To spur or not to spur

Discussion in 'Electricians' Talk' started by Michaelcod, Oct 1, 2022.

  1. Michaelcod

    Michaelcod New Member

    Just had a conservatory, i want 3 double wall sockets ,
    The closest socket in the house is only 2 ft away ,

    I wont be putting a tv, dish washer, tumble dryer etc in the conservatory,
    1 will be used for an electric heater on a timer/thermostat.
    The other socket will be an LED lamp and phone/laptop charger.

    The wall socket has 2.5mm twin and earth, i understand how to spur off this but 3 sockets i know its not correct way.
    Is it possible to add these 3 sockets into the ring just by going into the closest house wall socket.

    Or would spur ing off be ok as all the consumables i intend to use would not exceed 10amps if on all together.
    Thanks
     
  2. Teki

    Teki Screwfix Select

    You need to know the loading of the existing ring.

    You can then either extend the ring or spur off the socket with a fused connection unit and then add your three double sockets. The maximum load with a fused spur will be 13A.
     
    rogerk101 likes this.
  3. Michaelcod

    Michaelcod New Member

    Thanks, yes if i was to spur i would fuse the 3 sockets.
    how would i find the load rating of the ring,
    I would prefer to do it properly by extending the ring but if it means chasing out walls and lifting flooring then fusing the spur to 3 sockets sounds easier and the conservatory sockets will have have protection.
    Thanks again.
     
  4. MGW

    MGW Screwfix Select

    Theory and practice may not always match, in theory we should not really spur be it fused or not too close to the consumer unit as it could over load one leg of the ring final, and we should check how long the ring final already is, and if adding to it will take it over the limit.

    In practice we look at the size of the home, and in most cases realise we are no where near the limit, and we install the new sockets and then test and inspect, and 99 times out of 100 it passes.

    But with a forum we have no idea if you live in a mansion or the smallest house in Wales, which is in Conway, so we can't judge how close to the wind you are sailing. Likely no problem, but only way we can know is for you to tell us the loop impedance etc. And if you had the meters to test, you would not be asking the question.

    A fused spur is less likely to exceed limits than extending the ring final, so I would say use a fused spur.
     
  5. Rulland

    Rulland Screwfix Select

    Conwy :cool:, a beautiful town, been here 19 years and still never been in the smallest house, had a few pints at the Liverpool Arms, on the Quay a couple of yards up on numerous occasions though:p
     
  6. MGW

    MGW Screwfix Select

    Oh a true Welshman with Welsh spelling of Conwy, I am also a gog although live in Mid Wales now, and I have also never been in that house, loads to do and see around that area, so smallest house was low on the list.

    As to spur, a small extension from the main circuit or riders foot if talking about horses, the BS 7671 says "A branch from a ring or radial final circuit." but it is of a lower current carrying capacity to the original circuit, or it would just be a radial.

    The phrase ring final means any branch can't be considered as a circuit which is defined as "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)." so the Part P and the new circuit rule can't apply to a fused connection unit (FCU) off a final circuit.

    Seems more to do with English language than electrics, still not found a version of BS 7671 published in Welsh, so maybe it does not apply in Wales? However it is not law anyway, only advice as to what should be done to make things safe, and the laws refer to BS 7671 as an example of how to make things safe, and this is why the certificate from LABC is called a completion certificate not compliance certificate as the installation does not need to comply with BS 7671.

    I could wire a home to German standards and get a completion certificate, but not a compliance certificate.

    But really all we want is a safe home, and a MCB has two devices in one, a thermal and a magnetic trip, the latter is fast acting, a few milliseconds, the former is much slower, so for a short circuit we need to ensure the magnetic bit will work.

    A MCB has a letter B = 3 - 5 times, C = 5 - 10 times, and D = 10 - 20 times, so a B32 MCB needs 160 amps to flow for the trip to release on the magnetic part, add 5% for safety so 168 amp convert that to ohms 230/168 = 1.37Ω earth loop impedance, the problem lies with plug in testers with loop is they pass anything lower than 1.9Ω so even 25 amp MCB not covered, they only really test 20 amp radials which need 2.19Ω so to work on a ring final needs expensive equipment, for the RCD tester to measure 40 mS and the loop impedance tester to measure 1.37Ω we are looking at likely £400.

    So to comply with BS 7671 you need to get an electrician as too costly to get your own test equipment, however when protected with a 13 amp fuse, as with a fused spur, we were looking at 2.42Ω not sure if this also has 5% now, but it is well within what a socket tester with loop can test, and I know you can't measure 40 mS but if the test button works, one would be very unlucky to have a problem.

    So with lack of test equipment the fuse spur route seems the best of a bad job you can expect.
     

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