Large Rings - Cable size?

19 Feb 2009
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United Kingdom
Hi guys, for my first post I’m looking for some advice on wiring a ring for a large living room. The room is 11m x 5m, the floors are concrete, so the cable runs from the CU (32A MCB) to the 1st floor, through 2m of insulated cavity wall (all cables are bunched) and along insulated plaster board ceilings, there are 2m drops to each socket (of which there are 8 sockets), the total ring length will be approximately 70m. I’m a little confused as to what cable should be used, 2.5mm or 4mm, based on voltage drop, derating etc? I’d be grateful for any info. Thanks
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If you are installing a new circuit you should know how to do cable calculations.
The answer is in first year 2330 and in the red book.
Firstly, as this is a new circuit I presume that you know that you have to inform the council + it will require a electrical installation certificate, and of course that you have the proper testing kit for that purpose.

If the cable drop is not in an earted metal conduit or less than 50mm deep in the wall it should be protected by RCD.

The fact that the cable is run in an insulated area, already de-rates your cable to less than half, i.e if the carrying capacity of the cable is 27A it should be considered as about 13A, this without considering other de-rating factors.

If you are using a cable that is bigger than 2.5 it will solve this problem and if your measurements are correct and it is 70m it might help with the volt drop as well (for power circuits i think the volt drop allowed in 17th edition is 5%, I do not have the book in front of me).

You can go both ways,you can increase the cable size and reduce the CB value to 20A or even 16A, this will help with the volt drop and with the de-rating factors as you reduce the In (protective device rating), which is used in the "de-rating" formula.
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Thanks for the replies.

Flameport, yes 11m x 5m is correct; the footprint of the house is around 120 square meters.

Taylortwocities, I haven’t got that far yet. I wont be installing the new circuit as I’m not qualified, I’m just trying to understand what electrical work might be required as the house is being renovated. It’s obvious that a lot of corners were cut when the house was renovated several years back, so I want to be prepared before I get the pros in.

Thanks for the info Albert; Just to clarify my first post: currently the entire ground floor, all 120 sq. meters (including kitchen) is on one huge 2.5mm ring! So I’m looking into splitting this into several separate rings, including one for the living room and one for the kitchen. The drops are in plastic conduit and the circuit is RCD protected. I worked out that the current carrying capacity (of 2.5mm) is only around 10A after derating and the voltage drop is just under 5%, so I think 4mm maybe a safer option.
Remember that you do not have to use ring configuration, you can use radial. for the last 2 years I stopped using ring configuration (do not get me wrong it is not because it is not safe or not good). I find that a radial circuit although will require larger cable (in normal circumstances) it is easier to test, more simple to install and for maintenance, so if you need a larger cable anyhow there is no reason why not using a radial configuration for the new circuits.

Just to remind you; your lighting, oven, immersion, boiler probably are radial circuits

If you can separate the existing circuit to several smaller ones, I think, it is a very good idea.

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