Duct bank offset derating

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JJWalecka

JJWalecka

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New England
Why is there a need to derate 4100 Volt conductors in a duct bank that is offset below an obstruction? The offset would bring the duct bank below 30 inches to grade.


JJ
 
The deeper you go, the harder it is for the heat to get away from the conductors. The tables are based on a 30" depth.
 
JJWalecka said:
It doesn't get cooler to a certain depth?

JJ
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Heat is dissipated from the conductors following the principles of heat transfer by means of conduction and convection. Although the ground may be cooler at a deeper distance the cables still have to dissipate heat to the surrounding area. The main means of dissipation of this heat happens from the surrounding air in the atmosphere so the deeper the cables are the less heat is dissipated into the atmosphere because of the greater distance.

Think of this heat dissipation in terms of electicity. Think of the heat that needs to be dissipated as current and the depth of the earth the heat has to be dissipated through as resistance. Therefore the deeper the cables are the more earth (resistance) there will be and therfore less heat dissipation (current0 will take place. The earth heat transfer capibility (resistivty) is measured in rho and is different for different materials, soil, concrete, etc....

There is a method called the Neher McGrath method that explains how these calculations are performed for determininted heat dissipation and cable deratting. There is also software such as Ampcalc that assists with these calculations.
 
Look at Appendix B that describes how to estimate ampacity of cables in underground duct banks.

B310.15 (B)(3) (b) "Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6% per increased foot of depth for all values of Rho can be utilized." (Slight editing)

You may want to consider part (a)

"Where burial depths are increased in part of an electrical duct run to avoid underground obstructions, no decrease in ampacity of the conductor is needed, provided the total length of parts of the run increased in depth to avoid obstructions is less than 25% of the total run length."

While Appendix B is optional, it has some good engineering behind it. (It can also be misapplied easily).
 
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