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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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    If these cables are only to be used in a grid power failure, it MAY be acceptable to make rather optimistic assumptions about cable loading. For long term use, a limiting factor is the gradual heating up of the concrete and surrounding subsoil, not applicable for short term only use. Significant warming can take weeks or even months, how long will the power cut be ? And even if the cables do get hotter than normally allowed, this may be acceptable for short term use. Cable life might be reduced from say 50 years to say 5 years, and that is not 5 calendar years, but 5 years of power cuts. 

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  • 0

    If these cables are only to be used in a grid power failure, it MAY be acceptable to make rather optimistic assumptions about cable loading. For long term use, a limiting factor is the gradual heating up of the concrete and surrounding subsoil, not applicable for short term only use. Significant warming can take weeks or even months, how long will the power cut be ? And even if the cables do get hotter than normally allowed, this may be acceptable for short term use. Cable life might be reduced from say 50 years to say 5 years, and that is not 5 calendar years, but 5 years of power cuts. 

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