Ring or Radial

Discussion in 'Electricians' Talk' started by James Harte, May 9, 2016.

  1. James Harte

    James Harte New Member

    After starting a war with another thread I am a bit reluctant to ask for further advice, but here goes.

    I am constructing a house with a fairly large foot print. Consequently cable runs are going to be lengthy.

    Considering just one ring main there would be about 7m of cable before the ring actually started and the distance then to the furthest point would be another 20m or so. The return of the ring would follow the same route. So that would be around 54m of cable to feed only about 7 double 13A sockets. The anticipated total load will be next to nothing as the only permanent equipment plugged in would be a TV and sound system. The other sockets are for convenience and the use of things like vacuum cleaners etc. on an irregular basis.

    The questions are…

    Should I use 4mm² cable because of the length of the ring or will 2.5mm² cable do?

    Or

    To keep costs down should this “ring” be a radial. If radial what cable size?


    Thanks in anticipation of clear guidance.
     
  2. seneca

    seneca Screwfix Select

    I think I would do a 4mm 20 amp radial from the info. you've given. I have a similar on-going job which is an old coach house, loadings will be quite low but longish runs involved so I've installed several radials. Subject to calcs. for volt drop you could use 32 amp instead of 20a if you think it necessary.
     
  3. madhatter1uk

    madhatter1uk Screwfix Select

    I would do a 2. 5 ring, the cable is cheaper, more flexible and a higher current rating.
     
  4. seneca

    seneca Screwfix Select

    Yes the cable is cheaper but there will be twice as much of it MH!
     
    nffc likes this.
  5. nffc

    nffc Active Member

    A project like this should have a competent electrician carrying out the works. There are many other things to consider such as installation methods which could affect the type/size of cable used.
     
    BigEverz likes this.
  6. BigEverz

    BigEverz New Member

    Electrician to install a 4mm 32A radial all day long ;)
     
  7. JP.

    JP. Screwfix Select

    Hmmmmm interesting..its funny how extra long runs 'n' stuff promote lets just say ''lively debate''

    ''The anticipated total load will be next to nothing as the only permanent equipment plugged in would be a TV and sound system''

    It don't work like that James.

    :rolleyes::rolleyes::rolleyes::rolleyes:
     
    Last edited: May 9, 2016
  8. fire

    fire Well-Known Member

    The way i look at it, if you are installing a socket circuit regardless of what you anticipate on using on it it is a socket circuit and you must cater for the sockets being fully loaded.

    If it is a 32Amp ring final then it is a 32Amp ring final regardless if only a TV is plugged in or not. If you sell the house or you have family round, they will see a socket as just a socket. They will plug anything into it and certainly wont be thinking about how you designed the circuit.

    I would say split the circuits up into 4mm2 16Amp radials.

    To reflect, if you have say 2 bedrooms then run one radial for the bedrooms on a 16Amp 4mm2, run another for the kitchen(maybe a dedicated ring final for the kitchen sockets only, kitchen appliances are power hungry) run another 16Amp radial for the dinning room/living room and so on.

    By doing this you provide a lower overall resistance of the circuits. Cable lengths are shorter overall and if a fault happens on one it does not plummet all the sockets out of use.

    If in doubt about a very long run of cable even if using 4mm2 radial then just lower the breaker current from 20Amp to a 16Amp.

    You should really carry out a voltage drop calculation to see that on the longest run you won't drop bellow 220v. No matter what appliance you plug in at this location less than 220v will cause problems for that appliance.

    Here is a online calculator to give you a guideline of cable size to KW load vs length:

    https://www.tlc-direct.co.uk/Techni...230&length=20&submit=Calculate+Min+Cable+Size

    Assuming the kitchen is 10metres away from the consumer unit and you wanted to run a ring final for the kitchen only.

    Nominal voltage is 230v and less than 220v is not acceptable so no more than 5% voltage drop. Over a 10 Metre ring final @ 32Amp voltage drop is approximately less than 2% so more than acceptable for a 2.5mm2 cable.

    If the bedroom is 20Metres away from the consumer unit and you plan on a radial circuit this is a single run of cable and the online calculator set to your correct situation can be used.

    So a radial @ 20Amps breaker over 20 metres is a 2.1% voltage drop on 4mm2 cable (still acceptable).

    If we wanted to use 2.5mm2 radial then that has to be a 3.7kw circuit(16Amp breaker) which gives us a 2.5% voltage drop over 20 Metres. If you wanted 20Amp you can NOT use 2.5mm2 it is too small for a 20 metre run if the cable is to be installed in a wall, conduit, trunking etc. A 20Amp breaker on 2.5mm2 radial over 20metres just would not cut it.

    If you wanted a 32Amp radial on a 20 metre run then 230x32=7360Watts or 7.4Kw which would require 10mm2 cable which you would never be able to wire into the back of a socket fascia.

    Lets take your entire 50Metre run in account and use lets say a 16Amp radial. 230x16=3680 so 3.7KW

    That is a 3.7% voltage drop(still acceptable) use of 4mm2 cable @ 16Amp load/breaker and that is a full length of 50Metre 4mm2 cable run to sockets providing maximum of 3.7KW. 20Amp would also be fine for this but you are very close to the tolerances.

    Now can you see the benefit of breaking up the circuit rather than having one single socket circuit. Running a ring final in 4mm2 would be very hard to wire as that is the max size possible to fit into the back of a standard socket terminals.

    Running the entire houses sockets on a 50metre run of 4mm2 cable loaded to a 16Amp breaker is pretty low in current for every socket in the house. If you did run it as a ring final in 4mm2 then you could achieve more current loading but that is far more 4mm2 cable and allot of hard work getting that cable correctly tightened into each socket.

    A quick sense of maths, if one uses 2.5mm2 over the 50 metres as a ring final, this effectively doubles the loading capabilities of a 2.5mm2 cable so again a 16Amp maybe a 20Amp breaker can be used for that ring final. because 4mm2/2 is 2mm2 so the extra 0.5mm2 gives you a small overhead allowing for an entire ring final to be rated @ 20Amp. You are pretty close to tolerances though. a whole houses ring final at a max of 4.6Kw

    So to conclude, breaking it up into two radials and one ring final means you can supply 3.7KW to the bedrooms while also providing another 3.7kw load to the living room and a ring final for the kitchen you can provide a possible 7.4kw additional load.

    Hope you can see where this is going and how it is working. All the above are working examples to give you an idea of how it is done. By no means am i saying you must do it that way but you must take into consideration all factors involved within the design. If it has caused your brain cells to fizzout then get a electrician in to do the work. At least you now have an idea of how things are done.
     
    trebor47 likes this.
  9. BigEverz

    BigEverz New Member

    You have got a regs book in your downstairs toilet havent you !!
     
    seneca likes this.
  10. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    I am in the throes of doing a new build treehouse for Grand designs for broadcast September

    Its radials for all socket outlets and a ring final for the kitchen as the footprint for the three storey is large as well

    Last count was two DB's one with 12 circuits the other with 10 all on RCBO's

    Lets not p155 about and do it as it should be done~!
     
  11. JP.

    JP. Screwfix Select

    Flipping heck I'm only diy and I would have the job designed and done by now - dear o dear what a flipping palava..:)
     
  12. leesparkykent

    leesparkykent Well-Known Member

    A 20m run at 32 A would not warrant 10mm cable. iirc the mV/A/m for 4mm T&E is 11. 11x32x20/1000 =7.04V


    The maximum length of run for this circuit with the same cable size and type will be 11.5x20/7.04V=32.6m


    The maximum length of run with 4mm T&E on a 16A radial will be 65m.
     
    Last edited: May 9, 2016
    Brian_L, FatHands and Risteard like this.
  13. leesparkykent

    leesparkykent Well-Known Member

    why not use a double stack/dual row CU? will look neater that two CU's imo.
     
  14. stateit

    stateit Screwfix Select

    I find 4mm more flexible than 2.5mm every day of the week.
     
    Brian_L likes this.
  15. leesparkykent

    leesparkykent Well-Known Member

    With a ring final circuit it is assumed to have a load of 20A at the furthest point and the balance to the rating of the protective device evenly distributed so for a 32A protective device this equates to a load of 26A at the furthest point. You can now use this 26A as your design current (ib).The mV/A/m for 2.5 T&E is 18 we now use the formula mV/A/m x ib x L /1000 /4 so in your case 18x26x54/1000/4=6.318V. The maximum permitted volt drop is 5% for power 230/100x5=11.5V so your well within the maximum permitted volt drop.
     
    Risteard likes this.
  16. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    Two complete locales. .
     
  17. Risteard

    Risteard Screwfix Select

    Not strictly accurate.
     
  18. seneca

    seneca Screwfix Select

    I agree, it's like working with the old 7/029 again. (which I still come across quite often in my area)
     
  19. Bazza-spark

    Bazza-spark Screwfix Select

    To coin one of JP's phrases, flippin heck Seneca, you are going back some there lol.
     
    Brian_L and seneca like this.
  20. seneca

    seneca Screwfix Select

    Lovely stuff Bazza! ( like most things pre-EU, much better):)
     
    Bazza-spark likes this.

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