Hi – I want to spur off my ring main (4mm T&E, protected by 32A MCB) to provide power for an outside security light. I’ll tap into the ring with a 30A junction box, then feed the light via a switched, 3A FCU. I’ll then run a 1.5 mm T&E from the FCU to the light. Total length of spur is about 4 metres.
My question is: what cable size is needed from the JB to the FCU? I think I remember reading, possibly on here, that the cable on the supply side of the FCU must be rated as high as the ring (i.e. 4 mm). Why is this? Surely the cable from the JB to the FCU is also protected by the 3A fuse? I can’t think of a situation where the supply-side cable would carry more current than the load side cable. Or am I missing something fundamental here? Could I not just use 1.5 mm for the entire length of the spur?
Thanks in advance.
My question is: what cable size is needed from the JB to the FCU? I think I remember reading, possibly on here, that the cable on the supply side of the FCU must be rated as high as the ring (i.e. 4 mm). Why is this? Surely the cable from the JB to the FCU is also protected by the 3A fuse? I can’t think of a situation where the supply-side cable would carry more current than the load side cable. Or am I missing something fundamental here? Could I not just use 1.5 mm for the entire length of the spur?
Thanks in advance.