Spur SFU's off a ring main socket

[DetlefSchmitz]

But John, you picked me up recently on this. Apparently you ARE allowed to have two separate spurs from the same point on a ring. The point really relates to the cable capability.

Anyway, might make a difference in this case?

 

[JohnW2]

But John, you picked me up recently on this. Apparently you ARE allowed to have two separate spurs from the same point on a ring. The point really relates to the cable capability. Anyway, might make a difference in this case?

We're talking about different things. There's nothing in the regs which precludes having two separate spurs (with their own cables) from the same point in the ring, but nor is there anything in the regs that says that you can have two sockets (or FCUs,or whatever) attached to a single (unfused) spur cable - on the contrary Appendx 15 (albeit 'informative'), which only envisages 2.5mm² cables, explicitly says that you can't

I agree that, electrically speaking, having two sockets on a 4mm² spur from a ring final would seemingly be OK - but one would have to argue that 'from first principles', since it's not something which the regs mention doing with a ring final circuit.

Kind Regards, John

 

[DetlefSchmitz]

Just thought it might offer the OP an alternative.

And Iggifer said

You can take each FCU from a different point on the ring, or use one FCU to feed the other two, but two from the same point on the ring is not allowed

 

[JohnW2]

Just thought it might offer the OP an alternative.

Agreed - as I said, it seems electrically OK, but I don't think many people would want to have to argue ('electrically') about tha acceptability of something which was not directly supported by the regs (and which some regs-focussed people {including inspectors!} might even think was non-compliant with the regs - hence hassle and argument!).

And Iggifer said:

You can take each FCU from a different point on the ring, or use one FCU to feed the other two, but two from the same point on the ring is not allowed

He did - and, as I've indicated, if he meant that two spurs (with separate cables) from the same point on the ring is not allowed, then I don't think he'll be able to find anything in BS7671 (or, IIRC, even the OSG) which actually says that.

Kind Regards, John

 

[wildman147]

Hi John, Yes I understand that argument and your advice seems very reasonable. The first supply feeds a 10amp fixed load and the second is just for display lamps in a fireplace. Hence I see no reason why your alternative solution would not work. I will supply the second FCU from the load side of the first FCU(10A fuse) and fit a 2amp fuse in the second FCU and then feed the lights in 1.5 mm T & E cable. Thank you so much for your experienced opinion and I will take your advice in the near future. With fond regards wildman147

 

[EFLImpudence]

Plus, of course, there are 'special' rules which apply to our unique and somewhat illogical Ring Final Circuits.

A spur in 4mm² feeding several sockets may be alright at the centre of the ring but not if it were very near one end.

 

[JohnW2]

Hi John, Yes I understand that argument and your advice seems very reasonable. The first supply feeds a 10amp fixed load and the second is just for display lamps in a fireplace. Hence I see no reason why your alternative solution would not work. I will supply the second FCU from the load side of the first FCU(10A fuse) and fit a 2amp fuse in the second FCU and then feed the lights in 1.5 mm T & E cable.

In one sense, that would technically only 'leave' you 8A for the first load - if potentially 2A (I know it's unlikley to be that much in practice) was being 'used up' by the second FCU - would that be enough. Is there any reason why you couldn't have a 13A fuse in the first FCU?

Also, as I and others said, if you wire the lights in even 1mm² cable, let alone 1.5mm², cable you could (as far as the cable was concerned) run it from the load side of the first FCU, and not need a second one (that assumes that your lights don't 'require' a 2A fuse.

Kind Regards, John

 

[JohnW2]

Plus, of course, there are 'special' rules which apply to our unique and somewhat illogical Ring Final Circuits. A spur in 4mm² feeding several sockets may be alright at the centre of the ring but not if it were very near one end.

That's not really unique to the situation of using a single 4mm² unfused spur cable to feed several sockets (I was really only thinking of two, but I suppose it could be more). The same issue of 'bad design practice' (risk of overloading one leg of ring) would equally arise if one had two (or 'umpteen', were it physically possible) 2.5mm² unfused spurs, each serving one socket, attached at the same point, or very close points, near one end of the ring, wouldn't it?

Kind Regards, John

 

[EFLImpudence]

Yes it would. I did not mean that was the only way but an example of why the ring has special rules, some of which seem illogical - but then all circuits should be properly designed.

 

[wildman147]

Hi John, Yes I understand that argument and your advice seems very reasonable. The first supply feeds a 10amp fixed load and the second is just for display lamps in a fireplace. Hence I see no reason why your alternative solution would not work. I will supply the second FCU from the load side of the first FCU(10A fuse) and fit a 2amp fuse in the second FCU and then feed the lights in 1.5 mm T & E cable.

In one sense, that would technically only 'leave' you 8A for the first load - if potentially 2A (I know it's unlikley to be that much in practice) was being 'used up' by the second FCU - would that be enough. Is there any reason why you couldn't have a 13A fuse in the first FCU?

Also, as I and others said, if you wire the lights in even 1mm² cable, let alone 1.5mm², cable you could (as far as the cable was concerned) run it from the load side of the first FCU, and not need a second one (that assumes that your lights don't 'require' a 2A fuse.

Kind Regards, John

Hi John
Yes I can up the fuse on the first CFU but the load on the second CFU is so minimal I don't think that will be necessary (2x50 watt spots from a 50volt transformer) Thanks once again for your advice,
sincerely, wildman147