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Adding new Surface Mounted Socket Circuits to 2nd and 3rd Floors

Postby GeorgeMC » Fri Mar 03, 2017 8:02 pm

Hi everyone,

Myself and a group of friends have taken on a vacant commercial building with the intention of using it as artist studios and an education space.
However the top two floors (2nd and 3rd) have only been used as storage for many years and are lacking in power outlets (3rd has none at all) which will clearly be a problem.

The main constraint on this job is the fact that it is essentially temporary. We have the building on a reduced rent, for a year, possibly slightly longer, before it is totally gutted and turned into flats. This obviously limits the scope of what can be done.

As a result I want all sockets and cables to be clipped direct to walls so as to minimise the amount of installation required and make the materials recoupable when we leave.

There is spare capacity on the CU which is on the ground floor, to add circuits. I’m after advice specifically as to what circuit others would recommend. I anticipate that the sockets will be used mainly for low loads such as phone chargers, radios and laptops and small power tools but I would want to design in capacity for people using electric fan heaters in winter and occasionally large power tools. One room will also be specifically a music practice room so will be running amps. I think somewhere around 4 to 6 KW per floor should be plenty but feel free to say if you think otherwise. So my options appear to me to be the following -

A 2.5mm Final ring on a 32 A MCB from the CU.
The obvious problem is that I would use a lot of my allowed 100 metre sq area on the cable climbing to the floors and back.


Two 4mm Radials on a 32A MCB to each of the unsupplied floors from the CU. Taking into account the voltage drop allowance I could still be limited by length.


Running power up to a small CU on one of the upper floors and running two circuits from there. This seems the best solution to me as losing the 10m (20m if a ring is used) climb gives me greater flexibility in the new circuits.

Budget is also going to be a very limiting factor. I intend to install the work myself up to Part P specs and then get an electrician to run the necessary checks on it. Seeing as everything will be surface mounted and thus entirely visible I am hoping this would be relatively easy.

Any considered suggestions would be appreciated? Thanks
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Simply Build It

Postby ericmark » Sun Mar 05, 2017 5:33 pm

If no one lives in the building there is no Part P requirement, however still has to comply with health and safety of course. Insurance is main problem and that's up to you.

However in essence it is like a building site and I would treat as if it was other than using 230 volt rather than 110 volt.

Forget idea of a ring final, use radials for everything, I would install some sockets near the existing supply, 63 amp, 32 amp or 16 amp depending on load and these will feed a distribution unit on each floor with either fuses or MCB's the distribution unit could be old consumer units or could be fused connection units, or even grid switch units, it would depend what I could lay my hands on.

You will need cable able to take load and keep volt drop down, and be safe, it depends on the runs, YY cable may be order of day, or blue flex like used with caravan supplies, I would steer clear of twin and earth. Although depends on where it comes from.

Largest problem is reducing cable size near where to be used, I would look at cooker switches, cooker connection units, and shower isolators.

If you need to wire a bench then the ring final may work lollipop method, again duel appliance cooker connection unit is good for connecting a 6 mm cable to a 2.5 mm ring final. Even designed for three cables.

The big problem is to show you have the skill, I would hope nothing goes wrong, but people do daft tricks, so everything needs to be documented to show you did the risk assessment and gave everything due consideration. The installation certificate is more to protect you than any one else, it shows you planned it all and installed it all in a regimented fashion, then inspected and tested it.

If your certificate shows the ELI at furtherest socket is 0.96 ohms and latter some one is injured and it's retested and shows 40 ohms likely the courts will think it was 0.96 ohm when you did it. If you have no paperwork they will likely consider you were at fault.

Only person who can sign the installation is the one who was in charge of the work, it can't be done by some one else latter, even using a EZ150 plug in tester and writing down better than 1.5 ohm which is the first reading on the tester would be better than nothing, you need to show you did all in your power to make it safe.

And it goes without saying RCD on every socket and cable, you can get 16A sockets with RCD built in same with 32A sockets, there is only a need for one RCD on each circuit
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Postby GeorgeMC » Mon Mar 06, 2017 10:10 pm

Thanks for your helpful reply ericmark,

I didn't realise there was no Part P requirement if it was non residential, that is very interesting to know. The ground and first floor will function as a shop, gallery and educational space and thus be open to the public at times so this means we will be wanting to get public liability insurance and whatever else may be necessary. I assume attaining that would still involve an inspection of the building.

Out of interest, could you clarify why you would steer clear of Twin and Earth? I have never had much to do with YY cable, I will do some research into it.

I'm interested in the documentation procedure you suggest. Are you essentially saying It would be worth while keeping a detailed photographic record of me designing (i.e volt drop calculations, wiring diagram etc), installation and then testing the completed work. I hadn't considered this, I assumed as I am not an electrician I would in legal terms just be automatically considered to have no skill. Is there an actual certificate I could acquire and fill in as part of this or can they only be distributed by electricians?

Thanks again for your input
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Postby ericmark » Wed Mar 08, 2017 2:03 am

Certificates are a free down load from the IET web site. There is no qualification that makes you an electrician, it's down to your skill, there were 4 categories now dropped to 3 not sure on new 3 so I will list original 4.
Ordinary Person
Instructed Person - This would include an apprentice.
Skilled Person - He can look after his own safety.
Competent Person - He not only looks after his own safety but also safety of others, this has now gone.

What is being said is you may be skilled to fit a plug, but only instructed fitting a socket. An electrician who designs and installs a PLC controlled press will likely require a lot more skill than one wiring a house, and all that the regulations say is you must have enough skill to do the job safely. It does not say you must have a City & Guilds 2391 certificate.

However insurance companies may require some formal qualification, although I have never been required to produce it. I am trained to level 5 i.e. I have a degree in electrical and electronic engineering, however for many years my only qualification was in auto electrics. Latter became a radio ham, and then took some C&G exams. But no one said I had to take exams, fact I had worked as an electrician since 1979 was enough. OK I did know what I was doing, but paper wise I was an auto electrician.

I actually worked as an electrical engineer well before I had any exams showing I knew how to work with 230 volt (low voltage). In other words if you say you know what you are doing, people will believe it. OK to be fair I did know what I was doing, but I did not have any exams to prove it.

In court if you can show how since 1978 one has worked as an electrician then that shows you are competent without any bits of paper. But if I could not show how I was trained by SLD pumps then spent years in the trade, then I would need some paper to show I can do what I say I can do.

So you need something, even a C&G 2382 would show you have some skill, and all that exam really shows is you can read a book. And it is that book you need to follow. The IEE which became IET has for many years produced the wiring regulations, in 1992 it joined with the British Standards and BS7671 was born. This is the bible you have to follow, it's not law, but can be used in a court of law to show a warranty of skill was not used, so it may as well be law.

I don't like twin and earth because it's not protected by a earthed screen, so in theory every circuit using twin and earth needs to be RCD protected. Steel wired armourer and Alitube cable is officially considered as not needing RCD protection, although still need it for sockets, YY cable is not in the official list, but it's more flexible than SWA cable and does have a braid all around the cable, so any item cutting the cable will likely force the earthed braid into the live cores meaning a protective device will open and every thing will fail safe.

Problem with YY cable is water can get in over time and result in the braid failing, but for time your talking about unlikely. Anyway should anything go wrong, you can show you considered the problem and used your skill to reduce it to an acceptable level. In other words your showing the court you had some skill and did not simply connect everything up and pray to the lord, even if really that's what you did. Maybe better is SWA is used?

For me, I have done the work for so long I could guess on the readings and fudge up some results for the paper work, however you can hardly do that, and exceeding the permitted loop impedance is easy to do.

There is so much maths in electrical work, I know from experience what size cable I will likely need, so I guess I will need 6mm cable and once finished I measure and find I guessed right. Not that easy for you.

However by calculation and inquiry is a valid method. So with a incomer of 0.35 ohms and a 2.5 radial of 32 meters you can calculate the impedance at 1.14 ohms and volt drop of 11.5 volt so it should pass, just pass that's on the limits. Oh working on 20 amp design current.

With 6mm instead of 2.5mm the length increases. In fact 85 meters. The impedance drops to 1.08 ohms. Still working on 20 amp. I use a java script program to get figures, and in real terms if the figures you write down seem to fit then even if some one tests and finds it's not correct, you can argue it was OK when you tested.

However where you can see there is around 100 meters of cable used, if the figures on the paperwork have a daft result, then it's clear it was not tested.

Even with my qualifications when I wanted to rewire mothers house with idea of renting it out, I still used a firm to do the job and did not do a DIY, I realise no one would take my word for it being OK if any thing went wrong.

I would talk to insurers first, see what they want, then decide, as you say you need the insurance to be valid, just one person could claim 250k should something go wrong, so insurance is really important.
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