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Cables in buried duct - Table 4C3.

ianlloyd
Looking at the Regs, particularly Table 4C3, it would appear that only one multicore cable is allowed in a buried duct (i.e. "single way duct"). The 'Rating Factor' seems to be applied to the number of ducts, not the number of cables.


I've seen numerous installations where more than one cable is installed in a single duct - is this acceptable and, if so, how is the grouping factor applied?
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    There is an example for multiple cables in ducts in section 4.3.5 (specifically, page 54) of the Electrical Installation Design Guide, 4th Edition.


    Grouping factor is applied as per item 5.1.2 in Appendix 4 to BS 7671:2018 (page 378), but note you should take into account the following correction factors using the expression if Equation 2 in item 5.1.2 of Appendix 4
    • Ca for the Assumed ambient temperature

    • Cs for the particular soil thermal resistivity

    • Cd for the specific depth

    • Cf if a semi-enclosed fuse is used

    • Cc (0.9 for a cable laid in the ground where overload protection is required).

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  • 0
    There is an example for multiple cables in ducts in section 4.3.5 (specifically, page 54) of the Electrical Installation Design Guide, 4th Edition.


    Grouping factor is applied as per item 5.1.2 in Appendix 4 to BS 7671:2018 (page 378), but note you should take into account the following correction factors using the expression if Equation 2 in item 5.1.2 of Appendix 4
    • Ca for the Assumed ambient temperature

    • Cs for the particular soil thermal resistivity

    • Cd for the specific depth

    • Cf if a semi-enclosed fuse is used

    • Cc (0.9 for a cable laid in the ground where overload protection is required).

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