Ring circuit and spur/radial from same MCB ?

[LumpHammer]

I've been looking at a relative's 1960s house, and it looks like various people have done electrical work over the years. I've a couple of questions:

1) It looks like there is both a ring circuit (powering downstairs sockets) and a spur/radial (powering an electic fire via a FCU) attached to the same MCB within the modern consumer unit. I'm not going to open up the CU to confirm, but that's what it looks like. All cable is 2.5mm. Is that within regulations? I suppose it's no worse than a spur off the back of a socket, it's just a spur connection made within the CU.

2) It would be very convenient if I could create a spur off one of the rings, in order to power several low power devices. Is this permissable (although not ideal) if the whole spur is controlled by an FCU with a

13A fuse? Each device would be attached to it's own FCU.

[spuorgelgoog]

Yes (all other things being equal, etc)

Yes.

Owain

[Tim Watts]

A single spur off the MCB is perfectly correct.

Spurs are done roughly according to these rules:

1) No more spurs than there are devices on the ring 2) One spur max per device on the ring. 2a) Or you may spur from a junction box anywhere in the ring, noting condition 1).

Now once you've put a 13A FCU in, you can actually do pretty much what you like after that - you could run a radial off with several sockets - noting of course that the sum total load of the radial may not exceed

13A. If you mean your devices are hard wired, then yes, you can have a 13A fuse at the "head" of the spur and lots of flex connection plates downstream - but you must have a switch somewhere for isolation - either at each flex connection, or at the 13A fuse.

[Andrew Gabriel]

No such restriction, although the device must have the terminal capacity for the number/size of conductors in use. That varies with the quality of sockets/FCUs/etc.

The other thing is a ring should not have a heavy concentration of its max load near one end.

[John Rumm]

Yup, a spur taken from the origin of the circuit is perfectly acceptable. One just needs to take care that its not treated as a radial and then further extended.

You can create a fused on an unfused spur. A fused spur can then feed any number of further outlets / connections etc as required. A unfused spur has more limited application and is restricted to feeding one socket (single or double).

[ARW]

Now on the latest NICEIC inspection this was disapproved of. But as the NICIEC inspector did not want to talk to me (nothing personal - on the last inspection I passed with flying colours so he wanted to test someone else) so I was unable to find out why they do not like it.

[dennis]

You should be OK to take as many spurs off the CU as the terminals can take. The rules for distributing them around the ring is because its possible to overload a section of the ring. You can't overload the CU unless its junk.

[ARW]

I agree. Probably 4 cables at most on most MCBs IMHO. So two unfused spurs at most.

It's the unbalancing of the ring that matters (heavy loads on one end for sustained periods). But if you were to extend the ring instead of adding a spur then logically you could still be unbalancing the ring just as much as adding an unfused spur if you are using high loads.

[cl]

You have it right, as long as it's within what's allowed as a spur off a ring main then it doesn't matter if it's spurred off in the CU.

Yes, I don't see the problem.

[Tim Watts]

OK - but it was certainly a recommendation - perhaps from Whitfield and the 16th?

[Windmill]

From an electrical standpoint a triple socket, all of which nowadays seem to be fused with a 13A fuse, is equivalent to a single socket. But I wonder if the regulations would therefore allow two triple sockets on an unfused spur? It would be no different from one double socket, in terms of maximum possible load.

[Andrew Gabriel]

The whole no more spurs than devices on the ring is only a recommendation - it's never been in the wiring regs. It comes about because rings usually have no spurs initially, and if it's grown to the point where it's doubled in size with spurs, it's probably time to re-evaluate the suitability of the installation for current and future use.

However, as a recommendation, you can ignore it if the circumstances of that particular installation are still safely covered by the wiring layout.

[Tim Watts]

Actually it would, as a double socket is rarely rated above 20A combined load (look at the back of one :)

[Tim Watts]

Ok - thanks for the clarification. It was indeed "recommended" rather than "a regulation".

[dennis]

House holders don't look on the back, they just plug two fires in. If the socket melts blame the installer.

[John Rumm]

A double socket would be treated as a nominal 20A load, two triples as 26A.

It would not meet the regs requirements, but then again its also unlikely to cause a problem in real life. A single 2.5mm^2 T&E could be rated as high as 27A capacity (in plaster, or clipped direct), so the main issue would be if you could stack up enough sustained load on the sockets to overload the spur cable for a prolonged period. Probably quite difficult to do since you would need the best part of 7kW of load for an extended time.

[Windmill]

Just looked at a very large and clunky MK double socket from my junk box. Everything about it says 'over-speced' (must be pre-Thatcher) but the only thing it says on the back is 250V. 13 A.

[Windmill]

I suppose you could cheat and put 10A. fuses in place of the 13A. ones, if worried!

[John Rumm]

Indeed. (and if talking about the low end 4 way trailing sockets rather than real wall mounting sockets), that's not a bad idea anyway.