Mixing Cable Sizes

[colin H]

I am about to move a socket in my kitchen which is a spur off the ring main .
I bought some 2.5 t+e cable , but on closer inspection , the existing cable used is 4mm . Would I be o.k. to use the 2.5 (I need some of it elsewhere) , or should I get some 4mm .
Its just for general use on small appliences ect .

 

[holmslaw]

..

 

[ban-all-sheds]

I bought some 2.5 t+e cable , but on closer inspection , the existing cable used is 4mm.

Are you sure it's a ring final?

What size is the ring cabled in?

Has 4mm² been used because of circuit length, or because of the installation method resulting in a low current carrying capacity?

 

[PrenticeBoyofDerry]

I would always use the same size conductor as the existing, when extending a circuit.
The only time I would consider using a smaller CSA, if discrimination was being offered to that part of the circuit via a fused con unit.

 

[JohnW2]

I would always use the same size conductor as the existing, when extending a circuit.
The only time I would consider using a smaller CSA, if discrimination was being offered to that part of the circuit via a fused con unit.

That's a good general principle, but it creates some inconsitencies with common practice when it comes to spurs. Even running an unfused spur in 2.5mm² cable from a 2.5mm² RFC is not really 'same size conductor', since the effective CSA of the two ring cables is much more than 2.5mm² at many points in the ring. More straightforwardly, Appendix 15 of the regs (not unreasonably) explicitly allows a 2.5mm² unfused spur off a 4mm² radial circuit.

Kind Regards, John.