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Topic Title: Radial branches
Topic Summary: Does this need turning into a ring main?
Created On: 06 December 2012 08:06 PM
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 06 December 2012 08:06 PM
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quikjraw

Posts: 7
Joined: 21 May 2011

Hello,
I needed to add a couple of spurs to my house and started to investigate the circuit I was adding to (to make sure I wasn't adding a spur to a spur).

I found early on that I was dealing with a fully 4mm2 radial with a 30A fuse. It has a chain of 7 sockets but from one of the sockets there is a spur of one socket and from another socket there is a branch of 5 sockets.

So a total of 13 sockets.

I am now concerned now that I need to have this circuit converted to a ring but this is going to be extremely difficult I reckon as this circuit has three branch ends and the branch with the five sockets on is logistically as far from the consumer unit as it gets.

Are these two branches classed as spurs because if they are then how can this circuit have been passed off?

John
 06 December 2012 08:12 PM
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Avatar for OMS.
OMS

Posts: 19471
Joined: 23 March 2004

Think about it a bit more - you have 4.0mm2 cable protected by a a 30A (or 32A) protective device

It doesn't make any difference whatsoever, the configuration of the radial branch arrangements. It's poor terminology but you can take as many spurs from spurs in any order as you like as long as you maintain 4.0mm2 cabling (or add 13A fused spurs to supply limited numbers of sockets if you want to wire in 2.5mm2)

What you have effectively is a 32A tree topology - stick as many sockets as you like on it - you won't deliver more than 30A through it in any configuration

Regards

OMS

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Failure is always an option
 06 December 2012 08:16 PM
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daveparry1

Posts: 6106
Joined: 04 July 2007

Agreed!
 06 December 2012 08:24 PM
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aligarjon

Posts: 2802
Joined: 09 September 2005

Strictly speaking you need to watch the length and if the cable is installed in insulation. 30 odd meters from memory if its a breaker. you say 30amp fuse ? Is there an rcd protecting the circuit ?

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 06 December 2012 08:56 PM
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quikjraw

Posts: 7
Joined: 21 May 2011

OMS - thanks, I have since thought of a way you could get this back to the CU through the attic with not so much of an issue. Question is considering what you have said would it give me any advantages at all to do that, even something as simple as not confusing an electrician electrician if I ever need to have more work done.Or is this type of tree topology quite common for electricians to stumble across.
Gary - it is RCD protected by the way.
 06 December 2012 09:25 PM
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Avatar for OMS.
OMS

Posts: 19471
Joined: 23 March 2004

It's a completely standard conventional circuit arrangement - either wire your extra sockets in 4.00mm2 (and keep in mind that "standard" 13A sockets may not accept 3 x 4.0mm2 in the terminals) - or wire in 2.5mm2 from joint boxes via a 13A Fused connection unit.

Any electrician will undertsand the circuit topology - if he doesn't - get another electrician

Regards

OMS

-------------------------
Failure is always an option
 06 December 2012 10:30 PM
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Parsley

Posts: 960
Joined: 04 November 2004

I was looking at PIR today that had a code 2 recommendation which basically stated two lighting circuits on one protective device, I managed to speak to the inspector and asked why he considered the lighting to be two separate circuits, he said because it has two twin and earths at the MCB.

Regards
 07 December 2012 12:21 AM
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spinlondon

Posts: 4437
Joined: 10 December 2004

Did you point out the definition of a circuit in BS7671 to him?
 07 December 2012 08:41 AM
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Parsley

Posts: 960
Joined: 04 November 2004

Yes Spin, I did ask him where BS7671 details how a lighting circuit should be wired.

Circuit "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)."

There was also a code 1 for unfused spurs on a ring circuit (2 outlets) and code 1's for two other circuits both 4mm T&E on a 32Amp MCB feeding 1 x 3KW fan heater, the cables are installed in mini trunking so technically current carrying capacity inadequate for ref method but Ib obviously lower than In and pretty unlikely to overload the conductors unless the fan seized; but not a code 1 surely. Also a code 2 for ventilation ductwork that was not in contact with earth at any point, it just had a couple of grills through the external brick wall at high level followed by a couple of metres of ductwork x 2 to the heat recovery fans box section than another 2 x 2m lenghs of ducting.

Regards

Edited: 07 December 2012 at 08:50 AM by Parsley
 07 December 2012 02:15 PM
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Delbot321

Posts: 77
Joined: 06 November 2012

Sounds to me like you guys have been reading the same report as me

I see a lot of 'unusually' coded defects - sometimes because there isn't enough detail in the report to reach the same conclusion as the writer but usually because the writer is just not familiar with the categorising of faults against the level of risk they present.

They are often written by 'domestic installers' or the like who seem to believe that if a circuit doesn't match the criteria for a 'Standard Circuit' of the IEE On Site Guide or other similar publication where all the design work has been done for you then the circuit must not be compliant. This just shows the higher level of knowledge and experience required for carrying out EICRs.

(I am not saying all domestic installers are like this - just in my recent experience there have been a lot)
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