# Workshop Wiring



## Bodger7 (29 Jan 2021)

I am considering enlarging my workshop if my wife will grant me planning permission! At the moment my electricity comes via a 2.5mm spur taken off a house ring main using armoured cable so the power is limited, although I haven't had a problem to date. Before doing any building work I want to improve my electrics. In my garage I have an unused 10mm cable with its own connection to the consumer unit and I would like to run that to my workshop but I think that as I have managed so far with a more limited supply then a 6mm cable will be more than I need.
My thoughts are that I would like to link the 10mm twin and earth cable to 6mm pvc cable using a junction box in the garage ceiling cavity and then joining the 6mm pvc cable to 6mm armoured cable, using a junction box mounted on the outside of the garage wall. I would then bury the armoured cable underground and run it to my workshop where I would connect it to a small consumer unit . The 2.5 mm armoured cable can then be disconnected and left underground. 
Does that sound like a reasonable plan? Thanks for any advice.


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## flying haggis (3 Feb 2021)

strange to have two junction boxes unless that it is not possible to run 6mm swa all the way to the 10mm. if the junction box in the ceiling cavity is enclosed it needs to be a maintenance free version. I would if possible run all the way in 10mm the difference in price is negligible now and the work will be the same. what size breaker is the 10mm fed from now?


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## Spectric (3 Feb 2021)

You say you have managed, do you actually know the maximum load and volt drop for your installation. At the moment your maximum load will only be about 2500 watts depending how long the supply cable is. If you already have a 10mm cable from a consumer board then why reduce size to a 6mm cable? The correct way would be to run an armoured cable from the supply board all the way to your workshop without joints, you need to ensure the size can handle your maximum load and within allowed volt drop. I would also suggest running three core and have a dedicated CPC and not a two core and rely on the cable armour. This is also classed as notifiable and an instalation cert given to your local Building Control when finished.


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## RobinBHM (3 Feb 2021)

do you know the type of earth your house has?
that will determine if you are exporting the earth or not

armoured cable needs proper attention paid to terminating and continuity of earthing arrangment.

sometimes the armour can be used as the earth sometimes it needs to be a CPC


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## Bodger7 (3 Feb 2021)

flying haggis said:


> strange to have two junction boxes unless that it is not possible to run 6mm swa all the way to the 10mm. if the junction box in the ceiling cavity is enclosed it needs to be a maintenance free version. I would if possible run all the way in 10mm the difference in price is negligible now and the work will be the same. what size breaker is the 10mm fed from now?


Yes I agree and the electrician who first fitted the 10mm cable suggested running the armoured cable to the 10mm cable direct. I note what you say about the 10mm armoured cable. I hadn't considered that but having compared prices the difference as you say is negligible. The breaker for the 10mm cable is 20 amps. 
Thank you for your advice which I intend to follow. There seems little point in using the smaller cable for the small saving I would make.


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## Bodger7 (3 Feb 2021)

Spectric said:


> You say you have managed, do you actually know the maximum load and volt drop for your installation. At the moment your maximum load will only be about 2500 watts depending how long the supply cable is. If you already have a 10mm cable from a consumer board then why reduce size to a 6mm cable? The correct way would be to run an armoured cable from the supply board all the way to your workshop without joints, you need to ensure the size can handle your maximum load and within allowed volt drop. I would also suggest running three core and have a dedicated CPC and not a two core and rely on the cable armour. This is also classed as notifiable and an instalation cert given to your local Building Control when finished.


Hi Roy
My lathe is 2250 Watts, my bandsaw is 1.5kw and when cold I sometimes run 2 fan heaters at the same time as either the lathe or bandsaw (but never together). The ring main is used for the usual household items such as fridge, freezer, washing machine, tumble dryer and one shower as well as powering sockets in two bedrooms and dining room. One of my concerns is that when we are away if something goes wrong then we lose power to the freezer (once the light outside my workshop got water in it and activated the breaker. It was only good fortune that it happened shortly before we returned.) I also want to be able to have more lighting and more electric tools in the future. Following Flying Haggis's advice and your own I intend running 10 mm armoured cable direct to the 10mm upvc cable and joining it there. The prospect of taking up floorboards to route the armoured cable right back to the CU is too daunting (furniture moved and carpets lifted - SWMBO would have a fit!) I intend to run 3 core cable as you suggest - in fact I hadn't considered 2 core. 
Thank you for your reply. Helpful as ever.


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## Bodger7 (3 Feb 2021)

RobinBHM said:


> do you know the type of earth your house has?
> that will determine if you are exporting the earth or not
> 
> armoured cable needs proper attention paid to terminating and continuity of earthing arrangment.
> ...


Hi Robin
No I don't know what type of earth my house has and I intend to use 3 core cable. Thanks for your input.


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## Spectric (3 Feb 2021)

Bodger7 said:


> The ring main is used for the usual household items such as fridge, freezer, washing machine, tumble dryer and one shower as well as powering sockets in two bedrooms and dining room. One of my concerns is that when we are away if something goes wrong then we lose power to the freezer (once the light outside my workshop got water in it and activated the breaker.


Yes that is how we used to wire domestic properties except the kitchen would be on it's own circuit. Now we use more circuits each fed by it's own RCBO to ensure that one protective device cannot take out everything like freezers & fire alarms. I note you have a shower on a ring main, that is now classed as a special location and should be on its own circuit protected by an RCBO.


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## NetBlindPaul (4 Feb 2021)

It can’t be much of an electric shower if it’s on a “normal” ring final and hasn’t melted anything!


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## NetBlindPaul (4 Feb 2021)

Bodger7 said:


> Hi Robin
> No I don't know what type of earth my house has and I intend to use 3 core cable. Thanks for your input.


That’s the first thing that you have to establish.


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## Bodger7 (4 Feb 2021)

Hi Paul

Thank you for your replies.

It can’t be much of an electric shower if it’s on a “normal” ring final and hasn’t melted anything!
[/QUOTE]
No, it's not an electric shower, it's a power shower, so only the pump is powered by electricity. The electric shower in our house has its own separate supply from the CU.

As far as knowing the type of earth that my house has do I need to know this if I use 3 core (including an earth) armoured cable to connect to the PVC twin and earth cable fitted by the electrician who fitted the original cable and wired it into the consumer unit? I wouldn't be doing this at all if I didn't already have a substantial cable connected to my CU by a professional electrician. All I propose is extending and diverting the cable already fitted for a different use.


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## porker (4 Feb 2021)

The earth type may affect the earthing arrangement for any outbuilding. If you have a TT earthing arrangement (usually an earth rod outside into the ground) you will need an earth rod at your outbuilding. If you have TN-S or TN-C-S you can probably use the earth from your armoured cable as long as it is big enough.

*there are always exceptions


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## Fergie 307 (5 Feb 2021)

I wouldn't advise joining a smaller cable to a bigger one in the main supply, use the same size. Given the trivial cost involved surely better to fit an earth rod in the outbuilding.


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## Fergie 307 (5 Feb 2021)

Reading back over previous posts, can't help thinking of you are going to this trouble then it's got to be worth upgrading your consumer unit in the workshop. That way you can ensure everything is properly protected by the correct rating, and avoid the situation where one thing tripping knocks the whole lot out.


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## Bodger7 (5 Feb 2021)

Hi Fergie, Yes I should have made it clear that I will be changing the 2 way consumer unit in my workshop for a 3 way. That will enable me to have 2 circuits for power and 1 for lighting. Any suggestions as to size of breakers? At the moment I have 6 amps for the lights and 16 amps for the power but I can't remember why I chose those sizes. The CU has a 20 amp breaker. My father in law was an electrician so he may have advised me but he is rather out of date now (he is 97) although he still remembers the basics. With the lockdown I can't speak to him in person and can't have a proper telephone conversation with him as he is very deaf, and I'm not much better!
On your first point about cable size I have accepted previous advice to join 10mm armoured cable to the 10mm pvc. Thank you for your advice.


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## OldWood (5 Feb 2021)

My input would be to make your workshop CU have more capacity. I had a tablesaw that did work on the workshop ring main, but then the smoke got out after 25+ years and I fell heir to the motor from the local wood club workshop bandsaw - re-bearing'ed that and found that occasionally it would trip the ring so the saw had to go onto a seperate slow 'blow' 20A trip feed.
There have been several similar tales of woe here recently with new bandsaws.
Rob


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## Spectric (5 Feb 2021)

Hi

The CSA of the wiring determines the current rating of the protective device not the intended load. With a 2.5mm radial use a 20 amp device and with 4.0mm radial use a 32 amp device. I would run a 2.5mm radial but you must measure the loop impedances or prospective fault current in order to validate your design, the protectice devices will only provide protection if sufficient fault current can flow.


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## NetBlindPaul (7 Feb 2021)

Bodger7 said:


> ...
> As far as knowing the type of earth that my house has do I need to know this if I use 3 core (including an earth) armoured cable to connect to the PVC twin and earth cable fitted by the electrician who fitted the original cable and wired it into the consumer unit? I wouldn't be doing this at all if I didn't already have a substantial cable connected to my CU by a professional electrician. All I propose is extending and diverting the cable already fitted for a different use.


Yes you do need to know what supply type you have at your property because it’s actually very dangerous without written permission from your DNO to export a TN-C-S earth to an outbuilding.


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## Bodger7 (7 Feb 2021)

NetBlindPaul said:


> Yes you do need to know what supply type you have at your property because it’s actually very dangerous without written permission from your DNO to export a TN-C-S earth to an outbuilding.


Hi Paul
I have checked the paperwork left by an electrician who did some rewiring in my house 11 years ago and I can see that my supply is TN-C-S. I can understand that connecting things up in the wrong way could be dangerous but I can't see how the failure to get written permission makes it dangerous by itself. Does the written permission include a description as to how the installation is to be carried out perhaps? Why can it be very dangerous to export a TN-C-S earth to an outbuilding? Doesn't it depend on the circumstances and can't a separate earth be provided for the outbuilding if necessary? Thanks.


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## NetBlindPaul (8 Feb 2021)

To export a TN-C-S earth is dangerous because of the way that the system functions.
It goes against the requirements of BS7671 to do so as well.
If I have a few hours I can try to make time to write up a paper on the hazards of exporting a TN-C-S earth but I am going to struggle with the time in hospital.
Suffice to say that it’s dangerous and not to do it. Please just take the advice because if you do it and it goes wrong it could easily kill you.
You can TT the shed, but, care needs to be taken so that there is no exposed or extraneous-conductive-parts connected to the TN-C-S earth accessible in the TT zone.


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## guineafowl21 (8 Feb 2021)

This John Ward video covers earthing outbuildings pretty well:


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## Spectric (8 Feb 2021)

The video is quiet informative, but I always used to run the earth as a core within the armoured cable rather than the armour itself because the armour is not only steel but it is also zinc plated. The issue with the TN-C-S system for providing the means of earthing is that if for any reason the Neutral becomes open circuit then you lose your earth and are now relying on supplementary bonding but also any connected loads will cause the Neutral to rise towards 230V and therefore any extraneous metal parts like the steel frame of the building if used in it's construction. This can cause touch and step voltage hazzards. Now with industrial steel framed buildings the frame will be earthed at multiple points for lightning protection that is also connected to the main earthing bar, plus the supply will be a TN-S system so no issues.

For most domestic instalations it is easier to just run a 10mm 3 core SWA cable if the DNO will allow this otherwise just use a 10mm 2 core SWA cable and suitable earthing rods / mat to ensure the earth impedance is low ..

Again remember this type of work is notifiable and should be undertaken by a qualified electrician.


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## Bodger7 (8 Feb 2021)

Hi Paul
Thanks for your further input. My workshop is a concrete block building with timber cladding built on a solid concrete base and there are no extraneous conductive parts. The only conduit coming into the building is the electricity cable. I have watched the John Ward video and the explanation contained in it has clarified the position for me. Unless John Ward is wrong then there seems to be nothing to prevent the use of 3 core 10 mm2 armoured cable connected to the existing 3 core 10 mm2 pvc cable already wired into my CU by an electrician. I see that you are in hospital and I wish you a speedy recovery to full health.

Thank you GF. What an informative video that explains the position in simple language. As mentioned above my workshop is built from concrete blocks on a concrete base with no extraneous conductive parts ie no water pipes or any other pipes coming into the structure. I can't see any potential danger from using a 3 core 10 mm2 armoured cable and connecting all 3 wires to the pvc cable already wired into my CU.

Hi Roy
Yes, I found the video very informative. As you suggest, I propose to run a 3 core 10 mm2 SWA, connected to the 10 mm2 pvc cable, to the workshop and use the earth within the SWA. I see that John Ward makes the same point as you about the armoured part of the cable being steel. You will see from the above that there are no extraneous conductive parts so no bonding is necessary (or possible). I tried to contact my DNO, UK Power Network, today but the people who deal with questions such as these are not available at the moment due, I suspect, to a combination of Covid and weather. I will contact them before finalising my plans and will get an electrician to check my installation before going live. Thank you again for your advice.


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## NetBlindPaul (9 Feb 2021)

Since John did that video other hazards have come to light with TN-C-S systems that are currently being reported on prior to this report being presented to the IET, DNO’s, HSE & the ORR.
Once the report is complete both myself and JW will have the final copy for peer review, we were (myself and JW & a few other #e5 guys) on a video call with the guy that commissioned the research last night.
One of the things that has come to light is that the network operation can easily result in the N conductor rising well above the potential of true earth and thus the earth will also rise. Both may also carry fortuitous current and therefore pose an unexpected hazard.
I cannot recommend strongly enough, and I know that John would now agree that to export the TN-C-S earth is not recommended on safety grounds.


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## Bodger7 (13 Feb 2021)

Hi Paul, Roy, GF21 and others.
For obvious reasons I was concerned at the suggestion that my proposed installation could prove fatal, especially as I have young grandchildren who like to "make something" (albeit that they can't at the moment). As a result I have done some more research, sent an email to John Ward (that I have not yet received an answer to) and written to my DNO, Power Networks UK. I haven't been able to reach any firm conclusions but it seems to me that:-
1. The debate about exporting an earth from a TN-C-S system to an outbuilding is not new and has been rumbling around for 10 years at least.
2. There seems to be no definitive right or wrong answer and seemingly very competent electricians will disagree about the dangers inherent in exporting the earth from a TN-C-S system and the need to consult/seek permission from the DNO. For example, John Ward's video was produced during the period when this matter has been the subject of debate when others were advocating a TT system for all outbuildings. As far as I know that video remains on Youtube. Examples of totally opposed views can be found at, for example, Exporting the earth? 
3. The DNO, or at least UK Power Networks, do not have to be asked for permission to export the earth. Their answer to my question on this and other points raised was that "I do not know of any process that we have in place to provide written permission for works such as this to take place as our responsibility is up to the main head and for any equipment we have installed and maintain, or any alterations that have been carried out by ourselves." and "I have had a look at the various forums online and this seems to be a common debate in terms of whether to use a spike, export the incoming earth etc and the question or permission from the Dno seems to be one brought up on occasion but again shot down by others in the forums without a definitive answer."
4. I had asked the DNO to confirm "that there is no known danger from exporting a TN-C-S earth to an outbuilding (subject to there being no extraneous conductive parts)." To me this is a fairly simple question to which I would have expected a comment at least, even if the answer was "it depends.....". The question was not answered so I wonder whether opinion is also divided at the DNO? I am surprised that there is no guidance given for such an important safety issue.

Although there are arguments for both exporting the TN-C-S earth and creating a separate TT system for a workshop there doesn't seem to be any safety reason for not creating a separate TT system, so that is now my preferred option. I understand that the armoured cable should be connected to the TH-C-S system at the supply end of the cable but insulated from the circuits in my workshop to ensure that in the event of any accidental damage to the SWA there will still be an earth. The SWA will be cheaper as I will be using 2 core but will need ground spike(s) for the separate earthing system. Advice as to the size for the earth cable would be appreciated. At the moment the power circuit in my workshop is in 2.5mm and the lighting circuit in 1.5mm. The earthing circuits are therefore in 2.5 and 1.5 mm cable. 

When I asked the initial question I didn't realise what a can of worms I was opening but thank you to you all for the advice offered (even if it was bit worrying).


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## guineafowl21 (14 Feb 2021)

Bodger7 said:


> At the moment the power circuit in my workshop is in 2.5mm and the lighting circuit in 1.5mm. The earthing circuits are therefore in 2.5 and 1.5 mm cable.


If it’s twin and earth, the earth is smaller than the other two, e.g. for 2.5mm2 T&E, the earth is 1.5mm2.

Size of earth wire connecting TT rod to installation varies. For normal sheathed wire, it should be 16mm2 if buried, but 4mm2 if not. In either case, it can be just 2.5mm2 if mechanically protected. For the cost of a short piece, you might as well err on the large side.

John Ward also did a series of vids on earth rod installation (search “John Ward earth electrode”).

You must ensure your earth loop is below 200 ohm, ideally less than 100 ohm, and of course all circuits must be RCD protected.


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## Spectric (14 Feb 2021)

Bodger7 said:


> Unless John Ward is wrong then there seems to be nothing to prevent the use of 3 core 10 mm2 armoured cable connected to the existing 3 core 10 mm2 pvc cable already wired into my CU by an electrician.


You have stated an obvious potential problem, if that existing 10 mm2 pvc is say T&E then the conductors may be 10mm but the CPC will not. Now without extranous parts the bonding issue goes away but changing the size of the CPC is not good practice. Solution would be to run just a two core live & neutral and use earth rod at the workshop which covers everything, even if you do have extranous parts. Make sure the armour is earthed but only at the workshop end. As I have mentioned in another post the dangers stem not from yourself but at the supply with a PME system. If the Neutral fails at the supply side then you lose Neutral and Earth but still have live. Now if there is a load connected, it obviously will not work but it will mean the Neutral is now also LIVE, ( look at Ohms law) and with extranous conductive parts that are connected to Earth they will also become live, so you have a hazard. With an Earth rod this will prevent conductive parts having a higher potential than ground/Earth and so reduces the hazard of touch/step voltages.


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