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Substation Entry- Short section of cable in duct

Martin Walker

Hi I am in the middle of a design for a new substation and one of the supplies is circa 150m long and will run for 146m on a ladder in free air. The final 4m will be routed underground into the substation through a duct then the substation trench into the LV switchboard. 

when calculating for this if I say the cable is just on the ladder then I need a single 240mm2 cable, however if I say it’s buried in ducts then the cable requirement becomes 3 x 300mm2 cables in parallel which seems extreme in practical sense. 

does anyone have any thoughts on this or experience of a similar situation and how to comply with BS7671 whilst still being practical.

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    An effective 900mm2 vs 240mmm2 seems one heck of a de-rating (!) - or the 240mm 4 core is undersized, what are you assuming about cooling and ambient temps in the ducted section, vs assumptions in the open air run ? - and related you do not mention the design current ?
    In terms of cable ratings there is nothing magical about the fact it is a substation so the same rules apply as if it was a sub-main to a building.
    Note that the current ratings in the BS7671 annex are guidance and for a few example situations, they are not the last word in possible cable selection, mounting methods  and rating, and there are corner cases that are not a good fit to these 'reference methods' There are additional documents, based on work originally done by ERA that fill some of the gaps that may be worth looking at.

    An article about that
    electrical.theiet.org/.../

    regs Mike

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  • 0

    An effective 900mm2 vs 240mmm2 seems one heck of a de-rating (!) - or the 240mm 4 core is undersized, what are you assuming about cooling and ambient temps in the ducted section, vs assumptions in the open air run ? - and related you do not mention the design current ?
    In terms of cable ratings there is nothing magical about the fact it is a substation so the same rules apply as if it was a sub-main to a building.
    Note that the current ratings in the BS7671 annex are guidance and for a few example situations, they are not the last word in possible cable selection, mounting methods  and rating, and there are corner cases that are not a good fit to these 'reference methods' There are additional documents, based on work originally done by ERA that fill some of the gaps that may be worth looking at.

    An article about that
    electrical.theiet.org/.../

    regs Mike

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