Extending ring mains, cable sizes,voltage drops etc

[towman]

This is a bit of a theoretical one - I've been reading around, picking bits of info here and there and have been wondering about the advice on extending ring mains?

Most rings are in 2.5 mm cable which is rated at 32A (?) - but is this not in open air. Hence rating drops if cable is enclosed - and drops significantly. There can be very few installations where cables aren't buried in walls etc so the cable could be rated as low as 20 A or lower - which isn't a lot on 1 ring! Then someone extends the ring! I have a vague idea of the concepts of voltage drops etc and the recommended maximum cable lengths - it doesn't take much to have 50 m of cable on a ring.

I'm not an expert but it strikes me that extending a ring in this case must be a BAD THING - and in liklihood will be in most situations?

Any thoughts/explanations on this appreciated - not because I'm going to extend a ring ( I'm not sure I want to take that chance on my single ring supply my house!) - it has convinced me the best way forward is to put in extra circuits! I'd just like to know what the experts opinions etc are on this.

Thanks again
Towman

 

[steve190]

Just to start the ball rolling, as I'm sure the more learned will explain in greater detail.

With a ring the load share between both halves of the ring. A 32a ring is can supply up to 100 sq m of floor space (OSG pg 151 (brown edition)). This is for a domestic location and takes into account the correction factors and diversity.

One of those areas that is open to much debate. (As a theoretical question 2.5mm T&E using method 1 can carry up to 27Amps - so with the load split between both parts of a ring? But if through damage etc the ring becomes a radial?)

Just my mussings

 

[Damocles]

Indeed, breaking a ring is not a good idea as the remaining cable can be overloaded. A similar thing happens if all the heavy current draw is connected at one end of a ring. The resistance for the long way round back to the CU is much greater than the short way, so most of the current goes the short way.

This is also one reason for treating a kitchen to its own ring. The kitchen is likely to have a lot of load all at once. And if there was only one ring in the house, all this load would probably end up at one end of the ring.
Both these problems can be minimised by good design so that the sockets are distributed evenly around the ring. Then hopefully the risk of either causing a serious problem is minimised. But it is also why it is inportant to test the continuity of the ring from one end cable to the other. A radial circuit will show up a break because it plain stops working.

The 100sq m limit is just one of the design restrictions. All the others have to be taken into account. You are quite correct that anyone extending a ring should think about how long the ring probably is already. It is a pretty good risk that a short extension will not take it over the limit. You can test the length by measuring the loop resistance around the cable....while you are checking the continuity.