Cable size on supply side of FCU

8 May 2007
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United Kingdom
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.
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I know it's unlikely but suppose somebody was drilling a hole and shorted the cable supplying the FCU then the main fuse would be the protection offered on the cable before the 3amp fuse.

Are you sure the circuit is a ring and not a radial? It would be better to use the same size conductor upto the supply side of the FCU just to be on the safe side.
Actually, there is a set of conditions where the regs will allow a smaller cable between the spur off point and the FCU -let's see if I can find it...

473-01-02 and 473-02-02. It's a bit vague in that meeting 473-02-02 (ii) and (iii) are matters of judgement/opinion - "reduce to a minimum" does not mean "reduce to zero".

So, it seems to me that if one were, for example, to spur off a ring final using 1.5T&E, with a run of less than 3m to the FCU, and you ran the cable in steel conduit, then that would meet the requirements.

But to err on the side of safety, I would always wire in 2.5mm up to the FCU. But even that is only rated at 27A clipped direct, so is, technically, not totally protected by a 32A MCB (though the amount of under-protection is not really significant).

And better to place the FCU on the ring proper, if at all possible.

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