Confused about cable sizes when fusing down, please help

30 Aug 2006
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United Kingdom

this question sometimes baffles me, now if you had a ring final in 2.5 and wanted to spur of that to a light that will be switched via a fused spur, i know that on the load side of the spur it can be 1.5 but does it still have to be 2.5 from the socket to the spur as im comng straight off the socket

the same applies if i was running an extractor from the ring final its 1.5 from the load side but from the ring direct is it 2.5 as its the same loading etc

many thanks
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Cables feeding accessories should be rated to the device protecting those cables. So, if you have a 2,5 ring feeding a spur to an accessory, the cable feeding that must be 2,5 also.
I agree with your answer secure, but if you have a 2.5 ring on a 32A mcb then in theory a single length of 2.5 won't take 32A...

I realise you should only have a single 13A socket (single or double) or a fused spur on the end of the cable so that it won't be over stretched, but that is relying on downstream protection rather than the 'device protecting the cable' (the 32A MCB)

Is this just another of those inconsistencies in the accepted practise or am I missing something?

Its one of those standard arrangement things, those in the know have done some calculations and have decided that the margins are such that they are happy for a 2.5mm² to be protected against short circuit/earth fault by a 30/32C CPD, unless I'm mistaken you you could use 1.5mm² and comply with BS7671 in some cases but that wouldn't be a standard arrangement and you'd need to do calculations to check it was adequate (and note them with the EIC), it just becomes far easier and is accepted practice to stick to standard arrangements which save everyone time :)
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Fair enough - I'm all for accepted practice that saves me having to do calculations :p

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