Workshop Wiring

[Bodger7]

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.

 

[flying haggis]

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?

 

[Spectric]

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.

 

[RobinBHM]

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

 

[Bodger7]

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.

 

[Bodger7]

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.

 

[Bodger7]

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

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.

 

[Spectric]

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.

 

[NetBlindPaul]

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

 

[NetBlindPaul]

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.

 

[Bodger7]

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.

 

[porker]

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

 

[Fergie 307]

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.

 

[Fergie 307]

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.

 

[Bodger7]

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.

 

[OldWood]

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

 

[Spectric]

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.

 

[NetBlindPaul]

...
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.

 

[Bodger7]

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.

 

[NetBlindPaul]

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.