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Topic Title: Underground duct woes
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Created On: 04 December 2013 09:59 PM
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 04 December 2013 09:59 PM
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timothyarnold

Posts: 52
Joined: 12 January 2012

Hi All,

I'm looking for some advice please

I have five existing underground ducts between the switchgear room and some external generators. The duct distance is around 35 metres but the total run is around 65 (on ladder rack). I am struggling to get information on the original installation of the ducts so can't confirm bedding etc at this stage.

The ducts already have existing cabling to the generators (500mm singles) and I am looking to refactor and add additional to provide 2MVA/2800 Amps

Going through the cable calculations I get a Cg of 0.72 and Amtech suggests 8x500mm singles per phase.

I'm not that concerned with the number of cables but the termination at both ends would be a nightmare to say the least. I would also be concerned with the trefoil formation as I am not sure how this would be easily maintained in the duct during cable pull.

The alternative is to run new cable around the outside of the building and the distance increases to 90 metres but I only require four singles per phase. Bargain!

As I have some existing cabling that I could re-use the ideal option is to use the ducts but I am not happy with the de-rating. Can anyone offer any other suggestions?

Thanks
Tim
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