radial from ring main

[nick611]

hi there, would like your views on this.. Jb off a ring main and run two or three socket,s on a 4mm radial. I myself don,t mind it. no current capacity issues.and no zs problem,s if rcd protected.Any thoughts.

 

[chiplard]

You cannot do this.

If you intend to install multiple sockets spurred off a ring final circuit then you must install an FCU at the point of the spur.

 

[Risteard]

It could unbalance the ring.

 

[telectrix]

would be much better to run 2 legs in 2.5, and split the ring. then you would just have extended the ring. OK the thinking of load is not a problem, but it's not good practice. think of some poor spark might come along in a few years and think wtf is this

 

[mickytuff]

personally i would either fit an unswithed spur next to the ringmain socket (to protect the cable from overload) or i would extend the ringmain to include the additional socket/s, the latter would be more desirable, especially in a kitchen where the load would be higher than anywhere else in a typical house set up anyway, hope this helps.........

 

[malcolmsanford]

Or find 2 or 3 sockets on the ring and spur one socket off each. No need for 4mm^ cable either

 

[nick611]

not sure you all understood me.there are no overload problems because 4mm rated at 36amps.also the cable is stranded so no sparks would mistake it for 2.5..where are the technical problems with this?.

 

[Octopus]

I have to say I've always wondered why a fused spur has a limit of 13A, especially if you can add more than one socket and the cable will carry 20A!

Having a blonde moment!

 

[nick611]

balancing the ring is hilairious as well.no one can do that.we hav,nt a clue what the sockets we install will be used for.lets be honest

 

[rryles]

Isn't this shown as acceptable in 7671 appendix 15?

edit: I think the reason for fused spurs being limited to 13A is simply that you can only get FSUs and the fuses for them up to 13A.

 

[xraytek]

not sure you all understood me.there are no overload problems because 4mm rated at 36amps.also the cable is stranded so no sparks would mistake it for 2.5..where are the technical problems with this?.

I think the overload issue will be on the ring, if the additional radial is not at the midpoint.
If the radial is close to the CU then a large percentage of that current will take the short route and overload that part of the ring.

 

[nick611]

i must admit i would have no problems with this arangement. on an upstairs ring the total load would be very low.this would be ideal answer to roof conversion.one cable up, handfull of sockets in new bedroom.and as far as i can see no real reason not to do it.

 

[KFH]

The regs do not say you can do this. While you do not have to follow the regs you do have to be able to justify your decisions if something goes wrong. If you were feeding an attic conversion the regs would allow a FCU and then wiring in 1.5 T&E on the radial.


Yes it could unbalance the ring. With sockets on the ring it would be difficult to exceed 26A on any double socket; with the unfused spur you could have a max of 26A on each double socket on the spur, whether this would cause an overload problem on the shorter part of the ring is beyond my desire to calculate and that level of loading would be unlikely.


Not something I would do but I am not sure I would lose much sleep about it if I found it on an installation. Flag it up as a non compliance?

 

[dansk]

Its a spur of a spur, its shouldnt be done without FCU if your wanting to make it a radial of the ring - your call buddy, but if you dont follow the big red book and theres ever a problem, its you who has to justify your actions.....

 

[NMc]

You need to look at appendix 15 again. Page 362 is for ring main circuits. There is no mention on the diagram of what you intend

 

[jaresquire]

I think it's more a bad practice thing, can't think of the downside compared to a true ring except for testing and it's not the worst real world crime you find in most domestics of some age.
Wouldn't do it by design thou.

 

[chiplard]

IT IS NOT ALLOWED IN ACCORDANCE WITH BS7671!!!

END OF!!

It worries me when people come out with excuses as to why it wouldn't be a problem. Irrespective of how safe people think it is, what current might flow, what the load is etc etc. It is not an allowable method.

What else would people do in their own judgement of what is safe?

 

[jaresquire]

Ok, had a bit of a read of the brb, the issue seems to be;
Once the 4mm spur added, circuit is not a ring-final by definition, therefore need to be able to defend design, which is complicated because;
Need to consider the ring section as two conductors in parallel - requirements are balanced loading and no branches, so very careful examination of existing ring required.
Because of this I accept best to just extend ring or use fused spur.
Thats my humble pie for 2011

 

[marky sparky]

unfused spurs should only have 1 single socket running from them .ref 17 edition

 

[nickblake]

me i would stick a fused spur on it and wire in 1.5mm if its for just a couple of sockets in a bed room

 

[jaresquire]

"unfused spurs should only have 1 single socket running from them .ref 17 edition "


Not as simples as that, there are other circuit configs apart from ring finals eg radials where you can spur off multiple sockets. The standard ring in Appendix 15 is a guide that saves some head scratching as long as you stick to all the restrictions.

 

[nick611]

no thats unfused spurs in 2.5 not 4mm,this adition is not coverd in regs.so how can you know its not allowed?

 

[nick611]

IT IS NOT ALLOWED IN ACCORDANCE WITH BS7671!!!

END OF!!

It worries me when people come out with excuses as to why it wouldn't be a problem. Irrespective of how safe people think it is, what current might flow, what the load is etc etc. It is not an allowable method.

What else would people do in their own judgement of what is safe?

its not considerd in 7671 so how can you say its not allowed?quote me a reg that forbids it

 

[JulianC]

I think the only thing you'll find on this is in the on-site guide, 8.2.4 - "In an A1 ring final circuit.....a non-fused spur should feed only one single or one twin socket outlet"

However, given that the point of the fuse in a fused spur is to protect the cable in the spur, and, subject to installation conditions, 4mm can handle more than 32A and will therefore be protected by the 32A breaker protecting the ring, I am struggling to find a problem with what you are suggesting nick611. As you say, it is not considered in BS7671.

Mind you, I have been at the port.....

 

[Risteard]

I am struggling to find a problem with what you are suggesting nick611. As you say, it is not considered in BS7671.

It is not permitted because it would adversely affect the balancing of the ring.

 

[JulianC]

Aah, the mystical dark art of ring balancing. Not convinced. Where does it say it's not permitted? (playing devil's advocate)

 

[kmcgold]

u can spur off has many sockets has you like if you have fused it down to 13amps and the zs value of the circuit are ok
if no fuse then only one socket etc even if you used 6mm still only one socket etc

 

[kmcgold]

(risteard said It is not permitted because it would adversely affect the balancing of the ring.)

correct.

 

[jaresquire]

Not convinced of this ring balancing either, as we are talking of the same load at the same point of the ring, just different wiring method downstream.
Why am I defending something I've never done though?

 

[JulianC]

Just 'cos we've never done it, nor considered doing it, doesn't mean it's not right! Rather than blindly reading and (mis)interpreting the regs. it's good to think about these things sometimes.

It's clear that this is not covered specifically in the regs. One the plus side, there are no current carrying capacity problems using 4mm. On the down side there's wonky ring syndrome. Now correct me if I am wrong, but I could fit 4 fused spurs off 4 adjacent sockets quite happily and 'correctly' and unbalance me ring.

 

[nick611]

(risteard said It is not permitted because it would adversely affect the balancing of the ring.)

correct.

if the radial came off the center of the ring then it would not.so your answer dosent stand up. does it

 

[j.mawer]

Unless someone extended the ring in the future when it would no longer be in the center.

 

[Guest123]

Truly astonished by this thread.

 

[Octopus]

time to unsubscribe!

 

[kmcgold]

(nick611 said if the radial came off the center of the ring then it would not.so your answer dosent stand up. does it)

??

this is a forum for electricians ?

 

[nick611]

Well 34 posts and still no real reason why this is not allowed.A few random stabs at the regs none of which held water,and a few quite confused by the whole thing.Next week we shall discuss a 4mm ringmain on a 40a mcb. oh hold on hav,nt seen that in the regs book.

 

[kmcgold]

p362 of regs show a ring circuit which has been design for use with 2.5mm cable.
increase to 4mm to allow for voltage drop , etc but do your cals.(all 4mm cable).
you should not spur a radial of an existing ringmain in 4mm

 

[Adam W]

Truly astonished by this thread.

I agree - 4 pages of discussion over whether or not you can just invent your own kind of 'composite' final circuit just because appx 15 of BS7671 doesn't expressly forbid you from doing so.

I mean, seriously, is this a wind up?

 

[Paul.M]

I disagree with the above(kmcgold). At the design stage of this extended circuit we have to consider a few things. What is the flc of the 4 spur sockets? What load is on the existing ring? If a spur is taken from the ring how will it affect it?

By what info that has been posted I can't see a reg to prevent him from doing it, is it best practice is another question. I personally would fit a fcu rated for the 2.5mm cable supplying the radial regardless of what size cable the radial is in. This way if the spur is coming from the first socket of the ring and most of the current is being fed by the shortest section of 2.5mm the spur would not overload that part of the ring. The usage of 4mm for the radial seems excessive if its just for bedroom appliances and you could downsize this.

Do your potential flc calc for the radial, fit a fcu, make sure you don't overload the ring, explain your new extention on the minor works cert and maybe a diagram showing this circuit so no confusion for the next spark. Job done, but not the way I'd do it.

 

[Paul.M]

Double post.

 

[telectrix]

Just 'cos we've never done it, nor considered doing it, doesn't mean it's not right! Rather than blindly reading and (mis)interpreting the regs. it's good to think about these things sometimes.

It's clear that this is not covered specifically in the regs. One the plus side, there are no current carrying capacity problems using 4mm. On the down side there's wonky ring syndrome. Now correct me if I am wrong, but I could fit 4 fused spurs off 4 adjacent sockets quite happily and 'correctly' and unbalance me ring.

must be an awful feeling to have one's ring unbalanced.LOL> seiously. valid point there. it's only recently that the trade has been questioning the use of ring finals. rings have been used for socket circuits for years, just because that's the way we've always done it. ( well, since the war when copper was in short supply).it is good to question why things are done in a certain way. it's called progress.

 

[Paul.M]

Next week we shall discuss a 4mm ringmain on a 40a mcb. oh hold on hav,nt seen that in the regs book.

Table 6E1 OSG. 4mm, ref method B = 30a therefore on a ring main final it can carry 60a (on a balanced ring). But as we all know the maximum mcb/rcd can only be 40a in a domestic installation so this is fine. What can we talk about next week?

Ps, regards to regs for ring balance look at reg 433.4, 433.4.1 and 433.4.2

 

[Guest123]

So, to all that have posted that it is fine to spur from a 32A, 2.5mm T&E ring final to any number of socket outlets without an FCU beforehand, can you tell me how it is we arrive at the scenario whereby it is fine to protect a cable with a CCC less than that of the associated OCPD, and the fundemental principals that allow us to do this???

 

[Paul.M]

You can't. The cu mcb/rcbo must not be rated higher than the smallest csa of that circuit. In this case the OP must take this into consideration. That way the demand from all appliances on the 4mm can not be higher than the supplying 2.5mm cable if its the first on the ring and therefore not overloading any one section of that ring.

 

[Guest123]

You can't. The cu mcb/rcbo must not be rated higher than the smallest csa of that circuit. In this case the OP must take this into consideration. That way the demand from any ---- in the 4mm can not be higher than the supplying 2.5mm.

So why then do we have a 2.5mm T&E protected by a 32A MCB???

 

[Paul.M]

Worse case scenario, 2.5mm in three core enclosed in conduit in an insulated wall = 17.5a. So on a ring it would double to 35a and having a lower fuse to 32a to protect the cable. To spur off that ring without overloading a single section wild need investigation to the load on the ring before spurring off it. If your spurring off the first socket you will put the majority if the load on a single part of 2.5 on the ring and that's not good. (balanced ring).

 

[chiplard]

Can someone please tell me why Appendix 15 suggests a 4mm radial circuit should be protected by a 32A Breaker when the only installation method that this would suit is clipped direct.

Sorry to sidetrack the thread but i have always wandered.

 

[Guest123]

Seems a few people need to brush up on ring circuit theory.

 

[Paul.M]

Can someone please tell me why Appendix 15 suggests a 4mm radial circuit should be protected by a 32A Breaker when the only installation method that this would suit is clipped direct.

Sorry to sidetrack the thread but i have always wandered.

A 2.5mm ring = a csa if 5mm so any cable less than the ring can be protected by the same mcb. If it was a 2.5mm radial you can still increase the csa if the spur cable long as the main mcb is protecting the 2.5mm. Hence the reason for a fcu to protect the spurred cable and the flc for that spur.

 

[iconnect]

Ok my understanding don't shoot me !!! The idea of balancing a ring is crap the ring in its nature if installed correct is balance through each leg being as near as same length. If the first socket in that ring is used most of the current will go down the shortest leg and the rest the long way around correct!!! You can only spur once of a ring if no switched fused spur is used reason being you could connect 4x double socket on it and carry 32a on a 2.5 or 4mm but it would overload the shortest part of the ring it connects too. The reason we fuse it can not carry more than 13a due to the fuse therefore no different to pluging some thing in that pulls 13a!!! so the fact is 1x socket if spuring without fuse with fused spur as many as you want it will still only pull 13a. I hope that makes sense? You could do loads of stuff if you just look at one part of the regs it overlaps! The regs are for guidance they just prove or disprove you are breaking the law electricity at works !!