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Upgrading from ring to radial?

Discussion in 'Electrics UK' started by Ant1, 31 Oct 2020.

  1. Ant1

    Ant1

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    I am currently in the process of completely renovating my kitchen, I now have bare walls with sockets lying on the floor connected through a ring circuit.

    I have the opportunity to upgrade the electrics in that part of the house though I am unsure if this is totally necessary.

    I might have been watching too many videos of John Ward on Youtube, he seemed to imply that radial circuits were safer and more commonly used nowadays.

    Below is a photo showing the fuse where the ring is connected to, and a spare connector on the RCU.

    Is there any benefits going from a ring to radial?

    RCU.jpg
     
    Last edited: 31 Oct 2020
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  3. EFLImpudence

    EFLImpudence

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    For new circuits maybe, but for what is already installed the ring will be far more common.

    RCD - Residual Current Device.

    Why have you two CUs? Are you in the process of transferring the circuits to the new one?

    Not unless you are going to replace the cable with a larger one.


    Your new work - sockets -should be covered by an RCD.
     
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  4. Ant1

    Ant1

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    Thanks for the reply, I moved into the property in March so still learning about it.

    It is 150 years old, the newer CU is mainly for the storage heaters which were installed at a later stage before I moved in.

    To be honest my electrical installation is awful, I am stuck with a Comfort Plus White Meter from Scottish Power, there are 3 rates (day, night and control rate). The control rate is for the hot water. There are only very limited tariffs for that meter, and only from Scottish Power. They actually do not advertise tariffs online properly, I had a dispute with them for 2 months and now have a single rate for all day, night and control... weird... though this is a different story.

    I think I will simply stick with the ring if there is no benefit changing to radial.

    PS: if I add sockets, I assume this is still ok just to extend the ring?
     
  5. sparkwright

    sparkwright

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    A ring is good in that should a cable get damaged the circuit can be made into two radials.

    A radial is typically limited to 20amp, unless you use larger cable.
     
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  6. EFLImpudence

    EFLImpudence

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    Yes.

    Although, as I said, new work should be covered by an RCD.
     
  7. AdrianUK

    AdrianUK

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    But the spare is on the 'heater' side of the second CU, so it will only become live at night.....

    I don't the mrs is gonna like only being able to us ethe kitchen sockets after 11pm :)
     
  8. ericmark

    ericmark

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    For a ring final we take design current for circuit Ib as being 26 amp, there is no regulations or rules saying this should be the case, however that is how we arrive at 106 meters of cable being maximum for a ring final and it assumes 20 amp at central point and 12 amp even distributed on the ring final.

    Applying the same to a radial even a 16 amp one, we end up with 16 amp at end of run, we end up with a maximum length of 43 meters, so it is 86 v 106 meters if you can find a centre point, move to 20 amp and looking at 32 meters, so much is dependent on the loop impedance reading at centre of the existing ring.

    So 0.93Ω more than Ze is the limit to be within the volt drop limit, so with Ze at 0.35Ω looking at line - neutral impedance of 1.32Ω so first hurdle is have you got the test equipment to measure 1.32Ω? If no then really a non starter.

    But my kitchen I have two cup boilers I use to make coffee, slightly different sizes my wife does not like big cups, so that is 24 amp every time the adverts come on as I go to make coffee, the question is would a 16A RCBO allow me to do that? If not I want a ring with a 32A RCBO.

    As to a RCD and MCB been there done that and had to go out in snow to reset RCD no thanks don't what that again.
     
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  9. JohnW2

    JohnW2

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    Can you help me to understand that arithmetic - in particular the 86m (presumably 2 x 43m) and the 32m?

    If one uses the convention (for VD calculations) for a ring final of assuming that five-eighths of the total (20A out of 32A) will be at the furthest ("mid") point of a circuit, with the rest distributed along the length of the circuit, then why not do the same for a radial - hence assuming 10A at the end of a 16A radial or 12.5A at the end of a 20A radial (in both cases with the remainder distributed along the circuit)?

    In any event, as I always say, unless you can think of any loads whose "safe functioning would be impaired" by a low supply voltage, there is no requirement in BS7671 for the VD in a circuit to below any particular figure..

    Kind Regards, John
     
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  11. ericmark

    ericmark

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    Quite a long time ago I built myself a calculator
    Volt drop.jpg
    My worry was if I installed something which did not comply with volt drop requirements some time in the future some one could want me to correct it FOC so it is important to get it right, I was at that time regularly attending IET lecturers and the new edition allowed 5% instead of 4% and the lecturers said this equated to 106 meters of 2.5 mm² instead of from memory it was 88 meters, however in spite of using the correction factor Ct I could not get the figures quoted. So I asked, and was told for a ring final well the same would apply to a radial with 4 or 6 mm² we assume two sockets with a 20 amp load mid way for ring and end for radial and then the remaining is even throughout its length. And so we take the design current as 26 amp not 32 amp. And once I did that I got the figures they were quoting, since it uses square roots to work out the correction factor with the formula shown in BS 7671 it gets a little complex, not the sort of think you can work out on a fag packet, and my slide rule skills were never that good, so I used java script so I could use it with my phone.

    However once made I realised if you allow +/- 0.02 Ω with both the origin and mid point measurement then it would need to be a long way out before one could prove it was too long when installed, so if the incomer was 0.35 Ω and the midway point was 0.94 Ω yes I could say it was within limits, but if I measured 1.02 Ω midway point I could not be sure that varying loads on the supply had not caused that reading, loop impedance meters may show to 2 d.p. but they rely on a steady supply to be accurate.

    So before one could claim too much cable used you would need to have readings which were really well out, and although told 20 amp was what was used as centre load, if I consider it as 14 amp and 18 amp even spaced so Ib of 23 amp I get 123 meters, and 1.01 Ω and as you say nothing to say in the regulations I should not take 14 amp.

    However the point is be it 14 or 20 amp, if using a 16 amp radial the current is closer to the limit for cable so the correction factor become closer to unity, plus still taking the full 14 or 16 amp at end so less of the load is divided by 2 as shared along the length, and in the main radials are 20 amp so in real terms you need three radials to cover the same as one ring final, not two, so there is a point in using radials, but at £25 per RCBO it means a radial system is around £100 more expensive to a double ring final system. And if all supplied from same RCD then what is the point, if one trips so do them all?
     
  12. ericmark

    ericmark

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    As a thought I had considered 2 x 16 A linked (i.e. two phase type) MCB's feeding the ring, this would mean total still 32A but the maximum in any leg is 16 amp. And of could if any one trips it would auto trip the pair, as linked, however we have used a single 32A for years without a problem, and not sure it would comply, so never did try it. Lets face it MCB's do not trip that often so it would seem unlikely you would get a heavy load close enough to the CU to actually cause a cable overload without tripping.
     
  13. JohnW2

    JohnW2

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    I suppose one could consider that, but I don't think there's really much point.

    Although I realise that the regs allow a 32A ring final to have cable with a CCC as low as 20A but, in the very common situation of Method C 2.5mm cable, its CCC (27A) is so close to 32A that I'm sure no harm would come to it, even if (contrary to what one is expected to 'design against') and entire 32A load (itself unlikley domestically) were connected very close to one end of the ring. Even with cable having a CCC of 20A, I doubt that there would be any significant 'risk'.

    Kind Regards, John
     
  14. ericmark

    ericmark

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    My dad thought he knew it all, and everything forward and trust to the lord, so a spur off a spur off a spur he thought was OK, and with 7/0.029 cable likely he was correct it was 2.9 mm² so just slightly thicker than 2.5 mm² but I found 2.5 mm² cable with signs of melting due to being overloaded for an extended time supplying a 3 kW electric fire plus other items.
     
  15. JohnW2

    JohnW2

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    Maybe you've experienced 'an exception which proves the rule', but I've personally always thought that our concerns about such things are, in electrical (rather than regulatory) terms, probably not very justified in practice.

    Method C 2.5mm² cable is rated at 27A and that takes into account the fact that (if it existed) a 27A MCB would be required never to trip with continuous currents below 35.1A 30.5A (27A x 1.13). It is therefore deemed that such cable will come to no harm with less than 35.1A 30.5A (continuous, indefinitely) and that equates to a bit over 8kW 7kW. Many household might struggle to find 'portable' items with that total power at all, and the chances of more than 8kW 7kW worth of them being used simultaneously, for extended periods, all plugged into sockets on the same (unfused, hence non-compliant) spur would probably be 'vanishingly small'.

    Kind Regards, John
    Edit: Misbehaviour of typing fingers leading to consequent miscalculation correct, but my point remains. Thanks to EFLI for noticing this!
     
    Last edited: 1 Nov 2020
  16. ericmark

    ericmark

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    I think there is a mistake in the title, it should say down grading ring final to radial. And that fuse box does raise the issue what is potentially dangerous? Is the amount of asbestos found in those fuse carriers considered as dangerous? And can we permit that bus bar chamber in a premises in the charge of an ordinary person. And why some one spoilt such a nice job running one odd cable not through the grommet. Plus putting the consumer unit so close to fuse box you can't put a screwdriver on the earth terminal.
     
  17. skenk

    skenk

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    I agree with eric, it would be downgrading! If I were you I'd be gtting rid of that asbestos box as a priority. Unless you have the appropriate test equipment and know how to use it then I sugest getting an electrician.
     
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