4 cables in one hole

[rlfrazee]

Im currently reading Wiring Simplified by H.P.Richter, W.C. Schwan and F.P. Hartwell. In Chapter 11 they talk about ampacity restrictions in NM-B cable and how many cables can be run thru a series of single holes they say; "although the individual wires must, by NEC rule, have a 90 degree C temperature rating, the final ampacitiy must not exceed that given in the 60 degree column. This restricts the number of cables that are "bundled" for longer than 24". This includes routing more than one cable thru a succession of single bored holes. You need to consider this requirement in a basement, for example, where you might be tempted to run large numbers of cables back to the panelboard through a set of holes lined up through the floor joists and ending at the panel. As a practical manner, putting NEC 310.15 (B) (2) (a) together with 240.4 (B) (1) under this restriction means that for common applications you cant put more than 4 cables together through a common set of holes." Well I sort of understand the 60 degree restriction based on overcurrent protection limit on 14 12 10 awg wire as note to table 310-16 directs you. I dont understand 240.4 (B)(1) about fixture wire? Can someone explain this to me and how they come up with the 4 cables in one hole limit....RL

[suemarkp]

The 4 cables in one hole is the same idea as how many individual current carrying conductors you can put in a conduit. When you have between 7 and 9 conductors, you have to derate the ampacity by 70%. For #12 wire rated 90C, its ampacity is 30A. Seventy percent of 30A is 21 amps. This is the maximum number of #12 90C wires you can run in a conduit and still maintain 20A ampacity. The rule works the same for #14, but from #10 and larger, you need to recheck where 70% takes you. May need to backoff to 4 to 6 conductors with the 80% derating.

The same is true with bundled NM cable. You can have up to 9 current carrying wires, and since there are always at least 2 in a cable, four cables is the limit before you decrease the ampacity of the cable below the typical 60C rating or mandated maximum breaker rating.

If you're using 12-3 cables and the whites are true neutrals, then you can still have 4 cables in holes (the neutrals don't count against you). If the 12-3 cables are 3-way switch segments or switched/unswitched wires in the same cable, then you could only run 3 of these cables before having to breaker below 20 amps (neutrals count and 3 cables x 3 wires is 9 current carrying wires).

[Wgoodrich]

Two key points seem to be missing in the original post if this is what that author wrote.

First deration as suemarkup pointed out in his reply ignores NEC 240 and allows you to start your deration calculation in NEC Table 310.15.B.2.A using the 90 degree column amp rating for THHN conductors that are found in NM-B Romex. This means you start your ampacity at a higher rating than listed in NEC 240. Then once you have derated per the TAble found in 310.15.B.2.A you then must compare that derated answer to the ampacity limit found in NEC 240. You must use the lower amp rating of the comparison between your derated answer and the maximum allowed in NEC 240. YOU DO NOT PERFORM THE AMPACITY DERATION OF NEC TABLE 310.15.B.2.A USING THE AMPACITY RATING OF NEC 240. YOU START YOUR AMPACITY RATING USING THE COLUMN FOR THE TEMP RATING OF THE INSULATION OF YOUR CONDUCTOR IN THIS CASE BEING THE 90 DEGREE COLUMN THEN COMPARE THE TWO MAX AMP RATINGS AND USE THE LOWER AMP RATING OF THE TWO COMPARED.

Next key point is that if you are using bored holes through floor joists spaced 16" on center then chances are those cables are spreading apart between the two joists. This would not meet the definition of cabling. Definition of cabling would apply to cables tightly bundled limiting the ability to air cool where the separate. If you have bored holes through reptetive spanning lumber a lot would depend on the size of the bored holes and how loosely the cables are fit through those holes. If those cables are bundled tightly even between the joists then that author's concern is valid. If the holes are large enough to allow the wires to pass through the bored holes loosely and separate while between the repetitive spanning lumbers 16" or more separation then cabling in this condition of loosly spread cables between the joists would not meet the definition of cabling or bundling thus not being a concern. Tightly stuffed holes with wires pulled tight YES cabling may be of concern, loosely stuffed hles with cables loosely spreading between the joists NO concern.

Hope this helps

Wg

[rlfrazee]

Thanks Mark and Wg, I have for the most part worked out the deration of wires as Mark has explained and understand where the authors were going with the deration factor. However I understood the authors to mean "bundled" or "not bundled." It was this second part that was confusing me. The original post is a qoute of what the authors wrote. Reason I asked was first time I had heard anyone say 4 cables in a hole if not bundled, as I interpreted them to imply. By writing "this includes routing more than one cable thru a succession of single bored holes" to me says whether cabled or not. Matter of interpretation I guess. Anyway what you explain in your second key point ,Wg, is exactly what I thought they were not addressing. This subject has been discussed before in the forum and this was first time I had heard anybody say a definite number of wires in a hole. Gets me curious to find reason for their thinking. Gets me wondering if its one of those things "I didnt know, I didnt know" as Wg states so often...thanks.....RL

[Wgoodrich]

It would be hard to declare maximum number of wires in a bored hole. Are we talking about a 3/4" hole or a 2" hole. It would make a difference how tightly these cables are bundled. The key point is if the cables are able to spread not heating each other from their own heat without heat dissapation into the air void between the joists. Bundling can only occur if tightly bundled longer than 24". Otherwise heat can dissappate out the end of the short length of bundled wires.

Just my thoughts

Wg

[suemarkp]

Why does hole diameter affect the bundle. Won't the cables all sink to the bottom of the hole anyway? You're right that cables between studs typically don't snug up to the adjacent cables in that same run, especially when you want them nice and bundled for neatness. Also, wouldn't an insulated wall compound the heat problem from these wires, especially if you cut a single slit in the insulation to contain the bundle?

Finally, bundles can be quite different in size. My last house had a huge bundle of cables run in the attic along the bottom chord of the roof trusses. This bundle must have had at least 12 cables in it going to various places in the house. I would think the center of that bundle would be hotter than the cables on the outside of the bundle. No bored holes though, so it must be legal! However, I'd be hard pressed to consider 5 cables too much bundling to compromise heat dissipation (but logic doesn't always affect inspectors around here).

[Wgoodrich]

If the bored holes are tightly stuffed the cables can not relax between the two joist and spread out in that joist space. The hole with several cables say a 2" hole still allows the cables to relax enough to separate and cool not heating each other.

A tightly stuffed hole when cables are pulled through will retain the tension keeping the cables tight not allowing them to relax and separate.

Next time you see a stuffed hole versus an oversized hole look at the cables and how they lay between the joists.

Cables laying in an attic still will relax allowing air flow to cool. If those cables were tightly bundled by use of wire ties then you have a problem.

Insulation has not been mentioned to be a problem in cables heating each other. Not sure you don't have a good point there just never heard any comment. NEC allows Romex to be installed in insulation but forbids knob and tube wiring to be installed in insulation.

Hope this helps

Wg

[rlfrazee]

Just for information purposes this is the latest book of 'wiring simplified 40th edition'. Very common in the diy section at Lowes, Home Depot etc.. What we have been dicussing is in chapter 11 page 123. Not knocking the book for this, lots of varying interpretations of code, I learn something in everything I read. Just wanted to explore the subject and maybe add to everyones understanding of this subject.....RL

[Wgoodrich]

This post touched on a short paragraph in a complete set of writings by an author. While he said it right he only touched on the surface. The quote he said is correct per NEC rules concerning cabling and bundling. However only the AHJ can declare cabling and bundling concerns. Much like what is and is not subject to physical damage. The condition and level of concern with that certain bundling would make the call by the AHJ. If the cables are laying tight unable to expell heat without heating the neighboring cables then that AHJ will flag it.

Picture an inspector walking through a house. He sees a hole bored through joists that are laying loose thus separating clearly between the joist. While the 24" minimum is not reached before the cables separate anough in his opinion allowing the cables to cool not in his concern heat build up conditions yet that hole has 4 cables in it and the diameter of the hole is much larger not creating a tight stuffing or tightly laying cables. You as an electrician would have a fit if he rejected saying oops 4 cables in that hole. Find the number of cables in the hole maximum in the NEC and you will pull a miracle because the NEC does not say it that way.

If that inspector said oops these wires are taught not allowing the to separated between joists due to so many wires stuffed in a small hole creating a tightly bundled condition falling under the deration rule then you would be hard pressed to conflict with that ruling.

Bundling and cabling is a call of an AHJ much like subject to physical damage. Condition he or the electrician sees will vary by hole size, how tightly stuffed, how taught unable to relax cables between joists all would make this call yet different in each installation as discribed.

Author is not wrong but if you put the entire pages together of this discussion we got in detail on a subject that author just touched on.

Didn't mean to say he or she was incorrect, just meant to point out the subject is much deeper than a paragraph in that book. That author just touched in general on the subject without getting deep in to conditions that can exist.

Just my opinion.

Wg

[rlfrazee]

Wg I agree with your summation of what author intent was. I wasnt saying he was wrong either would have to write pages and pages of text to explain subject and every situation. My code knowledge is very minimal so would be rather foolish of me to question the author of an electrical book. Plan on taking a few semesters of code classes at local education center starting this summer. Its great when you relate a real life situation to code interpretation. Anyway knew author wrote just small paragraph about the subject he was addressing and much more to it for sure. My intent was purely to clear up some of the "fog" I quite often get trying to understand code and maybe generate some interesting discussion to help all who read the post.....RL

[Wgoodrich]

RL, you talk of taking an electrical class, yet you are in a real life class following this forum. When you take that formal electrical class I suspect you will be leaps ahead of most in that class because of the knowledge you have gained and been exposed to within this forum. I suspect you have learned already much more than you think just being an active part of this forum dicussion group.

One leap ahead for you.

Wg

[rlfrazee]

Wg you cant believe how this forum has increased my knowledge and understanding of electrical......RL