Spur from a ring main

Discussion in 'Electricians' Talk' started by Antdaz68, Nov 28, 2008.

  1. Antdaz68

    Antdaz68 New Member

    I've just read replies to a similar query but am confused!!


    I assumed that if you could spur 1 double socket off a ring then 2 singles at the same point would be the same net effect
  2. Charred Sparky

    Charred Sparky New Member

    I think the reg's only allow you one double socket, but the jury would be out, to interprit this as allowing two daisy chained singles!
  3. Charred Sparky

    Charred Sparky New Member

    More considered, I think your comment about the net effect being the same is correct, because ultimately the ruling factor would be the rating of the 2.5T/E cable feeding the first socket,that could see a max load of 26A. This would be the same for either double or twin.
  4. seneca2

    seneca2 New Member

    An unfused spur may feed one single or one twin 13a socket or one piece of fixed equipment, so say the regs.
    You can however daisy chain more than one socket from a spur if you feed through a fused connection (spur)unit first,
  5. sinewave

    sinewave Screwfix Select

    Might be prudent to 'hang fire' with the advise on ere for a while Charred Sparky being as your other post admits to being newly qaulified and all. ;)

    Then again there's * on ere who spout all sorts and have no quals at all! :^O

    [Edited by: admin]
  6. seneca2

    seneca2 New Member

    Do you dissagree with the advice I gave him then sine?
  7. Home Farm

    Home Farm New Member

    Most double sockets are rated at 16A not 26A.

    A short length of 2.5 will carry 26A for short periods under ideal conditions but is quickly derated according to installation method.
  8. unphased

    unphased Screwfix Select

    My understanding of the situation is that only one unfused spur is allowed from a ring circuit, this could be either a single or a double socket. However, if you install a fused spur on the ring then you can spur unlimited sockets from that on a radial. This rule applies to standard circuits as defined in the OS Guide.

    So to answer the OP's question no you cannot spur off a spur as you describe.

    I think the reference to single or double sockets is misleading, it doesn't matter. Regs. prohibit this.
  9. JP.

    JP. Screwfix Select

    Then again there's * on ere who spout all sorts and have no quals at all! :^O



    [Edited by: admin]
  10. Eye Squared R

    Eye Squared R New Member

    Appendix 15 (Informative) of the 'Red' Book only allows a single unfused spur however by taking the initial spur to a Fused Connection Unit (FCU) in 2.5mm^2 you can then use 1.5mm^2 to daisy chain as many sockets thereafter.

    I don't see this non-compliance as a major issue and BS7671 is not Statutory when it suits . . . .
  11. unphased

    unphased Screwfix Select

    This official use of 1.5mm2 cable off a fused spur for sockets is unusual. I think it came in with the 17th edition. It is protected by 13A fuse so obviously safe but woner why it was never officially recognised in earlier regs. Or was it?
  12. pcelectrics

    pcelectrics New Member

    Or was it?

    Yes, it was in the 16th OSG - but I'm too lazy to find the page number on a Friday evening. IIRC it was guidance rather than an actual reg.

    I believe the reason that one can use one 2G skt as a spur but not two 1Gs is, in part, that a 2G skt is considered a single point of utilisation and it is highly unlikely that anyone would plug in two continuous 13A loads (although the cable can just take it if RM1 and so can good quality sockets). Two 1G sockets, especially if a couple of meters apart or more, would represent two points of utilisation and so the risk of full load or overload is higher particularly if the RM isn't 1 (derated).
  13. JP.

    JP. Screwfix Select

    1.5 after FCU -

    Maybe it was not recognized in earlier regs due to the CSA of the 1.5 CPC?
  14. JP.

    JP. Screwfix Select

    For example:- the FCU is run within the Ring Final and the 1.5mm is taken from the FCU to appliance socket etc.

    A 13 Amp fuse is fitted in the FCU.

    Under fault conditions ie a dead short across phase and CPC and considering pre-arc characteristics of the 13 Amp fuse - will the Adiabatic equation hold good with respect to the 1.5mm?
  15. Coloumb

    Coloumb Well-Known Member

    It's not allowed as it wouldn't take much for some idiot to replace the single socket at the end of the chain for a double one.
  16. Antdaz68

    Antdaz68 New Member

    Thanks for all the responses. Whilst probably perfectly safe in my proposed format I accept that "some idiot" could replace the end socket with a double at a later date (+ it appears the latest regs dont allow it) ......... hence a 13 amp fused switch it will be!
  17. sinewave

    sinewave Screwfix Select

    Do you disagree with the advice I gave him then sine?

    Nah, bang on that mate!

    (I wasn't referring to you earlier!) ;)
  18. seneca2

    seneca2 New Member

    Thanks for reply sine.
  19. unphased

    unphased Screwfix Select

    The closing comment from Ant suggests he has been mislead by the single socket v. double socket references.

    There is a general opinion on this forum that a single socket is okay whereas a double socket is not in instances of spurs off ring circuits. Where are people getting this information from? Without checking the OS guide, it does not matter whether the socket is single or double. You are allowed one unfused spur off a ring circuit using conventional circuit design where diversity has already been taken in to account when sizing the cables. This seems to be overlooked by the vast majority. A double socket is absolutley fine on a single unfused spur.

    So to refer to spurs to single or double sockets is misinforming the OP. The fact he has two sockets spurred from a single unfused connection to the ring is wrong. Please look at the OS guide the people who are referring to single sockets. IF I am wrong then I aopologise but I will check later. ;)

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