Multiple cables/one hole

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  #1  
Old 01-19-07, 04:56 AM
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Multiple cables/one hole

I don't keep a copy of the NEC around as I rarely have a need for the numerous details. Came across this one yesterday during my rough electrical inspection in my basement remodel. I had 3 romex cables (14/3) running through the same set of holes through my framing timbers(3way to 3way on different walls). He didn't pitch a fit or anything - but asked that I drill separate holes for each cable and rerun the wiring. I've never run into this one before and thought I'd ask if it was "code" or his interpretation of something. Don't get me wrong - it's an easy fix to keep him happy since he signed off on the work - and, in my experience - that's what the inspection process is all about.
 
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  #2  
Old 01-19-07, 05:24 AM
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The NEC does say that when more than 3 current carrying conductors are in the same raceway or cables (like romex) are bundled leaving a total number of current carrying conductors over 3 we need to de-rate the conductors.

For example 1ea 14-3 and 1ea 14-2 would be 5 current carrying conductors.
We do not count the ground, or the neutral on a MWBC.

Three ea 13-3 would be 9 CCC. The table in the code says we need to de-rate the conductors to 70 percent of the original value. (4-6 is 80 percent, 7-9 is 70 percent 10-20 is 50 percent) To calculate this we take the value in the 90 deg columb for the wire size we plan to use on the corrrect chart from the code, and multiply by the adjustment factor. If the value in our answer is lower than the breaker we had chosen, we can either use a smaller breaker or a larger wire and try the calculations again. The other option, as your inspector said, is that we can re-run the wires to eliminate the need to de-rate.

All that being said, your inspector was wrong. You have 3 each 14-3 or 3 each 12-3. The calculation for de-rating starts in the 90 deg temperature columb. For 14 wire that is 25 amps and for 12 wire that is 30 amps.

Either one would still be above the original breaker size at the 70 percent mark, and therefore not need to be re-run or de-rated.

Someone once asked, if equipment or other rules never let you use X size wire above Y amps, why is there even a 90 deg columb. This is the answer. It is the starting point for reduction calculations.
 

Last edited by jwhite; 01-19-07 at 05:36 AM.
  #3  
Old 01-19-07, 06:28 AM
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I generally try to run no more than two cables through a hole. I do this because on a two by 4, at least, the hole cannot be too large, and running three cables through it might damage the cables.
 
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Old 01-19-07, 06:30 AM
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JW - I'm confused. I thought derating only applied to current carrying conductors contained in a traceway or conduit. I was under the impression that derating did not apply to NM conductors run through studs/joists.

I just helped my daughter install some lights in her basement. The electrician that wired the house when it was built drilled 1 1/2" holes though the joists and ran about about 6-8 NM cables the full width of the basement. I added a 14-2 NM cable to that run. The inspector didn't mention anything about derating. Did he miss this, or does the NEC requirement not apply?
 
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Old 01-19-07, 06:56 AM
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310.15 (B) (2) 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 stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(2)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a currentcarrying conductor.

That is from the 2005 NEC. The 2002 reads basicly the same.

I am not sure what year this became a requirement.
 
  #6  
Old 01-19-07, 07:34 AM
jn
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Wayne, I've seen many new houses wired similar to the way you descibe.
Many contractors (in my area) have switched to the engineered trusses that look like I-beams. I've seen sets of 2" to 3" holes run in line from one end of the house to the other.
All the wiring is paralleled through these sets of holes and then branched off as needed.
When I asked the local inspector about this trend and if he was derating, he said not as long as the holes weren't packed tight and the romex could be fanned out between the trusses so that each had air around it.
No cable ties or tight stacking.
In other words, as long as you could easily wrap your hand around a cable and slide it from truss to truss, he wouldn't derate.

With that said...I imagine any other inspector could interpret the code different and not allow this.
I must admit, it looks really nice.
Safer would be to put each in its own hole and separate everything.

JWhite--just wondering how you would interpret this kind of installation if you were inspecting it?
 
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Old 01-19-07, 08:06 AM
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jn,
I have been meaning to look up what the UL White Book or other sources have to say on this issue. Some pros think that this rule is completey bunk, others swear that it needs to be more strict. I would like to know how many fires have been found to have started due to bundleing of wires.

The only definition of "bundle" that I can find in the code is in article 520. Since the way the code is written (article 1-4 are the general rules and articles 5-9 modify or explain in more detail with regard to specific applications) this definition would not apply here.

It would seem from the wording of the section that I quoted that seperation would need to be maintained in some way in order for this to be legal. Any type of stopping material could not be used for this, as there is another section that says if you use fire stop you need to derate.

In my opinion it would be safe to do what you suggest, say in a basement or other place where air is not likely to get too hot, but you may have a problem if all the wires are choked above a panel where pass through the framing before splitting into different directions, or in a hot attic space.

Since I am not a member of the CMP (code making panel) my opinion only counts if I send it to them for review. Even then it would not be my decision to make.

BTW I also believe that 60 mph is safe on most roads that are posted for 55, but I can still get a speeding ticket.

If I were the type of person who would do something that was not 100% code compliant knowing what I was doing was safe and effective, I would still not be the type to put such infomation in writing and sign it, or post about it on a web forum for all to read.

My professional opinion is that we can do one of two things, follow the code or make suggestions to the CMP to have changes made.
 
  #8  
Old 01-19-07, 09:08 AM
jn
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Well said jwhite...
Now that I'm once again thinking about this issue, what are the techniques being used to avoid "bundling" when you exit the main panel and try to get up to the ceiling in that 9' high basement?
My inspector recommends using large electrical PVC from the top of the box (on each side) until above the ceiling line. This would almost certainly be a bundle wouldn't it? I would have to measure...maybe it is almost always less than 2' since the panels are real long and have a lot space in the top to hook up the feeder wires.
I also wonder if the use of "stack-ems" to stack your multiple cables would suffice as separation as you go up a wall. (Stack'ems are those clips that have 4 or so separate openings to hold down your wiring.
I'm just wondering what you all are seeing out there to avoid this...
 
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Old 01-19-07, 11:00 AM
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"..... or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways,"

Is this statement the key to the ambiguity? Passing cables through a 1 1/2" long hole in a joist certainly doesn't exceed the 24" limit.

What is the required "spacing" for NM cables?
 
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Old 01-19-07, 11:50 AM
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jn,
What I usually do is make the carpenter mad. The top plate above the panel, and the joist that sits on it look like swiss cheese when I get done drilling. Some have even taken the extra effort of boxing out an area above the panel for me, when they found out what I planned to do.

I then install dead wood from stud to stud above and below the panel so that I can staple my wires to it.

Yes, stack its count as the required spacing, but usually the panel has too many wires even for them.

Wayne,
I agree that there is some ambiguity there. I believe however that this part comes into play "without maintaining spacing". The 14 1/2 inches in between joists leave no provision to strap the wires so as to "maintain spacing". I also agree that there is no definition of the required spacing. I read this to mean any space as long as they are not touching. But, just like when you mix different systems in a control panel, the required space needs to be maintained. Just folding the wires into position and hoping that no one will ever bump into them again is not enough.

-----------------

While I understand having an academic discussion on this topic in order to better understand the code, I have never seen this as a huge issue with regard to the entire installation.

Larger cables like number 10 and up, most always are easier to install in their own hole to start.

Since we can start de-rating from the 90 deg c column, one would need to have over 9 ccc in a run for there to be a problem. That is 4 two conductor or 3 three conductor cables.

With a bit of proper lay out at the beginning of the job, it takes little more time to drill the extra holes needed. That time is usually made up for when one saves time because the wires are easier to pull.

So, from where I sit it appears that installations that meet code the way I am interpreting it is not more than an academic discussion.
 
  #11  
Old 01-20-07, 12:34 AM
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I just saw a recent install with about nine 12/2 cables thru a two and one-half inch hole, run for about 25-feet. At that point several of the cables branch off, while a few of them continue for another 30-feet or so, and then branch off further. No derating was done that I'm aware of.
 
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