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Cable calculations

MrJack96
Hi guys i wanted to calculate the maximum run when spuring of a ring final circuit is there any equations or is it just as simple as calculating VD to the furthest point in the ring and then VD to the end of the spur. The reason for the question is I want to spur of an existing socket using a 10meter run of 2.5 for an outside socket. Just want to Double check it’s sufficient 


thanks guys
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    Mike Gorman:

    Hi, additional the advice in other posts, outdoors your outside the building equipotential zone so will possibly need to consider appropriate protection via an RCD and safe trip current rating  housed in IPXX box. If this is not done well garden equipment ( which are notoriously dirty ) could take down the buildings ring circuit inadvertently and therefore any equipment connected to it. While it might be annoying it would be prudent to run a radial from the FB in  4mm2 depending on max load and VD and short circuit calc.        


    There is no "equipotential zone" for protection against electric shock. It disappeared in the 17th Edition.


    The requirements for RCD protection for general protection against electric shock in TN systems are in Regulations 411.3.3 and 411.3.4 ... RCDs may be required for other reasons, such as where cables are concealed in walls at a depth of less than 50 mm, or where necessary for special locations in Part 7.


    That doesn't mean they can't be used if none of these requirements apply ... and I agree with Mike it's a good idea whether BS 7671 specifies it or not for the particular application.


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  • 0
    Mike Gorman:

    Hi, additional the advice in other posts, outdoors your outside the building equipotential zone so will possibly need to consider appropriate protection via an RCD and safe trip current rating  housed in IPXX box. If this is not done well garden equipment ( which are notoriously dirty ) could take down the buildings ring circuit inadvertently and therefore any equipment connected to it. While it might be annoying it would be prudent to run a radial from the FB in  4mm2 depending on max load and VD and short circuit calc.        


    There is no "equipotential zone" for protection against electric shock. It disappeared in the 17th Edition.


    The requirements for RCD protection for general protection against electric shock in TN systems are in Regulations 411.3.3 and 411.3.4 ... RCDs may be required for other reasons, such as where cables are concealed in walls at a depth of less than 50 mm, or where necessary for special locations in Part 7.


    That doesn't mean they can't be used if none of these requirements apply ... and I agree with Mike it's a good idea whether BS 7671 specifies it or not for the particular application.


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