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Cables buried direct in ground...

Martyn

Hi Folks,

I have been giving a bit of thought to direct buried cables and was looking around for the ‘Best’ method for installation in reference to current carrying capacity especially with the rise in copper prices. 96A vs. 152A on something as small as a 25mm 4c or 365A Vs 592A on a 300mm 4c.

The differences between ERA & BS7671 figures are significant and they seem to come down to actual installed conditions, for example ERA have an assumed thermal resistivity of 1.2k.W/m vs. that of BS7671 which uses 2.5k.W/m.

I got to thinking is there an installation method that could be more readily assured to be better in terms of thermal resistivity?

Damp compacted sand would seem to be the best for decreasing thermal resistivity, but how much? would a liner in the base of the trench be of any use to reduce drying or migration of the sand or might this just add to the overall thermal resistivity?

I read this earlier https://ictinternational.com/casestudies/underground-power-cable-installations-soil-thermal-resistivity/ 

And then what about the ‘New’ Conducrete? could encasing in such material have significant benefits or might this risk damage with settlement?

Thoughts?

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  • 0

    AJJewsbury: 
     

    96A vs. 152A on something as small as a 25mm 4c

    (I think the 152A is for single phase 90 degrees C)

    Presuming the cable comes above ground to be terminated - there's also the (usually) clipped direct and 70-degree conductor temperature limitation - say 110A for 25mm2 4C - which might limit any benefits from improving the CCC of the buried section.

            - Andy.

     

     

    Both figures were ’run at 90’ 3ph, the only difference being installation method. 
     

    Clearly we cant run at 90 for most standard switchgear though…

    I have 25 4c run to 70 in ground as 82A to standard method…

     

     

Reply
  • 0

    AJJewsbury: 
     

    96A vs. 152A on something as small as a 25mm 4c

    (I think the 152A is for single phase 90 degrees C)

    Presuming the cable comes above ground to be terminated - there's also the (usually) clipped direct and 70-degree conductor temperature limitation - say 110A for 25mm2 4C - which might limit any benefits from improving the CCC of the buried section.

            - Andy.

     

     

    Both figures were ’run at 90’ 3ph, the only difference being installation method. 
     

    Clearly we cant run at 90 for most standard switchgear though…

    I have 25 4c run to 70 in ground as 82A to standard method…

     

     

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