32A protected Ring

[dabaldie]

I'm planning to add another two sockets to the ring upstairs.
I've checked back to the Cu and found a 16A circuit to the kitchen (ring), and 2 32A protected circuits (ring again), one to upstairs and one to downstairs.
Now my question is this..
The 16A uses 2.5mm cable and the other two (the 32A) use 4mm cable ( i think as its bigger than the 2.5 and smaller than some 6mm that i have, (not enough for this job tho))
I order to extend the ring upstairs by another 2 sockets , is it possible to run a 2.5mm spur from each of the two sockets already in the room, should i use a FCU to protect the 2.5, surely in theory i cant over load this cable as the max it will draw is effectively 13A anyway.

Also for some reason i'm struggling to source some 4mm^2 T+E. All the sheds sell 2.5 and 6 but nothing in between. (yet one of them actually recommends 4mm for a 32A Ring), and the one trade place had sold out, well had 4m left (which wasnt enough), and as far as i know all the local trade places (from experience in the past), are closed on sat!

Which is better?... 2 spurs, one from each socket or extend the ring to include the new sockets and use 6mm cable (if i cant get any 4mm)

 

[Big_Spark]

Terminating 6mm² cable into a socket will be a pain in the back passage and I would not recommend it.

4mm² is not required for most ring final circuits, and certainly not in a domestic, 2.5mm² is adequate for this requirement.

If you intend the new sockets to simply be spurs off the existing ring, then I would suggest that you install a couple of unswitched fused spurs adjacent to the most convenient sockets that willact as donors, then run 2.5mm² T&E from the output side to the socket in question. If the sockets you intend to add are close together, then one spur would be sufficient unless you intend to run loads of about 10A or higher on a regular basis from each point.

The spur will be required as your dropping cable size from 4mm² to 2.5mm², and whilst you could simply install the 2.5mm² into the existing ring without a spur, I would certainly not recommend or condone it.

 

[Lectrician]

It could be radial circuits. 2.5mm radial on a 16amp (should ideally be a 20), and 4mm radial on a 32amp.

I am saying it is, but do not assume.

It could also be that the 4mm you see is an imperial cable......does it have tinned (silver coloured) cores??

 

[dabaldie]

Ive checked all sockets and they all have 2 cables, plus the cables come back to the CU. Unless I have 2 radials off the same MCB! and a spur off the final socket in both cases!

Either way i have decided to install 2 2.5mm spurs off the sockets in the bedroom.

No harm in that?

The sockets will be bedside sockets (fitted units) and will only run an alarm clock and a lamp.. so intheory i could use 1mm cable!!!.... no i'm not THAT stupid, i appreciate that someone could in 10 yeras time plug their 3kw fire into the socket!

any idea why 4mm is hard to source btw?

 

[baldelectrician]

if it is a radial circuit (ckeck for lack of ring continuity) then you should keep the cable size the same.

I know you can spur a 2.5 from a socket on a ring , but you can't do the same on a radial- ring circuits have specific rules.

 

[ban-all-sheds]

I thought the regulaton which allows a 2.5mm² spur from a ring final was a generic one, that would permit any branch circuit with downstream overload protection to use a cable that was too small for the upstream protective device, as long as fault current protection was OK?

Can't be @rsed to look it up right now, I'm afraid, but I'm pretty sure I'm right...

 

[Lectrician]

Thats right, but the ring circuit spur has become a convention. Other forms of downstream overLOAD protection is still frowned appon, but perfectly acceptable.

 

[ban-all-sheds]

And as we know, a spur from a ring final can be taken from the MCB itself.

So it's either perfectly OK to have a 2.5mm² branch from a 4mm² radial, supplying 1 socket, or it is not OK to ever have a 2.5mm² spur from a 2.5mm² ring final.

And if the argument is "yes, well, but ring finals are established and spurs are accepted but it's frowned upon everywhere else" then that is nothing less than an admission that rings are an aberration, which only meet the regulations because the regulations twist themselves into an exception for them, and proof that they should not be allowed.

The laws of physics do not work on the basis of what is conventional and what is frowned upon. Either a circuit is safe, or it is not, and if it is not, then the fact that "it's allowed because its allowed" does not make it safe.