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Cables exiting LV Switchpanel into concrete trough below

fiftyhertz

Hi All,

Ive inherited a job, where we have a large LV Switchpanel sitting ontop of an open concrete trough.

I have approximately 18No. 95mm/120mmm SWA 4 Core cables feeding panel boards exiting into the trough below from the switchpanel for about 3 meters before it exits the trough and below the building but still above ground level (the buildings on stilts) onto a tray.

I am just struggling to find the correct de-rating factor in BS7671, 117-120 doesn't seem to quite fit the bill, but 119 seems to be the closest. But cant see it working, especially as it recommends 75mm apart, witch is 18 x 75mm = 1350mm and a maximum of 6 cables

This must be a reasonably standard arrangement, but i am stumped.

Any help would be much appreciated.

Thanks

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    well 60% of full current is (I squared R) 0.6* 0.6 of the heating so  about 0.36,  and  temp rise drops pro-rata more or less with dissipation.  You can have almost as many cables as you like horizontally spaced by two diameters or more, adding more runs to the edges does not really affect the middle ones. Not true vertically where the poor so and so at the top gets all the heat of all the cables below it so adding more keeps making it worse.

    Grid/ mesh floor /lid is good so is wider trench. Less clear on your insulated earth mat idea, that I may have mis understood

    Mike

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  • 0

    well 60% of full current is (I squared R) 0.6* 0.6 of the heating so  about 0.36,  and  temp rise drops pro-rata more or less with dissipation.  You can have almost as many cables as you like horizontally spaced by two diameters or more, adding more runs to the edges does not really affect the middle ones. Not true vertically where the poor so and so at the top gets all the heat of all the cables below it so adding more keeps making it worse.

    Grid/ mesh floor /lid is good so is wider trench. Less clear on your insulated earth mat idea, that I may have mis understood

    Mike

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