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Cables Buried Direct

me1899
Hi All


Hope all are well.


I was wondering if I could ask for some advice on cables buried direct.


Basically I am looking at a proposal where 10No.35mm 3 core cables are to run from a Ryefield (60A fuses) in an external cupboard to flats about 50 odd metres away (internal runs about 20m).


Electrician is suggesting all 10 next to each other with no spacing at 600mm depth. 


I had a quick look for derating and saw that the tables in the Regs cover 6 cables max like that.


I would have thought that they could be split into 2 groups of 5 with a gap of say 125mm between and that would be OK since I am looking at the old BICC values of 154A (there are short ducted portions where entering/existing buildings).


Am I thinking right or is there something to consider further?


Regards


Muhammad

  • 0
    How on earth is he expecting to put them 600mm deep? Is there a duct in the wall? All sounds a bit odd. 35mm cables from 60A fuses sounds like a significant derating already, and one could also apply some diversity unless there is electric heating. Buried cables method 57 in concrete are rating C, so why not just spread them out? The short ducted sections may be ignored for derating unless filled with insulation.
  • 0
    What's the rating of the supply feeding the Ryefield? That'll likely limit the average current across the group of 10 cables (even if one is drawing a larger current at any given time the others must be relatively underloaded so the overall heat generation balances out to some degree) - it may be that grouping factors don't make a significant difference.


       - Andy.
  • 0
    Are these cables underground outside  or buried under a concrete floor indoors ? You do not need to spread cables very much to get them out of each other's thermal influence, but some spacing is sensible, but as noted, even if the 60A fuses got upped to 100A, the cables should be running pretty cool. (especially if you consider the likely after diversity maximum demand will be a lot less than 60A per cable averaged out over the time to heat up a some cubic feed of earth or cement)

    note that the formulae for 4,5,6 cables all start to converge on a constant deratring factor whcih is why the tables do not keep going, as for any one cable the heat from the nearest neighbours dominates, and a cable does not see much heating effect from the ones far away, "looking through" the more dominant effect of the immediate neighbours.

    Mike.
  • 0
    Hi all


    Many thanks for responses all. 


    The cables are to be buried in earth outside. 


    Ryefield is fed by a 315A supply for 9 gas flats and 5 ev chargers. Bit weird and have queried. 


    The lengths of duct are a couple metres either end. 


    Also did not mention that the contractor wants for the duct runs to have 2 cables per duct. 


    The 10 cables are for flats. The EV charger canles are near the incomer position and follow a different route albeit they will have to cross under at one point. 


    Regards 


    Muhammad

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