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Topic Title: Cable size when spurring off a ring final...
Topic Summary: 2.5mm or 4mm for a spurred socket from a ring final...
Created On: 11 December 2008 11:46 AM
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 11 December 2008 11:46 AM
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JRPC

Posts: 8
Joined: 11 December 2008

First of all hello to you all, this as you may see is my first post. I often come here to look up peoples opinions on various subjects. I thought it is about time I get involved, unfortunately my first contribution is in the form of a question, but I hope to contribute to other discussions in the future. However enough of that drivel.....

I have been looking at a job where someone has spurred a socket off an existing spurred socket from a ring final circuit. I already know how I will rectify the situation, but it got me thinking about the cable sizes for spurs off ring finals. I have had a look around and as usual peoples opinions seem mixed, I want to know if it is considered ok to use 2.5mm T&E instead of 4mm T&E, and if so, why? Considering 2.5mm cable is not rated to take 32A. I know that 2.5mm is fine for ring finals as the total load is distributed around the ring and no one section of cable carries the full load, but would the cable feeding the spurred socket not have the potential to draw the whole 32A?

It's been a while since I was at college now learning electrical science, can some one enlighten me?

Regards,

J
 11 December 2008 12:01 PM
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MrP

Posts: 832
Joined: 24 March 2006

Welcome to the forum

Why would the cable be subject to 32A if the cable spur has got a double socket on the end the most it can draw is 13Ax2

Only here to learn

MrP
 11 December 2008 12:34 PM
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mark2spark

Posts: 1444
Joined: 15 November 2006

Also, to rectify the problem, a usual way would be to install a fused spur (FCU) at the point of 'tapping off'.
If that's so, then 13A becomes the protection, and 1.5mm can be used.

-------------------------
I am prone to talking complete bol***ks at times, please accept my apologies in advance.
 11 December 2008 12:52 PM
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OMS

Posts: 19529
Joined: 23 March 2004

You might also want to consider the k2s2 of the cable when contrasted against the I2t of a 32A CPD

OMS

-------------------------
Failure is always an option
 11 December 2008 02:31 PM
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JRPC

Posts: 8
Joined: 11 December 2008

Thanks all for the replies....

MrP - I totally forgot about the fact that a twin socket outlet is only designed to take 2 x 13A 1363 plug tops! But what if someone decides to use a 5 gang extension with this spurred socket, is there not a potential that the cable could be subject to more than 2 x 13A?

mark2spark - That is what I am going to do.

OMS - I'm pretty new to whole design process, I know that k2s2 is the amount of energy the cable can withstand and that I2t is the energy let through of the protective device, but where can these values be found (manufacturers data maybe?) or do they have to be calculated?
 11 December 2008 02:33 PM
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AJJewsbury

Posts: 11349
Joined: 13 August 2003

what if someone decides to use a 5 gang extension with this spurred socket

They'll plug it into the socket using a 13A plug ... which has a 13A fuse in it.

- Andy.
 11 December 2008 02:38 PM
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OMS

Posts: 19529
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But what if someone decides to use a 5 gang extension with this spurred socket, is there not a potential that the cable could be subject to more than 2 x 13A?


But the extension lead will also have a 13A fuse in its plug top so still 2 x 13A down to the dual outlet.

You can find values for K in BS 7671 (Typically 115 for a t&E cable) - S is the cross sectional area in mm2 of the conductor under consideration, I2t (ampere squared seconds) will be either given by the device nmanufacturer at the fault level under consideration (derived from the earth loop impedance and the phase - neutral loop impedance ) or calculated from the fault current above and the time of operation of the protective device.

Regards

OMS

-------------------------
Failure is always an option

Edited: 11 December 2008 at 02:39 PM by OMS
 11 December 2008 02:50 PM
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JRPC

Posts: 8
Joined: 11 December 2008

Thanks guys, ha yeah of course the plug top of the extension lead, what an idiot. I start thinking about these things too much and totally miss the obvious. Thanks for the clarification OMS.

Regards,

J
 15 December 2008 10:54 AM
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JRPC

Posts: 8
Joined: 11 December 2008

Just a thought.....

A 1 way single phase fuse box housing a 5A MCB is fed by 2.5mm single cables run in trunking, the 2.5mm cable is connected the blue phase (L3) of a 3 phase bus bar, the bus bar is being fed by 25mm singles backed up by a 100A BS88 switch fuse.

Now this looks instantly wrong to me as the 2.5mm cable is essentially backed up by the 100A switch fuse. But then why does the same logic from above not apply to this circuit? The 2.5mm cable can never draw any more than 5A as the 5A MCB would operate if it did.

Is this safe? Am I completely missing something here, or is it a regs issue?

Regards,

J

Edited: 15 December 2008 at 10:55 AM by JRPC
 15 December 2008 11:36 AM
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OMS

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OK - you need to get in the habit of thinking about circuits being protected from overload, from short circuit or both.

In this example, you are providing overload protection at the "load" end of the 2.5 so you meet that criterion. For short circuit protection again you need to ascertain, for both the earth fault condition and the short circuit condition, if K2S2 of the 2.5mm2 is greater than the energy let through of the 100A device at the fault level under consideration.

Regards

OMS

-------------------------
Failure is always an option

Edited: 15 December 2008 at 11:36 AM by OMS
 15 December 2008 01:06 PM
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ChrisGilbert

Posts: 136
Joined: 12 January 2003

I think the OP concern boils down to a common thought that many have had that a double socket on an unfused spur could draw 2x 13A with the 2.5mm cable only being rated to some 20A. In reality the most continuous load that might be a reasonable possibility is a plugging in of 2x 2KW heaters and then that would only be some 17Amps. I have once seen 2x 2.2KW Burcos in a works canteen which would draw some 19 Amps but not continuously. Of course, you can buy 3KW heaters and if 2 were plugged in that could give 26A. But how likely? And of course 20/21A is for cases with thermal insulation with 23A in conduit in plaster and 27A for clipped direct or embedded in plaster.

Edited: 15 December 2008 at 01:16 PM by ChrisGilbert
 15 December 2008 01:09 PM
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OMS

Posts: 19529
Joined: 23 March 2004

If you do manage to pin down a manufacturer, most would admit to the dual socket having a continuous rating of less than 17A anyway.

BS 1363 requires a twin socket to deliver 20A continuously whilst not experiencing a temperature rise exceeding 52K (usually deemed to be 4 hours)


Regards

OMS

-------------------------
Failure is always an option

Edited: 15 December 2008 at 01:16 PM by OMS
 16 December 2008 09:07 AM
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zeeper

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Joined: 11 July 2008

I want to know if it is considered ok to use 2.5mm T&E


You could always turn to page 362 of the 17th edition regs book they got pictures now.
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