True. Not sure how you'd get two of those babies into the back of a socket outlet 
wiring of a ring main? why is it 2.5mm cable? and the regs?
- Thread starter bob1234
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It is considered when working out ring main loading that 20A is drawn from a centre point and 12A is spread even though out the system. As a result when working out volt drop one uses 26A as the load. This is of course an approximate method and under certain unusual conditions the loading for each leg can exceed the 22A limit. Because of this any fixed appliance over 2KVA should be powered from it's own circuit and heavy users like washing machines and tumble driers should also be on their own circuit. Item like kettles are used for such a short time they can be discounted.
Anything less than 32A would result in likely hood of max draw very close to origin causing an overload so ring mains are only feasible where full 30/32A is used and using smaller cables with smaller MCB/Fuse just does not work.
So all ring mains use 1.5mm mineral insulated or 2.5mm for all other types of cable.
4mm cable with radials can often exceed to capacity of the cable and 6mm will only fit some makes of sockets so until the spec for sockets change and they are made able to take three 6mm cables there is a design problem using radials with 32A supply.
Using radials or any other system at less than 30/32A is likely to cause problems where overloading will open circuit the supply, because just two 13A appliances can cause an overload. So in practice using 20A MCB's and 2.5mm radials is just asking for problems with overload causing open circuits.
Using 6mm radials and spurring off for each socket from a JB would work, but the spur has to be less than 3 meters and the JB needs to be either maintenance free or accessible. So although this may work in commercial premises it is really a non starter with domestic.
The net result is the ring main is likely to be with us for some time. The old idea of a single socket on each circuit with it's own fuse may have worked when the norm was 5 sockets per house. But with the increase in socket requirement both the EU mainland method and ours have problems. But the EU mainland method has more problems than ours so of the two we have the better system.
The idea of a fuse at each outlet is only senseable option and the only question is should this fuse be in the plug or the socket. Having the fuse in the plug allows the use of thinner cable to each appliance so has some very good advantages. But fusing in socket means the plugs can be cheaper and smaller.
To me the fusing in plug seems far better method. But I also though 8 track was better than cassette and V2000 was better than VHS but in both cases market forces resulted in the cheapest rather than best being adopted.
I hope I have given a balanced reason why we use the ring main. But I am sure others will disagree and the debate could go on for years.
As to page 249 and the chart and table in most cases this is not important as the RCD will cover this and volt drop has more to do with limits than over current. But where fixed appliances are used and Ali-tube cable with no RCD protection then the earth loop impedance (ELI) and prospective short circuit (PSC) could still be the limiting factors so the table is still produced for those circumstances.
However this is not really a DIY question and maybe better asked on the IET forum if you really want a technical answer.
Anything less than 32A would result in likely hood of max draw very close to origin causing an overload so ring mains are only feasible where full 30/32A is used and using smaller cables with smaller MCB/Fuse just does not work.
So all ring mains use 1.5mm mineral insulated or 2.5mm for all other types of cable.
4mm cable with radials can often exceed to capacity of the cable and 6mm will only fit some makes of sockets so until the spec for sockets change and they are made able to take three 6mm cables there is a design problem using radials with 32A supply.
Using radials or any other system at less than 30/32A is likely to cause problems where overloading will open circuit the supply, because just two 13A appliances can cause an overload. So in practice using 20A MCB's and 2.5mm radials is just asking for problems with overload causing open circuits.
Using 6mm radials and spurring off for each socket from a JB would work, but the spur has to be less than 3 meters and the JB needs to be either maintenance free or accessible. So although this may work in commercial premises it is really a non starter with domestic.
The net result is the ring main is likely to be with us for some time. The old idea of a single socket on each circuit with it's own fuse may have worked when the norm was 5 sockets per house. But with the increase in socket requirement both the EU mainland method and ours have problems. But the EU mainland method has more problems than ours so of the two we have the better system.
The idea of a fuse at each outlet is only senseable option and the only question is should this fuse be in the plug or the socket. Having the fuse in the plug allows the use of thinner cable to each appliance so has some very good advantages. But fusing in socket means the plugs can be cheaper and smaller.
To me the fusing in plug seems far better method. But I also though 8 track was better than cassette and V2000 was better than VHS but in both cases market forces resulted in the cheapest rather than best being adopted.
I hope I have given a balanced reason why we use the ring main. But I am sure others will disagree and the debate could go on for years.
As to page 249 and the chart and table in most cases this is not important as the RCD will cover this and volt drop has more to do with limits than over current. But where fixed appliances are used and Ali-tube cable with no RCD protection then the earth loop impedance (ELI) and prospective short circuit (PSC) could still be the limiting factors so the table is still produced for those circumstances.
However this is not really a DIY question and maybe better asked on the IET forum if you really want a technical answer.
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True. Not sure how you'd get two of those babies into the back of a socket outlet
As long as they're no bigger than baby mice, it shouldn't be too much of a squeeze.
you keep saying 21A.. that's only for ref method 100..
clipped direct ( ie embedded in plaster on a wall and run under the floors with no insulation etc ) it's 27A..
clipped direct ( ie embedded in plaster on a wall and run under the floors with no insulation etc ) it's 27A..
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And for method 103 it's 13.5A....
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