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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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    Hi Martin,

    Did you find any supporting information that allows you to ignore the derating factor for short lengths of cable?

    SSEN G81 guidance allows 15m of cable in a duct for every 100m before derating is applied. Not sure if this comes from the ERA 69-30 method.

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  • 0

    Hi Martin,

    Did you find any supporting information that allows you to ignore the derating factor for short lengths of cable?

    SSEN G81 guidance allows 15m of cable in a duct for every 100m before derating is applied. Not sure if this comes from the ERA 69-30 method.

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