electrical ducts

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frenchcomp

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Annex B, NEC 2005 shows typical application with underground electrical duct bank configuration.
There are only 9 ducts max in figure B.310.2.
Which ampacities table can I use with a duct bank configuration of 3 horizontal rows with each 7 adjacent ducts (21 ducts total)?
 
Re: electrical ducts

I have NEC 2002, and in my annex B figure B.310.2, it is showing duct separation guidelines. I believe when you are talking about ampacities tables, you are referring to number of current carrying conductors in the same raceway, which is something different from B.310.2. The adjustment factors for number of current carrying conductors in a raceway can be found in Table 310.15(B)(2)(a). And this would apply if in the same electrical duct, you have more than 3 current carrying conductors together. I apologize if I am incorrect in my assumptions. :)
 
Re: electrical ducts

David, I think the question is addressing ampacities of the three conductors in each conduit, and not the de-rating related to having more than three current-carrying conductors in any conduit.

The simple answer is ?none.? There is no ampacity table that addresses a configuration other than those specifically shown in the figures.

I have already addressed the more complex answer in several previous threads. Start with my post of Dec 27, 1:44 pm, in this thread.
 
Re: electrical ducts

Wow, I was way off base :eek: Interesting subject! Has anyone heard of using a special "thermal backfill" instead of normal earth backfill for medium voltage applications to help disperse heating from underground conduits? We had a situation in Hawaii, where the Rho values calculated were extremely high, so the utility specified concrete encased PVC, and then used another concrete mix to fill in up to grade. These lines were utility owned and installed on an Air Force Base. The utility was saying this is what they had begun doing in all of their new underground intallations, because of problems they heard about relating cable failure to heat not being able to dissipate.
 
Re: electrical ducts

Thank to Charlie b
My client ask me to follow NEC 2005. If we still let 3x7 duct bank and perform cable calculation with software (under engineering supervision), cable section will be correctly sized but are we still conform to NEC 2005 :confused: ?
 
Re: electrical ducts

Yes. 310.15(C) has not changed in the 2005 version. That is the article that permits ampacities to be calculated ?under engineering supervision.? Please note (or I should say that this is my opinion) that the formula that is shown in that article is of no use whatsoever! :eek: It is not possible to look up the various variables, and punch them into a calculator, and come up with an ampacity. However, that is the basic formula that is built into the software packages that do this sort of calculation.
 
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