Underground electrical duct bank (de-rating??) 2

[ziggyfrst]

I have sched 40 rigid PVC (NEMA TC-2) underground duct bank buried in sand. Double tiered, some 6", some 4". It was buried mostly touching both vertically and horizontally. (no cradles were used) Some places it is wired to re bar with the wire completely encircling individual and/or multiple ducts. I have duct bank crossings
(East/West--North/south) interseting that are physically touching. There is up to 3.5' grade differences between vaults. (yea, standing water and sand) I don't know much about, and am having a hard time referencing de-rating factors. Voltages are 15 KV, 5 KV and 480 V. What do you guys think about this? By the way, I appreciste your input to my last thread.

 

[davidbeach]

Look at the IEEE Brown Book, it has a section about duct bank rating studies.

 

[umrpwr]

That is what cable ampacity software is for. I would strongly recommend that you either purchase a program capable of doing the calculations you describe or hire someone who can perform them for you. Some cable manufacturers offer that as a service. To try and perform the calculations you describe by hand would be difficult at best. The better cable ampacity programs like CYMECAP will also you to run different scenarios and get a reasonable estimate of the performance under various conditions such as the one you have described. It is standard practice at the company I work at to run cable ampacity calcs for all underground feeders.