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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.
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    OlympusMons:

    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?


    i found this calculator Temperature Coefficient of Resistance (gsu.edu) and plugging in a few numbers gives a rise in resistance of 20% for copper, initial temp 20, final temp 70. So in Mike's example above the worst case first leg of the ring of length 1M would have an equivalent length running at 70 deg C of 1.2 Metres, so hardly any difference at all to the load sharing calculations.


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    OlympusMons:

    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?


    Now there's a thing. The ring could be self current regulating. The short hot insulated overloaded length has an increase in electrical resistance so the supply shifts to the longer leg.


    Z.


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    Yes Zoomup, I saw your post on the old forum about this and wondered if it would have much effect, apparently not in Mike's example, if my sums are correct. The increase in resistance with the constraint of keeping max temp at 70 Deg C doesn't have enough leeway for self-regulation it seems. Good point though.
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    OlympusMons:

    Yes Zoomup, I saw your post on the old forum about this and wondered if it would have much effect, apparently not in Mike's example, if my sums are correct. The increase in resistance with the constraint of keeping max temp at 70 Deg C doesn't have enough leeway for self-regulation it seems. Good point though.


    I would like somebody to offer empirical evidence of any thermal cable damage to a 2.5mm2 P.V.C. cable on a ring final circuit where a socket is positioned close to a consumer unit and heavily loaded. Anyone?


    A one degree rise in temperature of copper = a 0.393 per cent rise in electrical resistance.


    A 2m length of 2.5mm2 copper conductor carrying approx. 27 Amps will create approx. 10 Watts of heat.


    Z.


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    mapj1:

    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.



     


    At the risk of appearing to be a dissenter, I can not believe that most sparks installing a ring final even bother to consider the positioning of sockets regarding equal loading. Has unequal loading even been shown to cause cable thermal damage?


    Having just re-read 433.1.204 I have concluded that the reg. is not about bunching of sockets or having them too close to the origin of the ring final circuit. The reg. is concerned with ring final circuits with or without unfused spurs. That is why the reg. requires that the load current in any part of the circuit (in ring or unfused spur) is "unlikely to exceed for long periods the current carrying capacity of the cable." This is especially important for unfused spurs. That is the main point being made.


    I do seem to be a dissenter. Oh ek!


    Z.




     


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    Ha!

    I chose the 1m example as one that almost never occurs, unless there is a socket in the cupboard with the consumer unit.

    I agree, I can't imagine that many sparks in the heart of battle (or any other southern town for that matter ) bother to think about spreading the sockets around the ring, although there are rules of thumb, like twisting the ring when you have a situation like a  long corridor so that sockets 1,3,5 are one  one leg of the ring, and sockets 2,5,6 are on the other.


    Even if you give it no thought at all, the risk of an overloaded ring in a domestic setting is very small - partly as to have 30 A of load plugged in at once is very rare, and to have it all sweating away within a very small distance of each other is rare still -  unless the building has air conditioning it will become insufferably hot pretty quickly.


    Also the cable is not at risk as much as it may appear -

    1) most houses are not a 30degree ambient - the loft in summer maybe, but then that is not when you start lugging electric fires about. A 20degree ambaint is more likely.

    2) a 70C rated cable does not rise up and seize you round the neck and throttle you at the instant it reaches 71C. Indeed pour bioling water over a cable offcut, or if on site drop it into you mate's coffee, and it does not melt the insulation like butter. Yes it gets a bit more flexible, but in the wall of a house it is not being flexed. You need to be well over 100C, more like 120-130 C before the plastic actually starts to move of it's own accord.  the current to reach that sort of temp relates to perhaps twice the intended heating per unit length, and so about 150% of the supposed max current.

    There will be a small temperature effect that raises the resistance of the most overloaded section, but it is small effect compared to the variations in cable length and so on..



    Repeated cycling of hot and cold to well above 70C may cause the plastic to age prematurely, but only in the sense of a 20 year life instead of a 50 year one, not instant failure.

    This is how we find showers wired in 2.5mm that have been there for years that look perfectly fine.


    Mike

    PS I am pro ring by the way - a perfectly sensible solution to getting power over a large area with low voltage drop and more flexible cables than the 4mm2 radial, and also allows a self test that earthing goes all the way to each point, even if you are not sure where the far point is. (bit like the foundation electrode having 2 terminals - you can verify the continuity, and you know both are connected to some metal)

    It needs understanding, but if you can't grasp ohms law and cable sizes, you really should not be wiring things up except to a preset recipe anyway. Which is fine for the bulk of situations.




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    " I can't imagine that many sparks in the heart of battle (or any other southern town for that matter ) bother to think about spreading the sockets around the ring, although there are rules of thumb, like twisting the ring when you have a situation like a  long corridor so that sockets 1,3,5 are one  one leg of the ring, and sockets 2,5,6 are on the other. "


    Hoy !      I do
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    Here we are again discussing rings, cable current rating, and plugging in several electric fires, washing machines, fryers, etc. There is a fundamental error in all of the comments which say "cable current rating" of which I cannot understand why this error is continuously made by electricians. I have discussed it at length several times before, but obviously, the point was missed or not understood. The tables of cable current rating and installation method are for permanently loaded cables, with a period of 24 hours. The properties to consider are power dissipation and thermal mass. The power is low and the mass surprisingly high. The only reason that the distribution network survives at all is this simple fact. It takes a long time to get the cable hot. Actually, you all know this because damaged cables in domestics from internal heating are completely unknown (implied by Zoomup asking for empirical evidence). The temperature at which PVC melts is 185 degrees C. It will go very soft at about 150-160 degrees but not fall away. So a safe temperature for a cable is MUCH higher than the tables in BS7671 suggest. There is loss of plasticiser to consider, but this doesn't matter much unless the cable is flexed (when it will be very stiff but can be fractured). This is the real effect of operation at excess temperature, loss of flexible life. A 2.5mm cable carrying 32A gets fairly warm but easily handled still. If buried in thermal insulation it will get hotter, but you will have to wait some time. There is no sudden failure, catching fire, or anything much else. I suggest that some experiments along these lines should be mandatory for all electricians, then a much better understanding of cable ratings should result, particularly for showers and cookers!
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    Good point, but should it designed and installed with being continuously rated in mind. I agree that such use would be unlikely, but if there were an issue the designer maybe called upon to justify why it wasn't.
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    Well I always thinks you should do the best yer can.

    A quick (note quick) reckon might nudge you to think "Oh this might be a bit better" judge the loads and frequency and duration of such - a quick mental thought not reams and reams of calcs and sometimes you come up with a slightly better arrangement for very little additional thought. I see quite a few do rings as a radial then bring a long run from last point to complete a ring which is a bit pointless.  Depends upon the setup. Especially a couple of kitchen & garage rings I did just a few alterations to get the heavy loaders in the mid 1/3rd instead of near one end might be worthwhile

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