Minimum length for spur?

[Checkfirst]

I want to add a spur to the 'reverse side' of an existing socket (to provide a new socket in the next room).
I have a recollection about a minimum distance of spurs (or spacing on rings) but have not found it in the Reference section here, nor elsewhere via Google. The closest I've found in this forum is from last year: a Hifi circuit thread
Apart from the wall needing to be thick enough to accommodate two flush sockets back-to-back, are there any electrical regs or 'best practice' to be followed? (I have seen the the safe zones for cables IEE leaflet)

Cheers
Checkfirst

 

[pdcelec]

I want to add a spur to the 'reverse side' of an existing socket (to provide a new socket in the next room).
I have a recollection about a minimum distance of spurs (or spacing on rings) but have not found it in the Reference section here, nor elsewhere via Google. The closest I've found in this forum is from last year: a Hifi circuit thread
Apart from the wall needing to be thick enough to accommodate two flush sockets back-to-back, are there any electrical regs or 'best practice' to be followed? (I have seen the the safe zones for cables IEE leaflet)

Cheers
Checkfirst

Depends on the wall thickness

 

[fido]

Why do it as a spur? If one of the existing ring cables will reach the new socket you can move it to there and just add a short link cable between the 2 sockets. In that way you would incorporate the new socket in the existing ring.

 

[Checkfirst]

Wall is 145mm (solid) and it can be done as a ring instead of as a spur.

So, are there any miminum requirements for the length of the ring capable, or is the criteria just whatever is within the vertical zone relating to the first socket and most convenient for fitting the new socket?

Cheers
Checkfirst

 

[Qedelec]

The only restrictions on length of ring cable would depend upon the resistance of R1 + R2.
This is phase and cpc combined.
As this is a ring, phase conductor r1 + cpc conductor r2 divided by 4 = R1 + R2
Very unlikely you will exceed the Zs value for the breaker but if you care to supply size and type of protective device and approx legth of cable we can work it out for you.

 

[HandyJon]

So, are there any miminum requirements for the length of the ring

I've not heard of any minimum length regulation between sockets on a ring. There are regulations for maximum length as qedelec has stated in high tech electro techno babel (in other words using words which the man in the street might not understand).

 

[Checkfirst]

Thanks Qedelec - the minimum would be if I add a 50mm backbox to the other side of the existing 50mm backbox, so the gap would be 50mm plus say another 50mm of cable within each backbox - a total of 150mm.

(I suppose offsetting vertically might be advisable to avoid leaving just a 50mm solid section in the wall, but I presume the minimium length is the worst-case)

At the moment the ring-main protection is a 35-year old fuse-box (fuse-wire) - this ringmain is on a 30A fuse. I realise this is hopelessly out of date - I moved in a year ago and my plan is to make the required socket additions, then get the fusebox replaced (by PartP electrician) with something more appropriate.

I realise that adding sockets could cause the ringmain to exceed the protection (I recall some reference to "approx 20 sockets per ring-main as a rule-of-thumb") although the type of usage/load is more important than X unused sockets.
At present there are two ringmains with 14/12 sockets on each, so I should have scope for adding 4-6 sockets on each.
Furthermore, my prehistoric fusebox does have a unused fuse-slot, so if (unlikely) the maximum potential load were to need to expand beyond a safe load, then I would look to split out a part (most likely the kitchen?) of one ringmain and use the unused slot.

The two ringmains have been wired to the fusebox in a way that strikes me as odd - they are not an upstairs ring & downstairs ring. They equate to north/south rings - each with upstairs & downstairs plugs! Each 30A fuse has two live connections, so when I have investigated a bit further I hope to find that each one has an upstairs and a downstairs connection.
That would at least allow me to re-assign the connections so all the downstairs sockets are on one fuse and all the upstairs on the other.

I've seen elsewhere on this site info about 'shared neutrals' but I hope this north/south arrangement is not an indication of such complications.

Thanks
Checkfirst

 

[HandyJon]

I recall some reference to "approx 20 sockets per ring-main as a rule-of-thumb") although the type of usage/load is more important than X unused sockets.

There is no limit to the number of sockets on a ring main. The limit on a ring main is 100 sq m of floor space within the ring. The figure 20 probably comes from the average number of sockets that are typically fitted on a ring.

 

[Qedelec]

Your Maximum Earth loop impedance for the fuses you have is 0.91Ohms, this is for a disconnection time of 0.4s

This means that the resistance of the loop from any socket, along the phase conductor (remember there are two in the ring circuit), along the supliers phase conductor, the resistence of the supply transformer, the supliers neutral conductor and your neutral conductor, must not exceed 0.91 Ohms

And handyjon, it is not techno babble it is the regs which are for your protection.