I was under the impression that it *IS* OK to place a 30amp junction box into the ring cable and take two 2.5mm spurs from it, so long as there are no more spurs on the circuit than there are sockets on the ring itself. I can't see that this would overload anything. Am I wrong then?
On 1 Apr 2004 07:47:40 -0800, in uk.d-i-y martin snipped-for-privacy@hotmail.com (Martin Pentreath) strung together this:
Each spur shall supply only one double socket, or two singles apparently. I would say it's bad practice to put more than one of anything on a ring. Better to extend the ring if possible.
As Lurch said, it's "OK", as in Regs-compliant, but less desirable than extending the ring (typically). Junction boxes are harder to get at for fixing faults/testing etc. than socketses, so spurs in the middle of a cable are undesirable. And since it's hard to know what loads will be connected to an arbitrary socket, putting sockets directly on the ring means worrying less about peak current draw, voltage drop, etc. Finally, it's a little easier working with just two cables in a backbox than three (2 ring and one outgoing spur).
None of these are "killer" reasons, and there are physical layouts where a remote socket is more sensibly supplied through a single spur than by extending the ring; but In General, extending the ring is cleaner than spurring off.
HTH - Stefek
Not sure that what he is proposing actually is compliant... it's certainly not common practice - you take just one spur from each point; socket or junction box, though I suppose you could put *two* junction boxes next to each other which amounts to a similar thing. If you're doing that however, why not split the ring and loop it through the new sockets?
AIUI the main reason for "one spur per junction and only as many spurs as original sockets on the ring" is mostly for load balancing in the halves of the ring. By limiting yourself to the above, it becomes physically difficult to put a load of extra sockets close together nearer one end of the ring. Without this rule you might find someone ending up with a layout like this:
+---S------S----+ | | /-+ S F | \-+ S | S S S | | | | | | +---S---J--S----+ | | | S S S(Where S=socket, J=junction box, F=fuse)
Which is undesireable because there is every chance that the currents in the legs of the ring will be unequal and may therefore (depending on the installation details) overload one of the cables. It is for this reason too that the OSG makes a point of telling you to ensure that "heavy" loads (heaters, washing machines etc.) are installed near the mid-point of a ring.
It's not so much to make it difficult for an installer, more to ensure that it is difficut for subsequent alterations to make the arrangement dangerous.
Hwyl!
M.
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