DIY Doctor

Draw question for loft spur in converted loft

Postby fuchsiashock » Sun Nov 08, 2020 5:41 pm

I could use some help and advice for this setup in a new house.
The loft has been converted and has the wiring as follows...
It's at the end of wire, which I believe is called a 'spur'.

Wire up from house (black, red, earth) TO
13a 3 way terminal junction box TO
2 plug wall socket AND TO
Wire (black, red, earth) TO
13a 3 way terminal junction box TO
2 plug wall socket where it ends.

What loads can be drawn on this setup?
Does it depend on what it put on the first wall socket?
Does it sound like the correct setup?

There was an incident where the first terminal junction box wiring melted inside, I think due to bad wiring as it was long wiring, all stuffed inside and I think it arced but now I wonder if it also overloaded.
Can a spur only take a smaller load than a main ring circuit?

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Postby ericmark » Mon Nov 09, 2020 10:35 am

A ring final is normally 32 amp, and there are two types of spur that can be taken from it, unfused your only allowed one device, single or double socket, but no more, the fused spur is really a radial and if can have as many sockets as you like, as long as the earth loop impedance and volt drop are not exceeded, and in the main we use 2.5 mm² cable.

We can also have radials, these often use 4 mm² cable and still 32 amp supply, but could use 2.5 mm² with 16, 20, or 25 amp supply.

Electric flex has a current rating, but non flex the current depends on how installed, so you can't go to B&Q and buy 20 amp cable, you buy 2.5 mm² cable and the amps vary depending how installed. This is because cable needs to get rid of heat, and so if in an insulating wall it can't carry as much power as in free air.

The are exceptions like mineral insulated cable, but in the main the ring final always uses 2.5 mm² the regulations say it must be rated 20 amp or more, so through an insulated wall could be 4 mm² cable.

It is unusual for cable to over heat inside a socket unless there is a bad connection, a loose terminal, and you can normally see how the damage is worse need the terminal.

Once cable has been overloaded there is no option but renew it, if just at the terminal you can cut it back a bit, as heat clearly due to poor connection, but if throughout the visible length, no option but renew.

Where we want to take power from the ring final job one is measure the loop impedance, your allowed 106 meters of cable in a ring final, you can work out how much cable already used by measuring the loop impedance, if low enough then best option is to extend the ring, my method is normally a LAP grid plate from Screwfix with two sockets in an existing double socket back box, you want no more than three wires in any terminal over three and one wire may not be fully clamped, and could over heat. So the LAP grid sockets double up on terminals and split the ring final so I can take two 2.5 mm² cables from there increasing the size of the ring, however with a ring if a wire comes out it will still work, so inspecting and testing is very important, so we check before starting the ring exists, and the loop impedance readings, we know how much cable added so we know what loop impedance readings we should get when complete, if too high we know a cable has become disconnected.

This is not how it should be done, we should test at the consumer unit, the R1 + R2 reading and record it, the form can be down loaded from the IET website, I find the filling in of the form is a prompt to do correct inspection and testing.

However of course to test and inspect you need the loop impedance meter, low ohm meter, insulation tester, and RCD tester often called 17th edition meters, although were required well before the 17th came out, we are now on 18th edition.

The more simple method is to use a fused connection unit, or a RCD FCU, if the supply has not already got RCD protection, and produce a fused spur, you are allowed 1.365625 Ω with a ring final for the earth loop impedance but with a 13 amp fuse around 2.3 Ω so you are unlikely to exceed the limit, and you can put as many sockets as you like after the fuse as the fuse will rupture if there is an over load.

Cheap plug in testers normally have first light at around 1.9 Ω so with a fused spur the plug in tester with loop can test OK, but with a ring final you need the full blown loop impedance tester which is over £100, where plug in tester only around £35 for cheapest up to around £60. Will also need a low ohm meter and insulation tester, I bought the latter 500 volt tester for £35, you should also have a RCD tester, but if test button works it is likely OK, down to a risk assessment, but I would not worry unduly.

However it seems you have and unfused spur from unfused spur, which is not permitted, and this has caused damage, so I would suggest this is not a DIY job, even if allowed without notifying under Part P.

I realise in these times there is a problem having tradesmen in your house, so have tried to explain as I suspect you will DIY whatever I say, so fitting a FCU or if no RCD already fitted a RCD FCU would seem to be one way around the problem, but if over loaded a fuse will not remove the over load, if will just rupture if it is overloaded and the question has to be how much load are you putting on the circuit?
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