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Fused spur or ring main

Discussion in 'Electrics UK' started by MKhan, 11 Mar 2020.

  1. MKhan

    MKhan

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    Hi,

    My room currently has two single sockets, each on adjacent walls and am wanting to add additional sockets.

    In January, I added two double sockets on one wall as fused spur from one original socket and another additional double socket from the other original socket as a spur. The wiring was placed in trunking and pattress boxes that looked ugly around the walls.

    At the moment, I am renovating the room and fitting decorative trunking around my room to house the T+E wires and using knockouts to house the sockets in the wall.

    Given that I haven't hired a qualified electrician nor can I place the wires in the wall or under the floorboard, I am completing the DIY job myself. Also, if need be to sell the house at a later date, I may like to remove the modifications given I have no certificate for installation or testing.

    So my question is, what are the differences between adding sockets as a fused spur or on the ring main And which would be better? I know the ring main will require me to run T+E cables in the trunking to the sockets and then back to the original socket whereas using a fused spur would mean running one cable from an FCU to each socket to the next.

    Any advice would be appreciated
    Thanks
     
    Last edited: 11 Mar 2020
  2. EFLImpudence

    EFLImpudence

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    Ok. How will the cable get from the trunking to surface mounted socket?

    Why can you not bury the cables in the wall if you are renovating the room?

    Testing is to ensure your work is safe. You should do it and record the results on a certificate (I realise few DIYers do).

    Well, on the ring means you put more sockets in the ring (draw it), rather than a spur whether fused or not.
    Neither is better. It depends on the positioning.
    https://www.diynot.com/wiki/Electrics:Socket-Circuits

    No, that would be stupid.
    Both legs of the ring in the same place means you don't need a ring.

    Yes, and then if you end up at a different socket which is also on the ring ???
     
  3. EFLImpudence

    EFLImpudence

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    Having reread your post I think it would be advisable if you kept to spurs.
     
  4. ajohn

    ajohn

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    What you can / should do depends on how the existing sockets are wired. The place to start is the consumer unit - the rating of the mcd or fuse etc that isolates the sockets. The next thing is cable size connected to the sockets and the number of cables going to each one. You might have a ring, a radial or the sockets might already be spurs. A radial may be either 2.5mm or 4mm cable. A ring 2.5mm. This assumes the usual wiring sizes. Extending a ring or a radial is done in the same way. Break into it, run to the new sockets and then back to the break in the cable which is no longer connected to anything.

    More than one socket can be added to a spur providing it's fed via a fused connection unit, usually fitted with a 13amp fuse.

    Cables can be run under floorboards. Notches in the joists to run them in should be close to a supporting wall. Less than 1/4 of the span from there and a depth of no more than 1/8 of the depth of the joists. The cable also have to be clear of any metal piping or plastic placed between them and the pipework.
     
  5. winston1

    winston1

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    That only applies if spurring off a ring. If coming off a radial the FCU is not required.
     
  6. EFLImpudence

    EFLImpudence

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    Depends what size cable is used.


    Ajohn: It is MCB - Miniature Circuit Breaker.
     
  7. ajohn

    ajohn

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    I don't read it that way as it doesn't make much sense to have one rule for rings and another for radials. The diagrams only show spurs on rings and I assume exactly the same thing can be done on a radial leaving me wondering if it should be done on a 2.5mm radial. Suppose it depends on loading and if overloaded it will trip.
     
  8. ajohn

    ajohn

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    LOL for some reason I sometimes replace the b with a d. Not sure why.
     
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  10. EFLImpudence

    EFLImpudence

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    Well, the rules are the same - protection against overload and fault current.

    The trouble is people quote Appendix 15 as if that is all there is.

    There are radial diagrams in Appendix 15.

    Sort of - it depends on whether overload will damage the cable before it trips.
     
  11. winston1

    winston1

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    Rings are protected by a 32 amp MCB so you can only have one socket on a 2.5 mm spur. Radials are usually protected by a 20 amp MCB so you are not limited to one socket on a spur using 2.5 mm cable. Occasionally radials are in 4mm cable on a 32 amp MCB and again spurs are not limited to one socket using 4mm cable.
     
  12. ajohn

    ajohn

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    Actually it can be a popular option. I favour rings but some electricians object to the extra checks needed on rings and the possibility that some one might have created a ring within a ring. For rewires that are pretty extensive such on rather older houses a 4mm radial may work out to be a cheaper option and turn out to be easier to install.

    One fact for sure is that people need to look at what they currently have before deciding on what to do. Our house, 1911 has 2 4mm radials plus economy 7 wiring. Others I have worked on had 2 2.5mm rings and one a single 2.5mm ring. I thought that all of the economy 7 was on spurs but eventually found that one was actually connected to one of the radials - spured of a socket. ;) Glad I had my neon screwdriver available when I removed this particular cable. It really is best to find out what isolates what before jumping in. Our economy 7 was supposedly officially disconnected years ago.
     
  13. plugwash

    plugwash

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    Rings are special because they are not overload protected in the normal way. They are typically breakered at 32A but are allowed to be wired with cable rated at only 20A (though in practice one can't find cable rated at exactly 20A so the cables used are usually rated slightly higher), provided the installer judges that the risk of sustained overloads is low.

    On the ring itself there are paralell paths, though the effectiveness of said paralell paths depends on the layout of the ring. On a spur on the other hand there is only a single path. For this reason the guidence in the appendix suggests limiting a spur to one single or double socket or one fused connection unit.

    In the case of a fused connection unit, the fuse in the fused connection unit protects the spur from overloading. In the case of a socket, the risk of an unprotected overload of the spur cable is not completely eliminated but it is pretty unlikely, probablly less likely than an overload caused by a concentration of load near one end of the main ring.

    The same applies to a reduced-size spur from a radial, if the main radial is rated at 32A but it is spur is only rated at 20A then you have the potential for an unprotected overload which needs to be mitigated.

    On the other hand with a branch from a radial wired with cable suited to the breaker rating there is no possibility of an unprotected overload. So no need for any special rules or guidence. The same applies to wiring on the output side of a FCU.
     
    Last edited: 13 Mar 2020
  14. bernardgreen

    bernardgreen

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    Rings also have the advantage that it requires two faults ( failed connections ) for sockets to lose their Earth connection to the consumer unit
     
  15. EFLImpudence

    EFLImpudence

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    ...but only one unnoticed fault to overload the Line or Neutral.
     
  16. ajohn

    ajohn

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    That in a nutshell is the only problem with them but correctly installed connections generally don't fail. A break in a radial results in dead sockets.

    The ring has 5mm of copper conductors. The loading aspect causing problems isn't a simple as many people seem to think. In short any load anywhere will have more than 2.5mm available to carry the current. The people that dreamt it up were not stupid and gave it a lot of thought,
     
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