unfused reduction in cable size

[user56565]

hi guys

i have been asked to add on an fcu via spur to a ring as usual. however when inspecting, i have seen that this ring has been wired in 4mm2 cable, and consequently there is not much room in the socket terminations to add on another set of 4mm conductors (not even 2.5mm. ps this is definitely a ring!)

im not entirely sure, but i think i recall reading somewhere, that all reductions in cable size can be fused within 3m of the reduction in cable size. is this correct?? if so, where is it stated? (as i have to be sure). i assume that this would then permit the attached diagram to be connected..

http://i7.photobucket.com/albums/y278/user56565/fcu.jpg

im not sure if there's a safety issue in this. short circuits due to a mis-drill etc on the 'unfused' part of cable would be easily handled without any cable damage by a type B. overload on this part of the cable wouldn't be possible because of the FCU. ps the 'unfused' cable is very short in length (10cm or so).

i apologise for asking. i have never come across a ring wired thicker than 2.5, and thus haven't been in this situation ever before. rather than get things messy trying to do it the typical way (maybe changing sockets, using junction boxes etc), i thought i should check on this first as im sure ive read this somewhere before.

many thanks

 

[securespark]

I have just got 3 x 4mm into a GET socket. Maybe change the socket and stick to the same cable.....

 

[user56565]

its what id prefer to do. however when suggested, they were put off by the extra cost (changing all kitchen sockets to match). but i don't know if there's a reasonable alternative.. thanks for your GET advice. not sure what manufacturer they currently have...

 

[ban-all-sheds]

i have seen that this ring has been wired in 4mm2 cable

What Iz has been assumed for a 4mm² ring final, and what regulation supports that?

 

[pdcelec]

hi guys

im not entirely sure, but i think i recall reading somewhere, that all reductions in cable size can be fused within 3m of the reduction in cable size. is this correct?? if so, where is it stated? (as i have to be sure).

This is true but you need to prove that the bit of cable will have the necessary capacity even at such a short length.

From table 4d5A it can be shown that 2.5mm cable when clipped direct can carry 27 Amps but 4mm cable enclosed in conduit in an insulated wall will carry 26 Amps.

 

[plugwash]

the main issue with such a run of reduced size cable is short cuircuit protection, but if thats not met for such a short run of virually any size then it probablly isn't bet for the rest of the cuiruict either. mcbs trip so fast that cable heating on a short is going to be virtually nonexistant.

the fcu downstream is providing overload protection. so i really can't see a problem with a layout like this.

afaict the normal reason for using 4mm rings is thermal insulation or an above average level of paranoia on the part of the installer. is the breaker on the ring still a normal 32A

 

[user56565]

thanks for the replies

i will go ahead, with this. thanks plugwash for similar thinking.

ps as far as i can gather, 4mm ring was used because of wool-fibre insulation under the floor..