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Is there a maximum length for a spur cable?

Former Community Member
Former Community Member
Good afternoon, hope you are all well.

Just two quick questions.

Is there a maximum length for a 2.5mm twin and earth spur cable terminated in a ring final wall socket?

Is there a maximum length for a 2.5mm twin and earth spur cable terminated in the Consumer unit RCBO which also supplies a ring final. I understand that there can only be one spur cable from the RCBO. So max three cables in RCBO. 


As always, many thanks in advance.

Simon

  • Former Community Member
    0 Former Community Member

    simon201:


    I understand that there can only be one spur cable from the RCBO. So max three cables in RCBO. 


     




    Hi sorry to be blunt but that’s incorrect, you can have more than one spur from an RCBO, the limitation comes from how many conductors can be  confidently and competently connected into any given terminal :)

  • 0

    simon201:
    Good afternoon, hope you are all well.

    Just two quick questions.

    Is there a maximum length for a 2.5mm twin and earth spur cable terminated in a ring final wall socket?

    Is there a maximum length for a 2.5mm twin and earth spur cable terminated in the Consumer unit RCBO which also supplies a ring final. I understand that there can only be one spur cable from the RCBO. So max three cables in RCBO. 


    As always, many thanks in advance.

    Simon

     




    Hello Simon,

                               see if you can have a look at Appendix 15 (Informative) of B.S. 7671. It is recommended that an unfused spur can be connected at the origin of the ring final in the distribution board or consumer unit, or from a socket outlet or from a junction box connected to the ring final circuit.  The unfused spur should supply just one single or one double (twin) socket outlet. It should be wired in 2.5mm2 P.V.C./P.V.C. T&E cable. There is no limit to the length of a spur, but Voltage drop considerations should be taken into account. It is not recommended that large loads like immersion heaters, large space heaters or similar loads be connected to the ring final circuit. It is unlikely that a spur will ever be too long in the average U.K. house.


    Edit. B.S. 7671 defines a SPUR as a branch from a ring or radial final circuit.


    Z.

  • 0

    It is unlikely that a spur will ever be too long in the average U.K. house.



    But might be if it's running down the garden to the shed....


    The OSG lists maximum cable lengths for rings for various situations - if you're within a quarter of that length for your spur originating at the CU you should be fine.


       - Andy.

  • 0
    Section 7.2.2 of the OSG says 'as a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring'.


    F
  • 0

    AJJewsbury:




    It is unlikely that a spur will ever be too long in the average U.K. house.



    But might be if it's running down the garden to the shed....


     

     




    If it's running down the garden to the shed it is not IN the average U.K. house.


    Z.

  • Former Community Member
    0 Former Community Member
    Hi everybody, thanks for taking the time to reply, as a non-electrician, I'm learning a lot.


    The spur from the socket is at the mid/far point of the ring final and will be approx 2m long, serving a home computer, which is turned off most of the time. The spur from the RCBO will be about 8m long as it will be running in loft space, so will definitely need to be metal clipped. It will be serving the TV and Sky box, which are also turned off a lot of the time. To be clear, I'm just running the cable in my small bungalow (78m square)  and I will get an electrician to connect everything as mentioned in my earlier post today about collapse of cable in floor voids in bungalows.


    Kind regards to you all

    Simon
  • 0
    Sounds perfectly sensible, and not worth reaching for the calculator for.

    A free voltage drop calculator where you put your load in, and it tells you the thinnest cable you could use is
    on the TLC website here .

    Even if you put in double the length, you have plenty in hand - another (rather rough) rule of thumb is that if you need two reels of 2.5mm (i.e. more than 100m of cable) for a ring and its spurs, then voltage drop on load needs careful thought. Much less than this, no thought at all.


    So you know what to expect, the TLC site has some info (based on an older version of the regs, so the RCD stuff is not right)here and linked pages

     

    There is a small risk that the existing ring actually is not, and is really a centre fed radial - while the covers are off and it is isolated, it is worth making a ring-round test to verify that it really is a ring (an ohm meter test from L-to L at any place where the ring can be interrupted, and then N-N and E-E). Depending how much hacking has been done before you got there, not all 'rings' actually are...


    Spurring from the consumer unit is fine by the regs, but be aware that many consumer units are a bit full, and in that case, placing a junction box nearby may be easier to do.
  • Former Community Member
    0 Former Community Member
    Good afternoon, 

    Thank you so much for all your replies, they have been most helpful. Having just looked under the floorboards I do see junction boxes so I am not confident that my original sockets are served by a proper ring either. It tested ok when the electrician installed the CU, but I would rather have a circuit layout I know and can put on a drawing for anyone to refer to. I have tried the voltage drop calculator for my situation, especially the long spur, and all seems ok. I will know for sure when it is properly tested by an electrician, but it's good to know in theory, what I want to do is ok.


    Kind regards

    Simon
  • 0
    Why install new ring circuits at all?




  • 0
    There is a high number of installers who think the limit is 2 metres (or 3) that is not correct. However limiting lengths of spurs is no bad thing and including all points to the ring proper is usually a better basis, leaving spurs for later additions if at all

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