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Concrete Duct Bank Derating Factor

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

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    Although it saves digging to place the ducts one above the other, the rating will be higher if they are side by side - cables in the upper layers will get heated to a degree by those below. I read the diagram as 150mm dia ducts on 300mm centres so the whole block is 1.2m metres wide ?

    According to literature a standard density concrete, at a temperature of 20°C, has a value between 1.33 W/mK and 1.95 W/m.K

    ERA 69-30 report assumes a soil thermal resistivity of 1.2 K.m/W, which corresponds with typical soil thermal resistivity for the UK in Annex A (A.22.2) of BS IEC 60287-3-1 . BS7671 uses a figure of about twice the thermal resistance.

    So to a good first stab, the concrete is a little bit more thermally resistive than typical UK soil, but even so a better thermal conductor than sandier drier soils.

    One way to look at the problems is to calculate the total (all cables) watts per meter dissipation, and then use that figure for the heating at a distance, but then solve the ducts, as if in that higher ambient.

    Mike.

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  • 0

    Although it saves digging to place the ducts one above the other, the rating will be higher if they are side by side - cables in the upper layers will get heated to a degree by those below. I read the diagram as 150mm dia ducts on 300mm centres so the whole block is 1.2m metres wide ?

    According to literature a standard density concrete, at a temperature of 20°C, has a value between 1.33 W/mK and 1.95 W/m.K

    ERA 69-30 report assumes a soil thermal resistivity of 1.2 K.m/W, which corresponds with typical soil thermal resistivity for the UK in Annex A (A.22.2) of BS IEC 60287-3-1 . BS7671 uses a figure of about twice the thermal resistance.

    So to a good first stab, the concrete is a little bit more thermally resistive than typical UK soil, but even so a better thermal conductor than sandier drier soils.

    One way to look at the problems is to calculate the total (all cables) watts per meter dissipation, and then use that figure for the heating at a distance, but then solve the ducts, as if in that higher ambient.

    Mike.

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