Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 29 replies
  • Subscribers 32 subscribers
  • Views 3387 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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
  • 0
    well its meant to be a discussion forum not a Q and A session with only one answer, so if he is anything like me he'll ignore it. It will go away....

    M.
  • 0
    mapj1:.


    BUT it sounds like no RCD at the origin, so be aware it is not ideal, you need some sort of RCD covering the socket and in the current regs to rely on an RCD socket to do this is strictly a non-compliance.


    Mike




    This needs expanding, the issues of having RCD sockets rather than RCD protection at the consumer unit:



    • You are relying purely on an over current device to disconnect an earth fault, so the earth loop impedance must be lower than if the circuit was 30 mA RCD protected.

    • There may be an issue with cables concealed in walls.

    • There may be an issue with cables within metal stud walls.

    • As Mike says RCD sockets are strictly a non-compliance and the proposal is to add another one to the existing circuit rather than upgrading the circuit protection by adding a 30 mA RCD.

  • 0
    So are you saying running an swa outside to feed an rcd socket isn’t compliant?
  • 0
    It's the position and type of RCD, a RCD socket outlet that's not listed in the regs and it's at the end of the circuit rather than the start. 


    It will not be particularly dangerous, but there's issues with doing it that way. 


    I suspect most electricians would consider doing it that way.
  • 0
    MrJack96:

    So are you saying running an swa outside to feed an rcd socket isn’t compliant? 


    Yes. Odd though it may seem, they are not a type of RCD that is approved in BS 7671. However, I certainly haven't rushed to change my 15th Edn installation at home. I have simply replaced defective SRCDs with new ones.


    If you search through the Forum, you will find quite a bit of discussion on the topic.


  • 0
    Well there’s the next problem it’s fed from  a Wylex fuse board with plug in type fuses. ??
  • 0
    MrJack96:

    Well there’s the next problem it’s fed from  a Wylex fuse board with plug in type fuses. ??


    Let's get something clear. BS 7671 is not retrospective. If the owner doesn't want to replace the DB, but is happy to pay for an SRCD, that is the safest option. Go with it!


  • 0
    This is new work, not a repair, so the bit about not being retrospective doesn't have anything to do with the requirements.
  • 0
    Nothing wrong with a good solid Wylex CU or indeed rewireable fuses - they're still recognised by BS 7671 after all. If you need additional devices (RCDs, SPDs, AFDDs) they can always be added in separate enclosures rather than replace the CU, if preferred.


    The problem with 13A sockets with built-in RCDs is that standard they comply with - normally BS 7288 - as BS 7671 now stiplates exactly which standards an RCD need to meet and while all the usual DIN-rail mounting types (e.g. BS EN 61008 and BS EN 61009) are on the list, BS 7288 isn't (and them that know tell us that it's deliberate, not just an oversight).


    You might find a SRCD that also complies with one of the listed standards, or alternatively fit a DIN-rail enclosure next to or including the socket - and provided there was no requirement for RCD protection upstream - those would be compliant solutions.


      - Andy.

Community Rules & Guidelines