Ring final with 4mm legs

[Risteard]

How about an unfused spur wired in 4mm but serving 2 double sockets, off a ring final wired in 4mm? Found this today. I don't think its actually dangerous, but does it comply? Ring final is about 20m total length and spur about 3m.

I have come across this arrangement a number of times recently (although with much more than two double sockets on it).

I would not in any way be happy with this arrangement as it has the potential to overload one leg of the ring unless coming right from the centre of it.

 

[Adam W]

I appreciate the reasoning behind it, but I wouldn't describe the situation as "fine". "Acceptable" maybe, but mixing different cable sizes in the same circuit seems to be inviting confusion. A reduced size for one spurred socket maybe, but say for example a chippy drilled into a cable, the customer didn't want their laminate flooring lifted and the only acceptable way to get power on is to split the ring into 2 radials, you could end up with what's effectively 2x 4mm² radials with several sockets spurred off in 2.5mm² without being fused down.

I wouldn't install a ring in this way out of choice either, and I certainly wouldn't install a radial like this. It seems like one of those "no other option" solutions - I don't see why the same size cable couldn't have been used throughout.

 

[7029 dave]

Must admit I have never mixed cable sizes on circuits, and don't see the point of it either.

 

[kingeri]

I have come across this arrangement a number of times recently (although with much more than two double sockets on it).

I would not in any way be happy with this arrangement as it has the potential to overload one leg of the ring unless coming right from the centre of it.

But how can one leg be overloaded if both legs can each take >32A and the OCPD is 32A, if it were a straightforward 4mm radial then a single cable would be taking all the load at all times anyway.

 

[Risteard]

But how can one leg be overloaded if both legs can each take >32A and the OCPD is 32A, if it were a straightforward 4mm radial then a single cable would be taking all the load at all times anyway.

I thought we were talking about a 2.5mm^2 ring final circuit with 4mm^2 conductors taken off it as an unfused spur. Apologies if I have misunderstood.

 

[kingeri]

I thought we were talking about a 2.5mm^2 ring final circuit with 4mm^2 conductors taken off it as an unfused spur. Apologies if I have misunderstood.

The ring is entirely in 4mm and the unfused spur is also 4mm. Cable lengths are short enough that it would comply on Zs and VD even with one return leg removed. Kinda makes the ring pointless.
I totally agree with you that if the ring was wired in 2.5mm and then an unfused spur serving multiple points but wired in 4mm was installed, then that would certainly be an awful design with the potential to overload one leg of the ring.

Risteard, what are your views on my OP, which was a different setup again?

 

[Risteard]

Risteard, what are your views on my OP, which was a different setup again?

If I'm now understanding this correctly, there is a mix of 2.5mm^2 and 4mm^2 legs within the one ring circuit. Both legs from the distribution board are in 4mm^2 and 2.5mm^2 is then used between socket outlets?

It's pretty rough I guess, but assuming the 4mm^2 conductors are of a similar length it shouldn't unduly imbalance the ring although this could still happen to an extent with the smaller csa conductors away from the centre point of the ring.

 

[Peter Monfort]

It would surely have been better to make it into two radials. These could have been supplied by two 4mm^2 radials and the 2.5mm ^2 could have been arranged in a variety of Spurs or radials. I think this circuit is more commonly known as a tree in other countries. It would also simplify testing as well as reduce the number of potential errors that ring circuits suffer from. The circuits could be protected at 20amps or alternatively in a commercial application you might protect at 30amps and use a timer control to zone sections of 4mm^2 radials or ring circuits in 2.5mm^2. There is a really good chapter in the book electrical installation designs which covers the history of the ring final circuit and more modern thinking on the use of tree circuits.