duct bank

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smarkv

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doesn't the concrete incasement around an electrical duct bank and the fact it's installed underground add to the derating factor.what am I missing here where does the concrete derating come in.
mark
 
Re: duct bank

Mark
You are correct. The installation of a duct bank does cause the cable to be derated. If you look at
figure 310.60 it shows several installations of duct. Then look in Annex B and begin with table B310.5. It lista the derated cable ampacities for the duct banks shown in 310.60. There are several post in the engineering site about this. These
cable derations are cacculated using the Neher
McGrath method. It is a very complex heat transfer problem rather that an electrical one.
 
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