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Ring Main Currently Spurs in 16mm Oval Conduit

MichaelBAC1-11

Hello Everyone and I'm new to the Site so Greetings and hope your'e all well.

I've recently moved house. The rings are continuous but the drops to most sockets (apart from the kitchen) are spurs via JB's with a single 2.5T&E drop in 16mm oval conduit. I'd figured on a certain amount of re-wiring anyway but as opposed to re-chasing and new conduit, which I've done before, I had the idea of jointing the 16mm oval via a 20mm extender/convertor to a circular conduit box and running in single core 2.5mm thereby removing the spurs and making each socket part of the continuous ring. I'd continue the runs in 20mm conduit/ single 2.5mm covering all the affected sockets which aren't many. Any new sockets could be added via 2.5 T&E and appropriate conduit. I calculate 2.5mm single stranded cable in less than 3metre straight 16mm conduit runs has a cable factor of 43 and 6 cables x 43 gives 258 which seems suitable for 16mm conduit which has a cable factor of 290.

Just a couple of notes:- The existing 16mm oval conduit protrudes above the plasterboard into the loft and the 2.5mm existing ring is 2.5 with a 1.5mm CPC. The 16mm conduit is continuous into the socket backboxes. It's a dormer bungalow and all the upstairs sockets are on internal stud walls so can be easily converted using 2.5T&E. The kitchen has already been re-wired.

Any comments or suggestions welcome as I've always chased out and replaced conduit in the past.

Best regards,

Michael

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    I too would be in the 'why bother?' camp - while the old rules of thumb suggested no more spurs than there were points on the ring, that was more to make sure people did actually install a ring and balance things around it reasonably, than for any particular electrical or safety reason you're likely to encounter with your layout. Obviously there's a limitation if you ever wanted to add extra sockets (and didn't want to fuse the spur) - but you could address that as and when.

    I must admit I've always thought of oval conduit as more of an improved version of capping than something I'd run unsheathed singles in - it always felt a bit thinner and flimsier than the proper round stuff. But I suppose as long as the manufacturer says it complies with the relevant BSs for conduit that BS 7671 refers to you should be OK. From memory my (now well out of date) copy of the OSG only gave factors for circular conduit - if that's still the case it might be worth double checking that the same factors do really apply to oval of the same nominal diameter - my gut feel is that the internal c.s.a. might be smaller and for larger cables the fill pattern might not be quite as easy.

      - Andy.

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  • 0

    I too would be in the 'why bother?' camp - while the old rules of thumb suggested no more spurs than there were points on the ring, that was more to make sure people did actually install a ring and balance things around it reasonably, than for any particular electrical or safety reason you're likely to encounter with your layout. Obviously there's a limitation if you ever wanted to add extra sockets (and didn't want to fuse the spur) - but you could address that as and when.

    I must admit I've always thought of oval conduit as more of an improved version of capping than something I'd run unsheathed singles in - it always felt a bit thinner and flimsier than the proper round stuff. But I suppose as long as the manufacturer says it complies with the relevant BSs for conduit that BS 7671 refers to you should be OK. From memory my (now well out of date) copy of the OSG only gave factors for circular conduit - if that's still the case it might be worth double checking that the same factors do really apply to oval of the same nominal diameter - my gut feel is that the internal c.s.a. might be smaller and for larger cables the fill pattern might not be quite as easy.

      - Andy.

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