Innerduct Current Carrying Conductor DeRate

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billyzee

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If I have a 4" outerduct with three 1" innerducts. Each inner duct has 3 current carrying conducts. Do I have to derate as if all 9 current carrying conductors where in the same conduit
 
Innerduct and Power Conductors

Innerduct and Power Conductors

OK so innerduct is generally used for teleco and communication cables. However, does the NEC disallow it's use for power conductors? If not how does the derating work? Can I consider the current carrying conductors in a single innerduct and then derate or do I need to look at the everything within the outerduct and then derate. I have used Maxcell flexible "condom" type innerduct in the past but it is very expensive. Any more suggestions for more economical vendors. Also, maybe the answer is that innerducts aren't "listed" for power conductors, but I don't believe that is the case.
 
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if the wiring method isn't listed under chapter 3 as an approved wiring method, then its not allowed by the NEC for power wiring (unless you are talking about class 2, 3, or another power limited method, or a chapter 8 method)
 
Yes, I realize that power conductors in an innerduct is not common. In fact I have yet to find a manufacturer that "lists" and inner duct or multi-cell systems for use with power. I have a customer that is enamoured with the idea of putting the power conductors in inner duct just like they do the communications. Thats as far as they've thought it through. At this point I am still in the "please customer mode" and have decided that the NEC doesn't disallow it provided a listed product exists. Therefore, I am also wondering how would the derating be handled if I could find a innerduct system for use with power. I have concluded that I would have to look at the entire system and not just the individual innerducts. Either way if anyone has specific information about this matter it would be helpful.
 
It's an easy answer, power and lighting wiring must be installed per a chapter 3 method. innerduct is not a chapter 3 method.
 
There is indeed an innerduct listed for power applications. there are several manufacturers offering a listed product.
UL Listed HDPE is compliant with the 2005 NEC Article 353
and is listed to UL 651

At this point, listed duct is the exception, and not the norm. i have seen it gaining more acceptance and being written into more specs recently. the ul listing adds to the price, adn the difference between listed and non is minimal. of the duct i sold last year for power applications, very little of it was ul listed.

Lance
 
since the innerduct isn't addressed in the code, I would have to guess that you would be required to derate the conductors just as you would if the innerduct was not installed at all (unless the innerduct is hdpe in which case it would not be allowed inside the building period)
 
Even if you use legal chapter raceways such as FMC, is there a prohibition on running two raceways inside each other (say three 1" FMC's inside a 3" EMT)? If not, how do you derate? I would think each of the smaller FMC's would be looked at individually. Is there a FMC fill limit for EMT? Is a small raceway inside a larger one secured and supported?
 
Even if you find a listed product for this application, I think you need to derate based on the total number of conductors in all of the innerducts because you will not be maintaining spacing between them as required by 310.15(B)(2)(b).
Don
 
I question the whole installation, but you do have 9 current carrying conductors in a raceway & subject to derating of 70%. When cables are installed in a raceway you count the ccc in each cable. Would a raceway in raceway be any different???
 
don_resqcapt19 said:
. . . I think you need to derate based on the total number of conductors in all of the innerducts because you will not be maintaining spacing between them as required by 310.15(B)(2)(b).
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That?s debatable, Don. The rule says to maintain spacing, but does not say how much spacing to maintain. If two sets of conductors are only a half inch apart from each other, but are kept at that same half inch distance for the duration of a 100 foot run, I would say that spacing was maintained. If you have a raceway within a raceway, then the inside raceways will maintain the spacing of the conductors they surround.

By contrast, if you put two 3-phase home runs in the same conduit, there is no way to control the distance any conductor will be from any other conductor, anywhere along the run. I think this is what is meant by ?maintaining spacing.?
 
mistermudd said:
Would a raceway in raceway be any different???
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If anything, I should think it would be worse than having all conductors in a single raceway. The idea of derating for multiple conductors in a raceway has to do with the ability of any given donductor to reject its heat to the surrounding environment. With a large number of conductors in the same area, two things happen. The first is that there is a large number of heat sources. The other is that with the area surrounding any given conductor now being at a higher temperature (due to heat from other conductors), it becomes harder for that one conductor to reject its heat. If you are trying to get rid of heat, it is easier if you are surrounded by cold.

If you have one set of conductors in a raceway, the heat is trapped within (until it can find its way - via air or dirt - to the outside world). But the heat generated inside one inner duct has to make its way not only to the outside of that inner duct, but also has to make its way to the outside of the outer duct. I think that complicates the heat removal process, and results in higher temperatures within the inner ducts.

I conclude that, as a minimum, you must treat this as a single raceway, for derating purposes. It might even be a better design practice to derate a bit more, just as an added safety factor.
 
Charlie,
The code section that I cited requires that spacing be maintained between conduits, tubing or raceways.
(b) More Than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.
Click to expand...
Yes it is very debatable as that is all the subsection says and it only implies that derating would be required if spacing is not maintained and it only does this by its location in the code.
Don
 
Well since I started all this I guess I should post one more time. The customer is adamant. They want a 4" outer pvc "stick", with three inner pvc "sticks". The conduit will be laboriously glued together in the dead of winter. Then we will proceed to fill each 1" up with wire. A parallel setup for communication is planned. So I am going to agree to the whole thing and derate the wire's as if they were all stuck in one big 4" conduit.

If anyone can point to specific NEC articles that stop me I'd like to know because I can't find anything that says this won't be up to code. Also I wouldn't mind hearing an air tight arguement about why I can derate based on the inner duct only. Anyway, it seems dumb, expensive and labor intensive with very little gain if any. At least it's Friday
 
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