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Ring Final

wessex
With reference to 433.1.204 and cable as installed has min capacity of 20A if protected by a 30A or 32A. If the protective device is reduced to 20A how is the new minimum as installed capacity calculated or arrived at ? I've been looking in the Electrical Installation Design Guide, but the answer is avoiding my eyes.
  • 0
    wessex:

    With reference to 433.1.204 and cable as installed has min capacity of 20A if protected by a 30A or 32A. If the protective device is reduced to 20A how is the new minimum as installed capacity calculated or arrived at ? I've been looking in the Electrical Installation Design Guide, but the answer is avoiding my eyes.


    Why would you want to reduce the protective device to 20 Amps from 30 or 32 Amps on a ring final circuit?


    Z.


  • 0
    In principle I believe that one could wire a ring final circuit in 1.5mm cable protected by a 20 amp MCB. It is not a standard circuit, but that does not mean prohibited. A 20 amp ring final in 1.5mm should be no less safe than the common arrangement of a 30/32 amp ring final in 2.5mm.


    But back in the real world, why on earth would anyone want such an odd circuit ? A standard 20 amp radial circuit would almost certainly be cheaper and simpler.


    BTW, I have installed  ELV lighting circuits as a ring in 1.5mm on a 10 amp fuse. To reduce voltage drop. A 2.5mm radial was rejected because a pair of 2.5mm conductors wont fit most light switches.
  • 0
    2.5mm2 cable installed to 101 giving an as installed current carrying capacity of 17A. Granted not much point in this installation choice and 4mm2 should have been used, but back to my question. How do you evaluate the minimum required current capacity of a cable installed as ring protected by a device.20A device. As the cable is installed it is 3 amps short, but there is a significant drop from 32A to 20A so I do not suppose there is a problem, but I was looking for a definitive answer other than volt drop which would still need an idea max current.
  • 0
    The 32A ring and 20A cable is based on someone else's calculations about load bunching, and is a well understood and time-tested design.

    However, with no risk at all,  the same assumptions could be made and scaled for any similar layout,  so a 64A device and 40A cable or a 16A device and 10A cable for example, keeping the ratio of MCB to cable rating.

    So a 25a MCB should allow you to use cable rated at (25/32)*20A ~ 16amps.


    But not all makers do a 25a MCB or RCBO however.

    (but for example Wylex do...)


    Mike.
  • 0
    That seems reasonable. Thanks for that. I do not see the reasoning behind the circuit, but never the less trying to validate it.

    Thanks all for the feedback.
  • Former Community Member
    0 Former Community Member
    min capacity of 20A if protected by a 30A or 32A.


    It used to be 0.67 x protective device rating giving minimum cable rating of 21.4 A but was "modified" to 20 A to coincidently satisfy the new Table 4D5 changes of CCC to suit newer building methods...............


    Hmmmm. Last time there was a temporary relaxation in CCC was during WW2 due to copper shortages,


    Regards


    BOD


  • 0
    The reasoning behind a ring final circuit is that the total load and therefore the OCPD can be greater than the cable rating becuase the currrent will be divided between the two legs of the circuit.

    The total load and the OCPD may not however be twice the cable rating because that would require that the total current be divided equaly between the two legs of the circuit.


    The original design was a 30 amp rewireable fuse, selected because that was already a standard size for electric cookers, and because a 30 amp fuse was the largest that would discriminate fairly reliably with a 60 amp suppliers cut out fuse.

    The cable originaly used was a then standard size.


    The design then evolved a bit to allow the slightly smaller metric cable 2.5mm and to permit use of 32 amp MCBs rather than 30 amp fuses. It is all bit empirical, but has a good record in practice.
  • 0
    Didn't the IET produce a paper validating the ring final. Giving some insight into how compliance was achieved and justifying it's existence. I seem to remember down loading it, but I would struggle to find it know.
  • Former Community Member
    0 Former Community Member
    The design then evolved a bit to allow the slightly smaller metric cable 2.5mm and to permit use of 32 amp MCBs rather than 30 amp fuses. It is all bit empirical, but has a good record in practice.


    But, it has been deemed that there is now a new qualifier in 433.1.204 regarding load distribution in the cable..........


    Regards


    BOD
  • 0
    perspicacious:
    The design then evolved a bit to allow the slightly smaller metric cable 2.5mm and to permit use of 32 amp MCBs rather than 30 amp fuses. It is all bit empirical, but has a good record in practice.


    But, it has been deemed that there is now a new qualifier in 433.1.204 regarding load distribution in the cable..........


    Regards


    BOD


    "...under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current carrying capacity (Iz) of the cable."


    I suppose that the overloading could occur in say a commercial kitchen in a restaurant or pub.


    Even the 1981 15th edition regs. recognized ring overloading, and probably previous editions as well. Appendix 5 required a ring final circuit to cover not more than 100 m2 in area in household installations, . Consideration was to be made of kitchens where a separate circuit may be required. 


    If used as a length of fuse wire, 2.5mm2 copper wire would be rated at approx. 80 Amps.


    Talking about fuse wire, 433.1.204 still allows a ring final circuit to be protected by a B.S. 3036 length of fuse wire rated at 30 Amps. That shows confidence in the time tested ring final circuit's design.


    P.S. J.W. does not like ring final circuits

    https://www.youtube.com/watch?v=hZN6hiGLtrE




    Z.


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