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Why are ring finals split up/down not side/side?

Discussion in 'Electrics UK' started by ericmark, 23 Aug 2019.

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  1. ericmark

    ericmark

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    Because of the 1/3 rule and a cable drop down the wall is longer than cable up the wall, when using two ring finals in a house splitting side/side means less cable, so better ELI and better volt drop, also more even loading, and in the case of a circuit failure no need to run extension leads up or down the stairs.

    Yet most houses seem to be split up/down why?
     
  2. plugwash

    plugwash

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    In a house with a suspended ground floor it makes far more sense to run the ground floor electrics under the ground floor than cut huge chases down all the walls.

    I suspect people just kept the circuit split that was considered "conventional", even when house design was such that it no longer made sense.
     
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  3. Notch7

    Notch7

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    Labelling an mcb ground floor
    And one for the 1st floor

    Is the most intuitive for a householder.

    Id guess thats why.
     
    Last edited: 24 Aug 2019
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  4. Keithmac

    Keithmac

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    As Notch7 says, it's much easier to turn upstairs or downstairs off rather than left or right.
     
  5. ericmark

    ericmark

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    One always needs to test, even turning off a consumer unit does not mean the house is isolated, with this house there is a consumer unit for flat and fuse box for house so it would not turn off anything in house turning off isolator on consumer unit, so the only advantage using up/down is labels already printed and included in consumer unit. But in the main electrically side to side better. Fact that side to side is used, one has to test anyway, however "North Ring" and "South ring" only works if you know were North is.
     
  6. EddieM

    EddieM

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    Why do we continue with ring finals is a question too.
     
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  7. ericmark

    ericmark

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    That is easy, it allows 106 meters of cable where with a 20A radial limited to 30 meters of cable, so you need three radials to replace on ring final assuming you are complying with volt drop limits and using a D20 MCB so all items that would work on the B32 MCB to 13A sockets will still work on the radial, like 110 transformers with high in rush. with the old 15A socket you have no option but 15A fuse so radial is best, and in Europe with 16A again no option but 16A MCB as that is the only protection, so common to find a board full of 16A bottle fuses one to each circuit.

    We have to use non reversible sockets, and our standard lamp can be wired in 0.75 mm² flex as we can fit a small fuse, if using a 15A socket it would need 2.5 mm² flex, so we use a plug with an integral fuse, so we can use a 32A supply to the sockets that supply those plugs, we of course could wire in 4 mm² but still limited to 40 meters, up to 6 mm² and then 64 meters, however have you ever tried connecting a spur to a 6 mm² radial?
     
  8. EFLImpudence

    EFLImpudence

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    Not really. Branches are possible.

    As for having to have three 20A radials, isn't that better? Twice the capacity and not all lost with one fault.


    Anyway, if there were 3mm² cable rated at 32A, then it wouldn't need to be a ring in the first place.
    The coming of MCBs rendered the ring final an anomaly. It could have a 40A MCB but as that is not allowed, it may as well be two radials of 25A.

    Yes, there might be derating factors, for other than method C, but that applies to all circuits so has nothing to do with the theoretical values.
     
  9. JohnW2

    JohnW2

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    That statement makes the compoarison sound a lot more dramatic than it actually is ...

    ... with roughly symmetrically arranged sockets, a 106 metre (total ring length) ring final would be roughly equivalent to two 53 metre radials - so the comparison is between 53 metres and 30 metres, not the much more dramatic difference between 106m and 30m.

    Furthermore, it is often quite difficult (sometimes impossible!) to work out how the OSG has arrived at the figures it presents, but I would point out that although it does, indeed, give the maximum length of a 32A 2.5mm² ring final as 106m, the figure it gives for a 20A 2.5mm² radial is 42m (not the 30m you mention - where does that come from?) - and the difference between 53m and 42m is not very much at all.

    Kind Regards, John
     
  10. ericmark

    ericmark

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    The 106 meters is volt drop limit, not really written in stone, 20 from centre and 12 amp even spread so working on a design current of 26 amp. I used same for radial 20 at end and rest even spread OK rest = zero.

    So easy to argue 26 amp should not be used as design current.

    I agree splitting the sockets into more circuits does reduce loss should a circuit fail, however main reason for circuit failure is RCD tripping so argument only valid if using RCBO.

    Most houses today have three socket circuits, up, down, and kitchen. But even with kitchen removed down stairs is likely to exceed up stairs most of the time, and heath and safety wise not a good idea having a temporary extension lead up or down stairs in the event of circuit failure.

    I do see a problem with lighting as you don't want lights and sockets on the same RCD in any room, however since most RCD faults are with items plugged in, having RCBO on two or three rings is not going to cost too much. However RCBO on 4 or 5 radials is starting to get expensive.

    As to if kitchen ring final is really a ring final or conductors in parallel not so sure? Often only reason for 2 x 2.5 mm is can't get a 4 mm and two x 2.5mm in the centre switch. So feeding either end of a grid switch with 2.5 mm is more about getting cables in terminals than electric good practice.
     
  11. JohnW2

    JohnW2

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    Indeed it is - although, as we have discussed before, although you heard it said at a meeting, I don't think that I have yet seen "20A at centre and the remaining 32A evenly spread" documented anywhere. I'm not saying that I regard it as being an unreasonable 'working assumption', just that I don't think I've ever seen it in writing.
    That doesn't seem particularly fair/logical/reasonable. If you are going to assume (not unreasonably) that 62.5% of the total load on a ring circuit is at the 'furthest point' (centre of the ring), with the other 37.5% equally distributed throughout the circuit, why not make the same assumption for a radial circuit? ... if you did that, you would assume 12.5A at the end of a 20A radial, with the remaining 7.5A evenly distributed along its length (and, although I've yet to do the sums, maybe that's how the OSG gets its 42 metres figure for a 20A 2.5mm² radial?).

    Kind Regards, John
     
  12. Notch7

    Notch7

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    I wonder how many houses have a ring final with a break...ie effectively 2 radials.

    I think I heard ring finals came into being after copper shortage following the war...not sure if thats just an urban legend though
     
  13. Harry Bloomfield

    Harry Bloomfield

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    It was developed between 1942 and 1947, using several designs of fused 13amp plugs. The flat pin design we use now became standardised 1960, but other designs of 13amp plug continued to be used for several years, in special circumstances. Hospitals tended to use the DS 13amp type, with a pin forming the replaceable fuse.
     
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  14. JohnW2

    JohnW2

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    Probably a good few - some 'accidental' and some deliberate. However, if its wired in Method C 2.5mm² cable (or similar), nothing catastrophic will happen - since 27A is not far off 30/32A.
    That's certainly the usual story - but, as Harry has implied, I think it was probably the introduction of fused 13A plugs (rather than the ring design, per se) that resulted in the most saving of copper - since it enabled multiple sockets to be supplied by a single circuit (protected by a 30A fuse).

    Kind Regards, John
     
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  15. EFLImpudence

    EFLImpudence

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    ... and that 30A rewireable fuse required, because of its fusing factor, "the next size up" from 30A cable (30 / 0.725 = 41.4A) meaning if it were not for the ring introduction the previous existing 15A (15 / 0.725 = 20.7A or even 30A) circuit would have had to be replaced to increase its capacity to 30A.

    So, the fused plug was devised. I don't think the plug design is relevant.
     
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