really "wrong". Some are on more of a quota system: If I can't find at least 20 things wrong, covering at least 4 categories (electrical, plumbing, structural, roof, etc.) then the buyer will think I'm not thorough and that they're not getting their money's worth. – manassehkatz-Moving 2 Codidact Mar 05 '21 at 22:29
  • Yes there is a change I remember reading it and thought oh great more nanny state because it makes no sense when you can have a 60% fill on a 24” nipple with no problems whatsoever, you must be on the 2020 code. Was there x 3 with ground conductors? I will try and find it. – Ed Beal Mar 05 '21 at 22:38
  • Current code is 3 cables per hole. Some inspectors allow 4 cables. Home inspectors have to "prove their value" like @manassehkatz-Moving2Codidact said. I did a major remodel on one of my rentals a few years ago and decided to sell it. New windows, flooring, cabinets, doors, trim, new roof, gutters, even re-did all the insulation and vapor barrier in the crawl. The flooring throughout was LVT, high end stuff. Inspector said in the bathroom a waterproof flooring product should have been used. LVT IS water proof. ....my rant is continued in the next comment.... – George Anderson Mar 05 '21 at 23:20
  • There was a wood deck that had some damaged boards that I replaced and "painted" the entire deck with a sanded deck paint, the inspector thought it was TREX. Then he called out the gutters, they were BRAND NEW, but there was a minor wind event and blew a few leaves off a nearby tree that ended up in the gutter, he called out that regular cleaning hadn't been done. It was OBVIOUS the gutters were brand new and the leaves fresh, not decayed or rotten in the least. So I fully agree with @manassehkatz-Moving2Codidact, there are conflicts of interest. – George Anderson Mar 05 '21 at 23:24
  • @ George Anderson, I think you are thinking of a home inspection for sale those guys are regularly WRONG on electrical. I think the question is on electrical inspection, but I agree and I regularly slam them hard and have in court multiple times. – Ed Beal Mar 05 '21 at 23:31
  • @manassehkatz-Moving2Codidact - for sure. I go through these things all the time. It is frustrating seeing things like a nail pop in the garage ceiling (to attic space) listed as a safety hazard... But this is electrical code where there shouldn't be much gray area... not residential building code or safety code which differs by city, state, time of day... – DMoore Mar 06 '21 at 01:08
  • @EdBeal - this was buyer's inspection. The city - which is beyond thorough did their and didn't say a word about the wiring (it was permitted and they had signed off on it). I do have another great "new" question from my city inspection... will load it up. – DMoore Mar 06 '21 at 01:15
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    As I said the buyers inspection those guys do not know electrical code very well at all. There is no reason that it could be flagged. – Ed Beal Mar 06 '21 at 02:10
  • @EdBeal -- yeah, my understanding most HIs get a very basic sketch of electrical stuff from their HI training and that's it, its up to them to further study the Code and work practice. There are folks out there who are working to help them get better, but it's uphill for sure – ThreePhaseEel Mar 07 '21 at 01:41

2 Answers2

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What they're probably trying to flag...

The usual thing that gets cited for this is the second paragraph of NEC 334.80 (text from 2017, but this hasn't changed much -- 320.80 is a newer extension of it that applies to AC/BX):

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.

...and why their cite's a swing and a miss

However, even though the 24" exception (which is the exception cited in the quote above) is ruled out in this case, because ampacity adjustments on NM cables are permitted to be made from the 90°C column ampacity in the table by the first paragraph of that very same section:

The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The allowable ampacity shall not exceed that of a 60°C (140°F) rated conductor. The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final calculated ampacity does not exceed that of a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable trays shall be determined in accordance with 392.80(A).

As a result of that fact, we can start with the 25A 90°C ampacity of 14AWG copper (or the 30A figure for 12AWG copper), and then start adjusting it accordingly, based on the number of cables (really, current-carrying conductors) we have present:

  • 3 /2 cables (6 CCCs): 80% adjustment, so 20A > 15A (or 24A > 20A)
  • 4 /2 cables (8-9 CCCs): 70% adjustment, so 17.5A > 15A (or 21A > 20A)
  • More than that: 50% adjustment, so 12.5A < 15A = now the adjustments limit ampacity

So, unless you're cramming 5 cables (whether /2 or /3, in most cases) into a single hole, or using archaic cables that lack 90°C rated insulation (all modern NM is NM-B, which means that the wire insulation is 90°C rated), then you're good to go.

ThreePhaseEel
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  • It was a rough inspection this last house had by buyers... Lots of things I was like ... what the hell (and they missed a few obvious things). This one made me blow my lid. I was like did I not check the NEC 2025 guide? It's dumb on so many levels. The option is drill a bunch of holes in your joists? – DMoore Mar 06 '21 at 01:12
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I believe the inspector may be interpreting the new 320.80.A wrong. When more than 2 armor clad cables are bundled together and installed so they are in contact with thermal insulation, calk, or sealing foam, and an air gap is not maintained between those cables to dissipate heat the ampacity of each conductor shall be adjusted according to table 310.15.C.1.

I asked the question why not 3 or more because 8 CCC of nmb, ac, mc will all still meet code for ampacity because they are all 90 deg rated. The person came back with what if they are not 90 deg I said check code they are required to be in the articles covering these wiring methods he then was stuck.

So make them cite it to write it, call your local building codes office and ask as this is probably being wrongly remembered as my comment above was wrong but I remembered bundling BS as I said.

Ed Beal
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