Cable sizing for new shed

Discussion in 'Electricians' Talk' started by psell1, Nov 1, 2022.

  1. psell1

    psell1 New Member

    Hi all, hoping for help sizing a new electric cable.

    The cable will run for 25m internally (direct from the CU), then 75m underground to the top of the garden to a new shed/cart lodge structure. Usual power tools in there, including a welder, nothing out of the ordinary.

    The current shed cable runs 25m internally, but then around 30m underground externally, is 2.5mm T&E then SWA (unsure on size, but think it's 2.5mm 2 core) - the lights tend to flicker if I'm running the welder or chop saw - possible a voltage drop given the length?

    Currently thinking I need to upgrade to 6mm or 10mm? Is 10mm enough/too much?
     
    Last edited: Nov 1, 2022
  2. arrow

    arrow Screwfix Select

    There is no need to join the cable, go to a wholesalers and get the swa 100 metres long or longer if required. Split the tails and fit a metal clad switch fuse of appropriate size and run swa straight from there, forget about running a bit of t&e and then joining on to it.
     
  3. psell1

    psell1 New Member

    Thanks for the quick reply. Unfortunately the internal portion needs routing through lots of nooks and crannies, so SWA isn't really an option. Plus it would be more expensive. I think it will be easy enough to joint using the existing junction box.
     
  4. arrow

    arrow Screwfix Select

    If you can get a t&e under the floor you can get an swa under the floor. Why put an unneeded joint on the cable for the sake of a few quid.
    Get an Electrician who is capable of running the swa the full distance, instead of unnecessary joints that could be a problem in the future.
     
  5. Coloumb

    Coloumb Screwfix Select

    I've not looked at this kind of thing for a long time but if you want to put a welder on a 100m run I would think 2.5mm is way under-specified. Having a little play with a volt drop calculator suggest that would only be good for a 1kw load before you start to get v.d. problems. You would also need to RCD the twin which isn't really permitted for a sub main. Then you would have to notify as its a new ctt. You would be better off getting a spark in to do this all for you.
     
  6. chesterw

    chesterw Well-Known Member

    A 2.5A swa supply over that distance would allow a total load of around 5A, and would need 5A RCBO circuit protection. And increasing the load, will cause an increase in volt drop which will reduce the effectiveness of the circuit protection.

    Meaning that in an electric shock situation you could be severely injured or killed.

    To avoid this situation, get an electrician to do the job properly, also its a legal requirement that the work is certified.
     
  7. Coloumb

    Coloumb Screwfix Select

    Why would it reduce the effectiveness of the circuit protection?
     
  8. chesterw

    chesterw Well-Known Member

    Read post #6
     
  9. Coloumb

    Coloumb Screwfix Select

    I have and I still don't see how it would. Could you explain pls?
     
  10. chesterw

    chesterw Well-Known Member

    The more resistance the lower the current flow, protective devices correct operation depends on low resistance and high current flow
     

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