New garage dual outlet

[-=sT3V3=-]

Good Morning gentlemen of the internet,
I have ground floor ring circuit I would like to use to supply a dual outlet in my garage. My understand is this is called adding a "spur"

Inside the lounge there is a dual outlet on a metal backing box mounted to the thermalight block of a cavity wall, I would like to drill through straight into the garage (crossing the 100mm cavity) where I could mount a 25mm dual outlet plastic box. (with a 450mm drill bit for example)

From the existing outlet I would wire a twin and earth cable and run it through the drilled holed in the "wall|cavity|wall" into the back of the surface mount 25mm plastic backing box.

please forgive me if my terminology is incorrect, I am new to the UK.
Are there any flaws to my idea?

 

[ericmark]

We have:-
A ring final which is normally supplied from a 32 amp overload wired on 2.5 mm².
A radial which can be supplied with anything from 16 amp to 32 amp, but the latter would normally be thicker cable 4 mm² or 6 mm².
A unfused spur, normally in 2.5 mm² and can only supply one item.
A fused spur could be 1.5 mm² to 2.5 mm² the fuse is normally part of a fused connection unit (FCU) and can supply as many sockets as you like.

There are two limitations, one volt drop, and one being able to supply enough current to trip the overload, a B32 overload will trip on the magnetic part in a fraction of a second, but needs between 3 and 5 times the current which will trip the thermal part. We also add 5% to allow for volt drop, so a B32 will trip at 160 amp ohms law 230/160 = 1.44 Ω add 5% = 1.37 Ω the problem is the plug in testers are designed for Europe even when they have a 13 amp plug, so pass at around 1.9 Ω which is really a fail.

The volt drop does not matter so much, it should be 5% so line - neutral is less than 1 Ω, but in the main it does not cause a problem, only item likely to have a problem is a freezer with a non inverter drive, or fluorescent lights with a wire wound ballast.

As to the other problem, in the main the RCD will ensure circuits are opened with a fault, where the problem lies is with rental property when getting an EICR done which will highlight errors.

As an electrician in the main I can look at a house and say, not going to be a problem, or that house is going to be on the edge, before I get out the test gear. But if you fit a fused spur your very unlikely to exceed the limits. I used a grid plate and socket plus fuse holder and switch to extend the supply when either already a unfused spur, or don't have the readings. The Screwfix LAP range will fit in a standard double socket box. That is assuming already supplied by a RCD or RCBO.

 

[oldbutnotdead]

No flaws but a few things to check.
First, is the circuit protected by an RCD or RCBO?
Second, is the circuit actually a ring final or a radial? If it is a ring final, check that the lounge socket is part of the ring and not itself a spur.
Third, make sure your gas and water pipes and main electrical supply cable are nowhere near (such things should not be routed in a wall cavity but check)
Fourth, since you are drilling through the cavity it would be wise to use a piece of plastic conduit through the cavity (seal round both ends with silicon.
Fifth, since you are breaching the firewall between garage and house, once you've fed the cable through the conduit you should fill the conduit with either pink (heatproof) foam or rockwool to maintain the firewall.
Finally, is this new socket going to be at a height and in a location where it could get damaged by careless parking or heavy items stored nearby? If yes then consider a metal clad surface socket or flush mounted plastic socket instead of the surface mounted plastic.

 

[-=sT3V3=-]

Thanks guys for the quick replies.

Eric, Are you suggesting a switched socket with an integrated fuse holder? (I have not seen one before)

1st - I can confirm there is a RCD and a B32 feeding the 2.5mm² supply to the circuit.
2nd - Whether it is an actual ring or not I am not sure how to confirm.(Fairly new house, 05 built)
3rd - I am able to look down the cavity from inside the roof and confirm nothing is in the way. (I will double check before works)
4th - Conduit sounds good, do I need grommets on the backing boxes or a fitting to join the conduit?
5th - can do

The new socket would be ~10cm from the edge of the wall inside the garage and 50cm from the ground (assuming the conduit is straight and my measurements accurate) Some very careless parking would be required to damage it. However a flush mounted socket will be considered.

 

[ericmark]

No not talking about a three gang socket, with a grid system you can build it up as you want and wire as you want.

A ring will have at least two cables, and testing between the two cables line to line will show connected together, same with neutral and earth, if a radial or spur they will not show connected together.

In theory you should not take a spur from a spur, so two cables means it is a ring, but not every one follows the rules, so you should test.

I have seen where an electrician did not take due care and he split a 4 mm² radial and extended it around the room with 2.5 mm² then back to start point, his inspecting and testing should have highlighted the error, clearly he did not inspect and test, he had just seen the 30 amp fuse and assumed it was a ring.

I quick look at 4 mm² and 2.5 mm² easy not to realise specially when mixture of imperial and metric as well. I use red, blue and yellow crimps, without crimping if the blue will not fit, likely 4 mm² or 6 mm² if tight in yellow 6 mm² if slack then 4 mm² trying to use a micrometre on the cable does not really work, go and no go with crimps works better.

 

[EFLImpudence]

Eric means one of these:

MK Logic Plus 13A Switched Fused Spur & Flex Outlet White - Screwfix

Order online at Screwfix.com. 13A. Double pole fused connection unit with flex outlet. FREE next day delivery available, free collection in 1 minute.

www.screwfix.com

[Actually the cable is the spur. Stupid people call these accessories 'spurs' because they think that is an abbreviation of Switched Fused Spur Connection Unit.]

If only one socket - single or double - on the spur, then you don't need one.

If more than one socket on the spur, then you will need one (them's the rules) but obviously you will then be limited to 13A.

 

[oldbutnotdead]

Thanks guys for the quick replies.

Eric, Are you suggesting a switched socket with an integrated fuse holder? (I have not seen one before)

View attachment 301276

1st - I can confirm there is a RCD and a B32 feeding the 2.5mm² supply to the circuit.
2nd - Whether it is an actual ring or not I am not sure how to confirm.(Fairly new house, 05 built)
3rd - I am able to look down the cavity from inside the roof and confirm nothing is in the way. (I will double check before works)
4th - Conduit sounds good, do I need grommets on the backing boxes or a fitting to join the conduit?
5th - can do

The new socket would be ~10cm from the edge of the wall inside the garage and 50cm from the ground (assuming the conduit is straight and my measurements accurate) Some very careless parking would be required to damage it. However a flush mounted socket will be considered.

Bit of a worry there's no insulation in the cavity.....
Yes grommets on the (inside) flush backbox and if you use metal backbox for garage socket stick a grommet on that as well. Surface placcie- no need. Since you're using twin and earth you don't need full containment from the conduit (so no need to mess with backbox adaptors unless you want to).
Up to you on the socket type, if you're happy that placcie isn't likely to get smacked then carry on.

 

[flameport]

I can confirm there is a RCD

What kind of RCD?
Can you add a photo of it?

 

[-=sT3V3=-]

So I will confirm it is in fact a 2.5mm² cable (thanks for your suggestion regarding using lugs eric).
I do not have a meter capable of measuring low enough to confirm if the circuit is a pure ring.

As for adding a switched fused junction before the single double plug I will consider - one double socket might not be enough if electric garage door dreams come true.

I thought it was odd the cavity was not insulated but I read filling the cavity with poly beads or anything else really has lots of risks with condensation - perhaps I will make a different thread for input there.

thanks for all the input so far.

 

[EFLImpudence]

I do not have a meter capable of measuring low enough to confirm if the circuit is a pure ring.

What does that mean?
You just have to confirm that, when separated, the two wires (of L, N & E) still have continuity.

As for adding a switched fused junction before the single double plug I will consider - one double socket might not be enough if electric garage door dreams come true.

You could, of course, use 4mm² cable (which can take >32A) for the spur and then you could have as many sockets as you want without using an FCU.



However, we are now in a realm where you really should know what you are doing.

 

[-=sT3V3=-]

To confirm a ring I though one would need to measure the resistance of both ends of the cable coming into/from the b32 breaker. Is that the resistance eric was referring to in post #2?
I just tested and there is continuity between the positives when removed from the b32 breaker. I did make sure to remove all plugs from outlets before checking. The cable is a solid core twin and earth 2.5mm²

While at the DB board I saw a B16 breaker feeding a single 2.5mm² solid care cable going to the decommissioned alarm FCU which may be another option.

thanks for the advice on the 4mm² cable. I cant think of a clean way of routing any addition plugs (for said garage door dreams) inside the garage, surface mounted conduit is the only thing I can think of.

 

[EFLImpudence]

To confirm a ring I though one would need to measure the resistance of both ends of the cable coming into/from the b32 breaker. Is that the resistance eric was referring to in post #2?

It's not the resistance of both ends but the resistance between both ends.
If they are two wires connected to the breaker (each other) there will be a resistance measurement.
If one end is not connected to the breaker it will be infinite.

I just tested and there is continuity between the positives when removed from the b32 breaker. I did make sure to remove all plugs from outlets before checking. The cable is a solid core twin and earth 2.5mm²

Ok, so it is a ring.

While at the DB board I saw a B16 breaker feeding a single 2.5mm² solid care cable going to the decommissioned alarm FCU which may be another option.

Possibly. Can't tell from here.

thanks for the advice on the 4mm² cable.

Ok.

I cant think of a clean way of routing any addition plugs (for said garage door dreams) inside the garage, surface mounted conduit is the only thing I can think of.

Can't be any different to anyone else's garage in which they have multiple circuits and accessories.

 

[-=sT3V3=-]

Thanks EFL.

Forgive my scope creep here...
If I use a 25mm surface mount double socket plastic backing box fed by a 4mm² cable, I can later add surface mounted 20mm plastic conduit to other outlets as needed, correct?
what cable size would be needed to the addition outlets?

 

[EFLImpudence]

If I use a 25mm surface mount double socket plastic backing box fed by a 4mm² cable, I can later add surface mounted 20mm plastic conduit to other outlets as needed, correct?

Yes, the type of back box and flush or surface-mount makes no difference.

what cable size would be needed to the addition outlets?

You will also need 4mm² if it feeds more than one socket (single or double).

 

[-=sT3V3=-]

Thanks a lot for the information gentlemen, I am curious as to what breaches of rules or laws I would be making doing this DIY vs hiring a contractor?