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Cable Sizing Derating Factors

M 90

Hello Gents,

I work in a project where an emergency diesel generator set is used to supply critical loads in case of normal source failure. As this generator set is located outside the building where the cable route shall cross a main road and as per the client request, the cables shall be laid inside a concrete encased duct bank along the whole route. My question is, Is there an installation method in IEC/BS for such installation ( concrete encased duct bank buried in ground) because I have found is ducts buried in ground directly without concrete or cables laid in in floor concrete trough or in building void. I also thought about calculating the total derating factor with conventional ways but I was a litte bit stuck with how to calculate the soil thermal resistivity factor. Also the grouping factor, as the cables will be laid in 2 or three layers.

Thanks in advance for your help

  • +1

    Depending on the length of the run the cable size may be set by current rating or voltage drop. It is quite likely that the exact arrangement you want to use is not quite  one of the standard arrangements. REF to the BS7671 reference installation methods

    And the odd article on what to do when it isn't for buried cables.

    However you should be able to interpolate between situations that are similar. There are also a lot more cable arrangements considered by the ERA (try ERA 69-30  ) among others, although that information is not freely available.

    If you can sketch what you doing and post that, it will be easier for others to advise without getting the wrong end of the stick as it were.

    Also it may be worth asking the moderators to move this question to the wiring part of the forum as it will be read by more relevant folk there.

    Mike.

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