UK Change in current capacity along run

[Tom_CV]

Hi,

I have a book on domestic wiring which says that everywhere there is a change in current carrying capacity along a circuit there must be some kind of protection (fused spur etc).

Suppose I need to run above an insulated ceiling (reference method 101) and then down a wall (reference method C). According to table 4D5, 6mm twin+earth has a capacity of 27A with method 101, and 2.5mm T+E is also 27A with method C.

Does this mean I can change cable widths where the method changes, with just a maintenance free junction box? Or is the rule really everywhere there is a change in wire type I need protection?

The situation is that the loft insulation has just been increased, and I don't mind putting a fat cable across the loft, but I don't want to have to chase it into the walls.

I've had a look through the wiring regs and can't find the relevant rule. Can you point me to the right place please? I only have a copy of 17th edition but don't mind a trip down the library if the latest edition has a change in this area.

Many thanks,

 

[mhar]

Don't think it says change in current carrying capacity, doesn't it say change in conductor csa (which will affect current carrying capacity? There is a 3m rule for backfusing in this circumstance

 

[littlespark]

look at the load at the end of the cable..... not what the cable can actually carry...

ie, if its just a single socket... the max it can take would be the 13A of the plug thats plugged into it.

 

[pc1966]

In the current book (18th, amendment 2) it is under regs 433.2 and 434.2

In most domestic situations the usual approach is the whole circuit is designed for a given load, and that is the supply MCB/RCBO rating. In some cases you can rely on a load not being able to cause overload, for example a fixed heater (normal load or high current fault) or a fused 13A socket (where the plug's fuse limits the overload possible).

If you fear the new loft insulation has lowered the circuit's CCC then dropping the related circuit MCB from (say 32A RFC) to 25A is a reasonable solution.