radial from ring main

[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???