Spur off ring to double socket and lighting

Discussion in 'Electricians' Talk' started by Dave Leicester, Oct 10, 2020.

  1. Dave Leicester

    Dave Leicester New Member

    Hi

    I plan on fitting a new outside socket and would just like to check my understanding before I commence.

    I plan to create a new spur from one of the sockets in my kitchen ring. The plan is as follows:

    2.5mm T&E spur from kitchen socket to 13A FSU indoors
    Spur extended to exterior wall with 2.5mm T&E ending in IP66 double socket
    Spur extended from double socket with 1mm T&E to IP66 5A FSU to power lighting

    Is this reasonable, and in line with regs?

    Thanks
     
  2. terrymac

    terrymac Screwfix Select

    Is the circuit protected by an RCD ?
     
  3. unphased

    unphased Screwfix Select

    No you can't take 1.0mm2 off a socket. You should spur off the outside socket to the 5A spur in 2.5mm2. The rest seems okay. Obviously follow rules for 30mA RCD protection.
     
  4. Banallsheds

    Banallsheds Well-Known Member

    A better way would be to spur off the ring to your double socket without the FCU. Take a second spur off the ring via a FCU for the light or even take the light from your lighting circuit with no FCU.
     
  5. Dave Leicester

    Dave Leicester New Member

    Yes it is

    OK so all fine except use 2.5mm throughout?

    To be honest I'm trying to avoid the additional mess of a 2nd spur and the lighting circuit is nowhere nearby to access so that would be a real pain. I want an FCU inside so I can isolate the circuit when I'm away.
     
  6. unphased

    unphased Screwfix Select

    You could consider two spurs inside. Spur off the inside socket to a 13A spur. Spur off the 13A load side to an outside socket AND another spur fused at 3A. Then run 1.00mm2 off the 3A to your light. Preferably not clipped outside. Use conduit.
     
    nigel willson likes this.
  7. Dave Leicester

    Dave Leicester New Member

    OK, so just to be clear, what you are suggesting is (forgive my paint skills :) ):

    upload_2020-10-10_20-17-8.png

    I was planning on conduit outside, of course.
     
  8. unphased

    unphased Screwfix Select

    Yes. To avoid an infused spur off spur you must put all loads on the first 13A fcu. Everything is then on a fused spur.
     
  9. Dave Leicester

    Dave Leicester New Member

    Perfect, thanks
     
  10. Banallsheds

    Banallsheds Well-Known Member

    You could avoid the 13amp FCU by taking both spurs from the same kitchen socket.
     
  11. Coloumb

    Coloumb Screwfix Select

    That is a reg breach. Dangerous advice.
     
    MrWLJ71 likes this.
  12. Banallsheds

    Banallsheds Well-Known Member

    No it is not. There is no reg saying two spurs cannot come from the same point. Please provide proof if you think there is.
     
  13. Coloumb

    Coloumb Screwfix Select

    How about reading the regs? 433.1.204.

    Oh wait you can't.
     
  14. Banallsheds

    Banallsheds Well-Known Member

  15. Coloumb

    Coloumb Screwfix Select

    Try reading what I said properly. I said there isn't a reg but it's n.p. because of 433.1.204. If you don't get this go back to school and learn basic physics as you don't seem to understand the basic concept of ohms law.

    n.p. = not permitted, basic english.
     
  16. Banallsheds

    Banallsheds Well-Known Member

  17. unphased

    unphased Screwfix Select

    You cannot overload a ring final circuit because the OCPD won't let you. If the circuit has too much load plugged in the device will break the circuit. You can, however, overload individual components installed on the ring. A BS1361 13A socket, for example, can have two 3kW loads plugged in to it. If both those loads were used at the same time the socket is pretty much on its limit. Let's say it had a spur off it, and a 3kW load was also plugged in on the spur. Then there is 9kW of load at one point on the ring all being sustained by the double socket. That is now being pushed. Let's say there were in fact two spurs off the socket and another 3kW load was plugged in to that. So now we have 12kW of load being carried by the double socket. That is totally unsafe and if the OCPD hasn't tripped by now the socket will be burning out. By limiting the amount of spurs to ONE per point on the ring eliminates this potential scenario. I contributed to both of those threads you linked to. I am phantom9.
     
  18. Banallsheds

    Banallsheds Well-Known Member

    12kW is something like 50a so the MCB would soon trip. You could have 4 3kW loads with one spur, 2 in the original socket and 2 on the spur socket. But, and this is important the full load is not being carried by the original socket, only the cable in the terminals.

    There are other examples of this. Say a 4 mm radial on a 32 a MCB carrying full load. The first socket on this radial would have 32 amps through its terminals but no one has said this would overload the socket.
     
  19. unphased

    unphased Screwfix Select

    The socket is rated at 13A, (26A in practice can be plugged in), bas, and any cables connected to it are loading the terminals with current. It is effectively a junction box in this case, same as a 32A rated junction box. The heat generated by the current is transferred in to the terminals. If the terminals aren't designed to carry 32A plus of current, they burn out. Since a ring final circuit is only designed for 20A of current and can carry more because of short term loading and the fact a 32A ocpd is used on the circuit, it doesn't need much imagination to understand that an accessory wired on to the circuit must be limited to how much current it is allowed to carry, hence the rule, one spur per point. Yes, you can have 12kW passing through a point when a double socket is spurred off a double socket with four 3kW loads plugged in, hence the need for a 13A fuse on the spur. In practice the circuit OCPD would trip, and the situation would not exist for long. I hope that helps you understand why we limit spurs to just one per point on a ring. The loads on the point must not be such that local overload can generate dangerous overheating and fires. Both the circuit fuse and the limitation on the loads at one point are factors to be considered.

    A radial circuit can have multiple spurs because that is what a radial circuit is, but there are still limitations to be considered. In that case the limitations are the capacity of the terminals to accept the number of conductors pushed in to it, and of course the lower rating of the OCPD preventing any significant over current when there are multiple spurs on a point, or the increased size of the cables. You would not put 2.5mm2 spurs on a 4mm2 radial circuit but the principles of potentally overloading a point are the same. Unless the accessory or junction box is designed to carry more current without exceeding the current capacity of the circuit then the number of spurs must be limited.The ring final circuit is a special case. The reason it is able to carry more current with 2.5mm2 conductors than its equivalent radial circuit wired in 2.5mm2 is because a ring circuit is a parallel circuit and the conductors in a parallel circuit have vastly reduced impedance. V=IZ. Its just simple maths. It is taking advantage of the behaviour of conductors in parallel to be able to use smaller conductors than would otherwise be necessary.
     
    Last edited: Oct 11, 2020
  20. Banallsheds

    Banallsheds Well-Known Member

    There is no rule of one spur per point. Please don’t lie. The terminals don’t have to carry the current, it’s the cables crimped together that carry the current.
    A 32 a radial circuit on 4mm could well have 32 amps passing through the first socket terminals. If, as you say, they will overheat why is this allowed.
    If 2 spurs from one point is as dangerous as you imply why is it not forbidden.
     

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