Jump to content


ebuild is sad to announce its closure - it has become too time and resource intensive to develop, manage and maintain.

However, ebuild will remain on-line in archive mode (ie no posting facilties) for several weeks so that users can use it as an information resource.

Diy Electric Circuits


  • Please log in to reply
28 replies to this topic

#1 vberg

vberg

    Regular Member

  • Members
  • PipPipPip
  • 197 posts

Posted 03 February 2016 - 08:53 PM

Do I need an electrician to do all the wiring, putting up switches/socket, fix lighting inside the house or is it enough if the spark just connects the house to the grid and makes a nice fuse box then I could do the rest myself? Not neccesarily want to , rather just interested - done similar DIY work before - I've put up switches, connected them to the wires etc.

#2 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 03 February 2016 - 09:53 PM

This is all covered in the links I've given before, in this case you need to look at Part P of the building regulations.

You can, in theory, do the electrical installation yourself (to the regulations contained within the 17th Edition of the wiring regulations, BS7671). You will need to follow every part of those regulations, including cable sizing, placement, grouping factor and insulation derating factors, making the correct allowance for diversity on circuits etc. In addition you are required to place all switches and sockets at heights given in Part M of the building regulations and must follow some other accessibility rules, like having all fuses or resettable circuit breakers in an accessible location. The main consumer unit must be, as a minimum, a metal unit that complies with the 17th Edition in terms of having a minimum of two residual current devices and miniature circuit breakers sized for the cable they are protecting in each circuit.

After all this do it yourself work you can, in theory, get your building control body to test and inspect the electrical installation for compliance. In practice, there are very few electricians who have the additional accreditation to test and sign off work done by others, so building control bodies can find it hard to offer this service. Our building control body cannot find a suitably accredited electrician to do this test and inspection sign off, for example, so we had no choice but to use a Part P accredited electrician to do our electrical installation and sign it off.

Edited by jsharris, 03 February 2016 - 09:54 PM.


#3 Crofter

Crofter

    Regular Member

  • Member Blogger
  • PipPipPip
  • 592 posts

Posted 03 February 2016 - 10:12 PM

I found myself asking a similar question back when I was doing my loft conversion. It seems that you can alter an existing circuit yourself (within limits) but not run a new one.
I did initially wire in lights and sockets myself by tapping into existing wiring, but it was all a bit dodgy and figuring out what each cable was in the existing wiring was a nightmare.
In the end I got in a sparky for a day and we redid the whole thing, running new circuits back to the fuse box. I laboured for him which cut the costs down, and in fact I did all of the crawling around in awkward spaces whilst he just fed me cables through the holes. Running a whole new circuit was far, far easier and simpler than tapping into an existing one- so I'm not sure why it isn't allowed on a DIY basis but I guess they have to draw the line somewhere.

#4 vberg

vberg

    Regular Member

  • Members
  • PipPipPip
  • 197 posts

Posted 03 February 2016 - 11:37 PM

I'm not sure about how others do electricals, but on the plans I would have all the wires, sockets, cables,etc. so from a fuse box the job would mostly be running the cables in tubes that are already in the walls, so it's motsly labour. I wouldn't be comfortable working with the fuse box or the electric inlet itself but the rest is fairly simple I think in a new build especially if you have a well developed plan for everything.

#5 cjard

cjard

    Regular Member

  • Members
  • PipPipPip
  • 292 posts

Posted 04 February 2016 - 07:46 AM

View PostCrofter, on 03 February 2016 - 10:12 PM, said:

I'm not sure why it isn't allowed on a DIY basis but I guess they have to draw the line somewhere.

It's not allowed because the circuit has to be designed for the current it will draw, and the installation of the cable has to support that current. Most people aren't competent to do this. Modifying an existing circuit in certain ways is allowed because it doesn't change the design current so it's hard to introduce a fault at will lead to a fire hazard. The only proble you get is that, legally, upon making a minor mod to one circuit you may be required to bring other aspects of the installation up to a particular standard. While this is well intentioned, it causes a problem illegal DIY electrics because an electrician is duty bound to say he can only add an extra socket under the stairs if he upgrades the entire crappy old fuse board to a modern consumer unit (and a load of faults will then appear because of the increased sensitivity of modern protective devices) at a cost of about 600 quid.

Very it's not really enough to put a wire on a plan and assume this means it is safe and sensible to run that cable that way. If you're building in the UK, and want to save money on the electrics, I recommend you find a spark like mine - one who wants to keep his turnover low to avoid registering for vat, so you buy your own materials, then work with him as his labourer running the wires before plasterboarding and possibly connecting them up under his supervision afterwards. He can then tell the truth on the certificate he has to sign that he has designed and implemented the electrical system

#6 temp

temp

    Advanced Member

  • Moderators
  • 10,200 posts

Posted 04 February 2016 - 08:34 AM

What jsh said plus.... If you want to DIY and get Building Control to sign it off then you have to notify them before you start and check when they want to do any inspections.

#7 billt

billt

    Member

  • Members
  • PipPip
  • 56 posts

Posted 04 February 2016 - 09:52 AM

View Postcjard, on 04 February 2016 - 07:46 AM, said:

It's not allowed because the circuit has to be designed for the current it will draw, and the installation of the cable has to support that current. Most people aren't competent to do this.

I bet most electricians don't "design" circuits. They'll just use 2.5mm for rings, 1.5mm for lighting, and 4mm or 6mm for high current circuits (where you might want to check voltage drops and temperature rise). But it's not terribly difficult, despite the best efforts of the trade bodies to make it seem so.

If the non professional does his research and exercises reasonable care there is no reason to think his installation would be any less safe than that of a part P registered electrician.

Incidentally, most of the injuries caused by electrical faults are not due to the permanent wiring, it's the things that are plugged into it that cause issues. And the very few cases that are due permanent wiring faults have almost invariably been installed by professionals.

Of course, the fact that LAs don't seem to be willing to do their job and inspect DIY electrical installations means that, in practice, the only way to get it done is to employ an electrician to do it if you want the relevant piece of paper. IOW the trade bodies have managed to create their closed shop.

Edited by billt, 04 February 2016 - 09:52 AM.


#8 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 04 February 2016 - 10:57 AM

I can't say what others do, but the electrician I used didn't just use any cable without checking to see it was OK. We upped the size of the hob feed cable, for example, because he pointed out that it was running in a conduit in the heated floor, so was subject to derating. The same went for some cables that went through a length of insulation. He also did a quick diversity check on one of the spurs to make sure that the cable rating was OK. This is all standard Part P stuff, so I'd guess most suitably qualified electricians should do the same.

#9 ProDave

ProDave

    Self build in the Highlands

  • Moderators
  • 5,960 posts
  • LocationScottish Highlands

Posted 04 February 2016 - 11:06 AM

View PostCrofter, on 03 February 2016 - 10:12 PM, said:

I found myself asking a similar question back when I was doing my loft conversion. It seems that you can alter an existing circuit yourself (within limits) but not run a new one.
Don't forget that Scotland and Northern Ireland don't have Part P, so anyone who is competent can do your own wiring still.

There are a few "gotchas" like a full rewire in a flat or 3 storey building requires a building warrant for instance.

#10 Crofter

Crofter

    Regular Member

  • Member Blogger
  • PipPipPip
  • 592 posts

Posted 04 February 2016 - 01:32 PM

View PostProDave, on 04 February 2016 - 11:06 AM, said:

Don't forget that Scotland and Northern Ireland don't have Part P, so anyone who is competent can do your own wiring still.

There are a few "gotchas" like a full rewire in a flat or 3 storey building requires a building warrant for instance.

Thanks- perhaps I had been misled by English regs.

As we're on the subject, I need to install a permanent supply to my shed. It currently runs off an extension lead that I have fed up through the floor and just plugs into one of my ring main sockets (via an RCD). I have now buried a 2.5mm2 SWA cable to do the job properly, and want to wire this into a new circuit of its own. There is a spare space on my CU. Would you think I'd be breaking any rules if I wired in this new circuit myself?

#11 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 04 February 2016 - 01:43 PM

My guess it that you're probably not supposed to wire a new external circuit to a CU, even in Scotland, but who's ever going to know if you do and make a proper job of it?

Worth checking if 2.5mm SWA will be OK, though. Also, how are you going to terminate the SWA at the CU? It's not easy to fit an SWA cable gland to a plastic CU, so perhaps best to use a metal adaptable box at either end, to allow the armour to be properly earthed at both ends.


I ran 4mm SWA to my garage because, at 30m, the voltage drop was getting a bit high. There is a voltage drop calculator here: https://www.tlc-dire...oltageDrop.html that makes it easy to see the effect of long runs.

Mind you, I needed to run a 16A car charging point at the end of my cable, plus leave enough spare capacity to run lights and machine tools (one at a time) and a small (1.5hp) compressor. I was getting nearly 12V drop (allowing for diversity) using 2.5mm SWA, which is on the limit; by upping the size to 4mm the voltage drop came down a fair bit.

Edited by jsharris, 04 February 2016 - 01:45 PM.


#12 Crofter

Crofter

    Regular Member

  • Member Blogger
  • PipPipPip
  • 592 posts

Posted 04 February 2016 - 02:18 PM

Thanks for the link, I should be fine with the 2.5 (bit late now anyway!) as I only need enough power to run my table saw, everything else is less power than that.
I'm going to use a junction box to swap from SWA to T&E once it has entered the house, as you say the SWA isn't the easiest to work with!

#13 ProDave

ProDave

    Self build in the Highlands

  • Moderators
  • 5,960 posts
  • LocationScottish Highlands

Posted 04 February 2016 - 03:08 PM

I share the view that 2.5 is probably on the small side, unless it's a very short run.

The issue is not will it carry enough current, 2.5 SWA is rated for about 28A (so put it on a 20 or 25A mcb max)

The problem is likely to be voltage drop. since you will be running a light in the shed, you are supposed to limit volt drop to 3% and it won't take a very long run of 2.5 before you reach that with the table saw going, or the heater, or kettle that are likely to migrate to the workshop.

#14 joe90

joe90

    Regular Member

  • Members
  • PipPipPip
  • 847 posts

Posted 04 February 2016 - 03:23 PM

My brother in law used to rent a workshop that was obviously fed by too small a cable as when the table saw was switched on it took forever to get up to speed and all the lights used to dim a lot, if the kettle was on as well it got quite dark !!!

#15 Markblox

Markblox

    Regular Member

  • Members
  • PipPipPip
  • 136 posts

Posted 04 February 2016 - 03:48 PM

View PostProDave, on 04 February 2016 - 03:08 PM, said:

I share the view that 2.5 is probably on the small side, unless it's a very short run.

The issue is not will it carry enough current, 2.5 SWA is rated for about 28A (so put it on a 20 or 25A mcb max)

The problem is likely to be voltage drop. since you will be running a light in the shed, you are supposed to limit volt drop to 3% and it won't take a very long run of 2.5 before you reach that with the table saw going, or the heater, or kettle that are likely to migrate to the workshop.

I must admit I get quite sick of people thinking it is all a closed shop and oh so easy. As was just said, bit late now anyway, obviously not designed and that circuit never will be either. To give you an example, the other day I saw some metal wall lights wired in bell wire. When the switch is turned on the lights work and so most people are quite happy with that. How many of you would do something like that and not even know what is wrong with it? Ignorance of the regs is not justification if you don't know what you are doing.
I once worked in an equipment hut which was in the centre, between the runways of Bristol international airport and there was a requirement for air con. Trouble was, the zircon couldn't run. If you put a kettle on the lights went out. The circuit was about 300 metre and the supply was in 2.5mm.

#16 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 04 February 2016 - 04:48 PM

I think there are stacks of "gotchas" when it comes to electrical work, 99% of them things most people wouldn't even think about. One of the ground works guys asked me why I was running 4mm2 SWA to the garage, as it seemed a bit thick to him. When I mentioned that the load was going to be around 20 A one of them (who thought he knew a bit about electrical installations) replied that 2.5mm2 would have been fine, as it was OK for well over 20 A. I explained that at 20 A I was on the acceptable voltage drop limit with 4mm2, so would have had a lot bigger drop with smaller cable, he couldn't get his head around it, just kept muttering that 4mm2 was overkill at around 40 A rating underground.

Edited by jsharris, 04 February 2016 - 04:49 PM.


#17 Crofter

Crofter

    Regular Member

  • Member Blogger
  • PipPipPip
  • 592 posts

Posted 04 February 2016 - 05:06 PM

I don't think my 'workshop' lives up to the standards people are assuming :)
Very drafty cold shed, no point ever putting a heater or a kettle in there. If I can run a double socket and a light then that's fine.
For the past couple of years it's been running off an extension reel laid across the garden. The SWA is both a higher rated cable and takes a considerably shorter route.

#18 Markblox

Markblox

    Regular Member

  • Members
  • PipPipPip
  • 136 posts

Posted 04 February 2016 - 05:36 PM

View PostCrofter, on 04 February 2016 - 05:06 PM, said:

I don't think my 'workshop' lives up to the standards people are assuming :)
Very drafty cold shed, no point ever putting a heater or a kettle in there. If I can run a double socket and a light then that's fine.
For the past couple of years it's been running off an extension reel laid across the garden. The SWA is both a higher rated cable and takes a considerably shorter route.
So it's an improvement but is it good enough. Very common to hear people say that it is OK because it is just a shed. The standards are there no matter what is the installation, other than in some areas extra regs apply for good reason. The electrons don't know where they are and don't care either. Amazing that most people won't touch gas but quite happy to play with electrics.
That is why I am reticent to advise on the subject because a little knowledge can be a very dangerous thing, plus I am crap at explaining things. :blink:

#19 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 04 February 2016 - 05:39 PM

View PostMarkblox, on 04 February 2016 - 05:36 PM, said:

That is why I am reticent to advise on the subject because a little knowledge can be a very dangerous thing, plus I am crap at explaining things. :blink:

I don't mind explaining things, but then I used to teach electrical engineering science to South West Electricity Board (as it was then) apprentices. It was a bit of a thankless task, at times................................. :mellow:

#20 Crofter

Crofter

    Regular Member

  • Member Blogger
  • PipPipPip
  • 592 posts

Posted 04 February 2016 - 06:07 PM

View PostMarkblox, on 04 February 2016 - 05:36 PM, said:

So it's an improvement but is it good enough. Very common to hear people say that it is OK because it is just a shed. The standards are there no matter what is the installation, other than in some areas extra regs apply for good reason. The electrons don't know where they are and don't care either. Amazing that most people won't touch gas but quite happy to play with electrics.
That is why I am reticent to advise on the subject because a little knowledge can be a very dangerous thing, plus I am crap at explaining things. :blink:

I don't want to eschew good advice, so what's the worst case scenario here? I assume that the RCD will trip and I'll have to go and reset it?
Or maybe I will get a few years down the line and wish I'd stuck a bigger cable in because I've bought some bigger toys and/or want to run them simultaneously?