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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.
  • Former Community Member
    0 Former Community Member
    I suppose that the overloading could occur in say a commercial kitchen in a restaurant or pub.


    The Regulation is not referring to the total load but for typical poor design where the CU is in the kitchen or utility room and the route of the cable calls in first to heavy loading and then wanders off round the house (or rest of room) before returning to CU. 


    Hopefully I don't have to explain current and path of least resistance.........


    Regards


    BOD
  • 0
    The assumpmtion is about bunching. The worse case is when all the load is near one end, say within the first 10% of one of the limbs.

    By the miracle of resistance scaling with length of cable, and the voltage drop from origin to load being the same both the long and short way round, the current split is always in inverse proportion to the ratio of the lengths.


    So if the load is equally far from each end, then the load is shared perfectly, and 16A goes each way.


    But let us consider an example of the bad layout case..


    It is possible to have say 3 sockets each pulling 10A  at 1m, 2m and  5m from the origin and then a 20m return leg with no load.


    Now the first 10 amps is spit in the ratio  of 1/25 to 24/25, those being the tow path lengths to that socket.

    The second 10A is split 2/25 to 23/25

    the third 5/25 and 20/25


    So as the common denominator is 25ths of 10A is 400mA 'units'

    Going left we have (24+23+20)* 0.4A  = 26.8 amps

    going right we have  (1+2+5)* 400mA = 3.2A

    Quick sanity check total = 30A.

    So that last metre of cable from CU to first socket  takes nearly 27A, and would be fine in plaster or clipped direct, but may age rather faster than we would like in thick insulation. It is the only stressed cable on the ring, and making it longer,  or moving one load to the return leg would restore order, as would making that first socket a spur off the MCB, so its current is not adding to the short leg current.

    (This is often a quicker fix)

    If you can avoid having more than one socket in the first 20% of the ring length on either end, the the problem vanishes even in quite thick insulation.

    Mike.



  • 0



    The Regulation is not referring to the total load but for typical poor design


    The regulation falls under the heading of PROTECTION AGAINST OVERLOAD CURRENT.


    It is impossible to design in good use of a ring final circuit as it is impossible to determine just what the user will plug in and where.


    Z.


  • Former Community Member
    0 Former Community Member
    The regulation falls under the heading of PROTECTION AGAINST OVERLOAD CURRENT.


    And your point is?

    It is impossible to design in good use of a ring final circuit as it is impossible to determine just what the user will plug in and where


    There's normally quite a good clue from where the plumber has left connections for the washing machine and dish washer....


    Regards


    BOD
  • 0
    perspicacious:
    I suppose that the overloading could occur in say a commercial kitchen in a restaurant or pub.


    The Regulation is not referring to the total load but for typical poor design where the CU is in the kitchen or utility room and the route of the cable calls in first to heavy loading and then wanders off round the house (or rest of room) before returning to CU. 


    Do we have rings in commercial kitchens?


    At this stage, I am going to retire with terminal embarrassment. Double socket in utility room about 4 ft from CU. Washing machine and tumble drier plugged in. Relaxed


  • Former Community Member
    0 Former Community Member
    Hello Chris


    This quote I suppose that the overloading could occur in say a commercial kitchen in a restaurant or pub.is Zoom's, hence my italicisation of it!


    Twenty years ago I inspected and tested some 50 or so pubs and couldn't believe that a manufacturer made a twin deep fat fryer each having a 3 kW element and it came with two separate 13 A plugs. These would inevitably be plugged into the nearest double socket-outlet. The manufacturer's logic being that they wouldn't sell any if a dedicated 32 A circuit had to be installed as per the traditional cooker.


    I also learned to only have packeted condiments with a pub meal having witnessed the ever so pretty dish with brown or red sauce in it being brought back from the table part used and poured back into the gallon bulk container for the next eager diner to sample.............


    Interesting wiring of yours but fairly typical I'm afraid.................


    Regards


    BOD


  • 0
    perspicacious:

    Hello Chris


    This quote I suppose that the overloading could occur in say a commercial kitchen in a restaurant or pub.is Zoom's, hence my italicisation of it!


    Twenty years ago I inspected and tested some 50 or so pubs and couldn't believe that a manufacturer made a twin deep fat fryer each having a 3 kW element and it came with two separate 13 A plugs. These would inevitably be plugged into the nearest double socket-outlet. The manufacturer's logic being that they wouldn't sell any if a dedicated 32 A circuit had to be installed as per the traditional cooker.


    I also learned to only have packeted condiments with a pub meal ...


    Interesting wiring of yours but fairly typical I'm afraid.................


    It was just a general question about rings in commercial kitchens. Yes, there must be 13 A sockets at the very least for small equipment, but radial may be a better option.


    I wonder whether the fryers' installation instructions specified two single 13 A sockets.


    I noticed when I was in Italy once, that those complimentary bowls of nuts in bars always had a teaspoon so that customers kept their fingers out.


    I do not quite see how my wiring could be any different - the CU is where it is and where else would you put the washing machine and tumble drier? Once again, radial may be the better option.


  • 0
    It is impossible to design in good use of a ring final circuit

    I agree you cannot control the plugged in loads (well, the total may be limited  by mcb choice I suppose.)

    However, there are things you can do that eliminate the cable overload risk altogether, though this is only worth it if you know it will be heavily used

    1) you can make sure that no more than 13A of load is within 20% of the MCB end (so if the ring uses 50m of cable, the first 10m on each end should only have  1 socket each.)

    , to avoid the double socket and adaptor thing as well, you could  simply have no sockets that close to the CU, or wiring the ones that have to be close as direct spurs from the MCB rather than mid-ring.

    Or

    2) you could arrange it so the cable's current rating of the starting end  of the ring is higher either routing the first bit of the ring as surface wiring or plastered direct,  or doing the first 20% or so in 4mm cable.

    M.
  • 0
    Hi Mike, in your example the leg carrying 27A would be running hot, perhaps 70 deg C, therefore would have higher resistivity than the cooler longer leg. I can't do the maths but would this make a huge difference?
  • 0
    Well, I have done my sums and things are not so bad after all.


    Ring is about 40 m long in total. Drier is plated at 2850 W; let's assume that the washing machine is the same, so 25 A total. 39/40 of the load goes the short way = 24 A.


    RM B - plastic trunking so CCC = 23 A. There is another ring and a couple of lighting circuits flopping around in there so derate by 80% = 18.4 A.


    Is this a problem? No because the overload is only of short duration. Phew - glad that's sorted! ?

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