Wiring in an Internal Porch Light and Dead Socket


Postby Credulous » Tue Nov 15, 2016 11:02 pm

Hello all.

Time to learn some new DIY skills.

I need to wire in an internal porch light and reactivate a dead plug socket.

Hopefully the good people of this forum can help me do this in a manner which fulfils the associated regulations/standards. As I progress I plan on asking my questions here to keep me on the right path.

In terms of opening questions:

Plug Socket
- My DIY book states the easiest way to add a new socket is as a spur from an existing socket and there is no limit on the number of sockets other than the floorspace served by the circuit is <100m. My DIY book is a few years old now so has anything changed in this regard?
- I'm likely to run the spur if this is permitted from a socket in the hall under the ground floor suspended wooden floor. Are there any requirements for trunking/sheathing cable when it is running under a ground floor like this, or can you just clip to underside of joists? Is it good practise even underfloor to run parallel or perpendicular to joists?

Light fitting
- My DIY book advises 8 100W bulbs on a single lighting circuit is the commonly perceived limit. Most of the lighting downstairs in our house is ceiling spotlights. Is it as simple as adding up the wattage for all these bulbs to see if it is less than 800W before adding a new light, or more complex than that?
- One of the ways I could wire in the new porchlight would be wiring it from a ceiling rose in the loft space above the extension on our house. The reason I would consider this is that there is a now 'defunct' cavity wall between the original house and the extension which would be great for running the cable down to the porch, but I have read that running cables in cavity walls is usually frowned upon, but thought might be different in this case as not really a cavity wall any more.

Further questions to follow as I actually start this job, but thanks in advance for any help which people can provide.

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Postby kbrownie » Thu Nov 17, 2016 7:09 pm

You can only take a single or double gang socket from the ring final as a spur!
If it was a fused spur taken from the origin of the socket only then can multiply sockets be added.

The 100m ring final circuit

Cables should be fixed at a distance that prevent, stress to cable and terminal fixing. This would normally be around every 300mm to 400mm.

Yes adding the number of watts of every lamp on that circuit, will give you the total load demand and it would not be 8 x100 watt lamps on a standard 6A lighting circuit.

The reason why running cable within cavity walls, is that is left unsupported and unprotected from falling debris.

Your DIY book is out of date and even then giving you false information to regulations that have long time applied.

drop the book in the bin and purchase either or both the on-site guide or BS7671 2008 amd 3 (2015)

Does you DIY book tell you how to identify a ring final circuit and to prove end to end continuity is present on all conductors? .
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Postby Credulous » Fri Nov 25, 2016 10:27 pm

Thanks for the reply kbrownie.

Sorry my original post was slightly ambiguous. The DIY book doesn't state that you can have multiple sockets on a spur, and I'm only planning on adding one single socket to the porch (double might be excessive for a porch!), but I can see how you read my post that way. I was trying to ask if the only criteria for the number of sockets on a circuit was that the floor area served is less than 100m2 which is what I've read in several places now. Looking at the version of the onsite guide I have downloaded that would appear to tally as long as on a ring main (A1 type of circuit).

I'll read the onsite guide to try and understand how you work out the capacity of a lighting circuit but may have further questions on that.

I can see that onsite guide includes requirements around curving cables and fixing vertical cables but I think I can clip at top and bottom of run and stick within these rules. If I also put within some kind of trunking would that be within requirements? Can't see anything that implies it wouldn't.

My DIY book does not tell me how to identify a ring final circuit and verify end to end continuity is present on all conductors. Having read the onsite guide I'm not sure that does either! Tell me more.

Thanks in advance,

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Postby kbrownie » Sat Nov 26, 2016 11:45 am

A a ring final circuit can take any amount of sockets you chose to install on it. The 100m2 area is a rule of thumb and the terminologically withing BS7671:2008 (Amd 2015) has changed with regards to previous editions.
The 100m2 area is now deemed as an historical adopted sq area.
(which means it was in the past considered suitable)

In the real world the ring final should be designed to prevent load currents exceeding the current capacity of the cable.

Achieving this would be by dividing the load demand around the circuit evenly.
And not having high load appliance on the circuit, such as heaters and any appliance above 2kW.

The on-site guide does other guidance on the methods involved in testing circuits. (Which edition do you have?)

When adding sockets or load to a true ring final circuit, then you have options, and they are:
1) Extend the ring as a ring, keeping the integrity of the circuit as a whole
2) Add only one outlet (single or double) to the origin of any part of the ring as a spur.
3) At the origin of the ring, install a FCU (fused connection unit), which will act as a fused spur. Then downstream of that you can install as many sockets as you wish, but you are restricted to an overall load of 13A.
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Postby ericmark » Sun Nov 27, 2016 5:09 pm

As already said the DIY book seems to have over simplified what can be done.
The main point is the ring final circuit can be overloaded and also any spur can be overloaded unless either only one device is fitted or a fuse is used.
The ring final instructions recommends you don't use any fixed items over 2kW from the ring this will prevent overload, however it is not the only method, in the centre of the ring overloading is not a problem, only near the ends can large loads cause an over load.
There are also rules on RCD protection, and for the DIY guy the easy way is often to use a fused connection unit (FCU) once this is fitted there is no limit to number of sockets, and using a RCD FCU gets around the problem when the main board does not already have RCD protection.
I have been doing work on my mothers house, now with all RCD protection, but many sockets were disconnected during the rewire, so needed to use a fuse to re-connect strings of sockets, easy method was to use grid sockets, fuses, and switches so I could use the double socket box as both a single socket and FCU without having to ripe it out and use a twin socket back box.
It also makes it easier to extend a ring, basic two single sockets but looks like a double socket. I used LG socket grid system but there are others.
So yes you can do what you want, it's the how which presents the problem.
The other problem with a ring is the inspection and testing, with a radial or spur it works or does not work, and a simple plug in tester will show if there is an earth. But with a ring you can have a disconnected wire causing an overload with out any signs of a problem until it goes on fire. So testing is important.
I have questioned testing with DIY, as testing at the socket is done with socket removed, and replacing the socket could cause a wire to disconnect, however testing at the consumer unit often means working around live wires which you can't isolate.
Only you know how good you are. And I would not be at ease directing you as to how to test, as any errors could cause danger.
However the EZ150 plug in tester does have a basic loop test and at around £50 not too expensive. The full blown tester is around the £500 mark.
So question is "Is it worth it?" to DIY. If you have a lot to do then you could either buy or hire test gear or get an electrician to do an electrical installation condition report (EICR) after.
But often cheaper not to DIY.
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Postby Credulous » Tue Dec 20, 2016 9:45 pm

Thanks for the replies all.

I replaced the plug socket a couple of weekends back, and it seems to be working fine. I bought a socket tester and used on the socket I added, and the two on either side (i.e. the ones at risk of wires being disconnected due to me pulling wires out) to check for continuity etc and all seemed ok. The spur was added from a junction box already there under the floor (which was being used just to connect two cables within the ring), so I suppose the risk of the ring main being broken exists, and having invested in the onsite guide (latest version to reply to earlier question - the yellow one) and watched John Ward's excellent videos on YouTube, I could theoretically do the tests required. However for a DIYer such as myself the risk of a broken ring main feels less than opening up a consumer unit and starting to play around inside to do the 'figure of eight' test etc.

Today I prepped the porch light, including running the cables from the second floor ceiling rose down to the switch position (and light fitting positon and the installation of an accessible junction box to send off and retrieve the switch cable. Interestingly the local DIY shop sold me twin red and earth for the switch cable, which should have disappeared years ago right? (It's definitely red not brown).

All the hard work basically done now, just need to mount the wall fitting, and connect the wiring to the existing lighting circuit (I've calculated will be well within load).

When under the floor a couple of weekends back was surprised all the cables were just loose trailing on the floor, rather than being clipped neatly to the joists (as my handiwork was!). This is despite house being substantially rewired before we moved in a couple of years back.
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Postby ericmark » Tue Dec 20, 2016 11:12 pm

There are a number of ways to test a ring is complete, the CU is likely the most fool proof way, but you can also use a loop impedance meter, and a continuity meter.

Basic method measure loop impedance (twice line - neutral and line earth) dismantle socket and test using continuity meter if sound then you can take the loop impedance measured as being correct, reassemble and retest loop impedance which should be the same as at start this is not correct method but if you do have a wire come loose the loop impedance should rise so in centre of ring reasonable test.

Without the loop impedance meter then select a socket which is easy to work on which is on the ring, and test from that socket.

There is nothing to stop you over sleeving old red cable with brown sleeving.

Leaving cables loose is common for a re-wire they are just fished through.
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