Quick question (hopefully)

[FunkyJive]

For a conventional UK ring-mains socket, is there any such thing as a standard 'minimum' guaranteed cable acceptance to the screw terminals beyond 2 x 2.5mm?

I'm laying some wiring in the wall cavity of an outbuilding and was wondering whether 2 x 6mm CSA (i.e. a 6mm ring daisy-chain) could be expected to fit in each screw terminal of conventional ring sockets (yet to be ordered), or whether I'd need to reduce the CSA to 4mm.

I've got plenty of 6mm T&E but would order 4mm if advisable/necessary, and hate clipping strands to make wires fit. Silly perhaps, but just me

Many thanks,

FJ

 

[ericmark]

This says MK sockets will take 2 x 6mm but not sure on other makes.

 

[PrenticeBoyofDerry]

The standard socket outlet will struggle to take 2 X 6.00mm T&E, sometimes it's a battle getting in the 4.00mm T&E.
If it's a RFC (ring) is there a reason why you do not or can't use 2.5mm instead of buying the more expensive 4.00mm?
You can still put a 32A breaker on 2.5mm, providing there are no de-rating factors to calculate in to the design.

 

[bernardgreen]

This says MK sockets will take 2 x 6mm but not sure on other makes.

But it still requires carefull dressing of the conductors to get them to fit into the back box and still get the front screwed to the box.

As a cost saver ( using surplus 6mm ) it may not be worth the extra hassle in the back boxes.

If it is to reduce voltage drop along a long run then it might be cost effective to use 6mm for the long run(s) and 2.5 for the short ones.

 

[PrenticeBoyofDerry]

I've got plenty of 6mm T&E but would order 4mm if advisable/necessary, and hate clipping strands to make wires fit. Silly perhaps, but just me

I would not advice clipping strands neither, as you will be reducing the safe current carrying capacity of the circuit, this may not be an issue as it stands as far as your concerned, with 6mm T&E but you will be compromising the cable's rating.

 

[seasickstevie]

... use 6mm for the long run(s) and 2.5 for the short ones.

That would be unlikely to comply

 

[DetlefSchmitz]

... use 6mm for the long run(s) and 2.5 for the short ones.

That would be unlikely to comply

Bet you can't quote a regulation...

 

[bernardgreen]

... use 6mm for the long run(s) and 2.5 for the short ones.

That would be unlikely to comply

It has been accepted as compliant with BS7671 which states minimum cross sectional areas for conductors and does not ( apparently ) state maximum cross sectional areas for any conductor.

It would not comply in the minds of some cable installers whose "calculations" determine the size of cable that is the only size to comply.

 

[seasickstevie]

Can't find the reg (if it is a reg!), but have read somewhere that a cable of reduced CSA can be of 2m (from memory - might be wrong) length, and must be installed to eliminate chance of damage (or words to that effect).

 

[imroberts]

Not got my BRB or BGB with me, but I think I know the reg you're thinking of.

You can run a length of upto (3m iirc) of reduced CSA so long as it is fitted with a protective device downstream.

In essence, you could connect a length of upto 3m of 1.5mm cable on a 50A breaker so long as it was fused appropriately at the other end of the cable.

In respect of wiring a ring main with 6mm and 2.5mm cable though, this wouldn't apply, as the protective device (probably 32A) would be sufficient for both cables so there'd be no "undersized" CSA to consider

 

[JohnW2]

Can't find the reg (if it is a reg!), but have read somewhere that a cable of reduced CSA can be of 2m (from memory - might be wrong) length, and must be installed to eliminate chance of damage (or words to that effect).

I don't think any of that applies if the OPD is appropriately rated for the smallest CSA of cable that exists in the circuit. In other words, if you had an OPD rated appropriately for 2.5mm² cable, then I can see no reason on earth (and no regulation) why you could not have bits of 4mm², 6mm², 10mm² or even 16mm² cable in 'the run' - which, as Bernard suggested, one may sometimes want to do for reasons of voltage drop. Only if the cable size ever reduced below 2.5mm² (hence not adequately protected by the OPD) would there be an issue.

I suspect what you are thinking of is what the regs says about how long a cable may be following a CSA reduction if the circuit's OPD has too high a rating to protect the smaller cable and the down-rating OPD (e.g. an FCU) is at the downstream end of that length of reduced CSA cable.

Kind Regards, John.

 

[seasickstevie]

Yes, it's all making sense now. Thanks.
As imroberts says, I believe it is 3m max. if stepped down below appropriate size for OPD.

 

[holmslaw]

..

 

[JohnW2]

Yes, it's all making sense now. Thanks.
As imroberts says, I believe it is 3m max. if stepped down below appropriate size for OPD.

You're welcome and, yes, I think it's 3m - but there still has to be an OPD (e.g. an FCU) appropriate to the reduced CSA at the downstream end of that ≤3m of cable, and I think the cable has to be in such a position that it is unlikely to sustain mechanical damage (I don't have the regs immediately to hand).

Kind Regards, John.

 

[JohnW2]

The reg is 433.3.1, and its probably the most misquoted. To comply the cable must be less then 3 metres long, but there are other conditions that must be applied. The cable must have reinforced mechanical protection, fire protection must be provided and the cable must be installed in such a way that removes the inherent dangers from persons.

433.3.1 relates to situations in which OPD following a CSA reduction may be omitted. As I wrote, I believe that what people were thinking of was probably 433.2.2, relating to a length of reduced-CSA cable with the (reduced current) overcurrent protection at it's downstream end. It requires that the cable is installed in such a manner as to 'reduce to a minimum' the risk of mechanical damage, fire or danger to persons - conditions which are probably usually satisfied (albeit 'reduced to a minimum' is obvioulsy open to interpretation!).

I can't imagine any domestic situation where 433.3.1 could be applied

That could be true - but every FCU connected by a spur cable to a ring final circuit (of which there must be millions) (rather than having the FCU wired into the ring), and every unfused spur from a ring (more millions) is only compliant by virtue of 433.2.2.

Kind Regards, John.