Back boxes for Dob & Dab?

[DonutHole]

Hi,
I've built a new kitchen extension, which will have a new ring main. The internal walls are Thermolite (Aerated) blocks, which I would like to dob and dab plaster, thus hiding the cables behind the plasterboard, as the floor is solid screed. The new Kitchen will have a free standing gas stove but with electric clock and oven light, and electric extractor hood, dishwasher and washing machine.
What type of back boxes, metal or plastic and depth would you recommend, and how deep should I be chasing them into the Thermalite block, or how proud of the Thermolite should I leave them? I think I need 2.5mm twin & earth cable, Screwfix reviews say that Prysmian is very good, but if you could confirm that as well please.
Thanks guys,
Andy.

 

[rjm2k]

If you are doing a lot and have an sds it may be worth getting a box sinker , otherwise its a matter of drilling and chiseling out then filling back in a bit to get a level box. Flush metal boxes.

 

[Taylortwocities]

You absolutely will need 2.5 T&E. What else??
Please note that as this is a new circuit it will need notifying to your local authority and signing off. You'll need a registered electrician if you dont have the experience, competence and the test equipment. Involve the electrician before you start. They just can't/won't come in and sign off work you have done yourself.

What depth boxes will you use. I would recommend 32mm, not 25mm as the shallower ones are a squeeze with cables and do not work at all with flat plate sockets etc.

The depth will depend on how thich your plasterboard is plus a bit for the D&D so leave about 20mm proud of the blockwork, so then you'll have 12.5mm plasterboard, 4mm for the D&D and a plaster skim on top.

 

[PrenticeBoyofDerry]

What type of back boxes, metal or plastic and depth would you recommend, and how deep should I be chasing them into the Thermalite block, or how proud of the Thermolite should I leave them?

The depth would depend on the face plates being installed, some plates only require 25mm depth other could need 47mm. The boxes are best left level or slightly recessed to the finished wall.

I think I need 2.5mm twin & earth cable, Screwfix reviews say that Prysmian is very good, but if you could confirm that as well please.

2.5mm2 T&E would be the minimum size you require but there could well be other de-rating factors to consider that would increase the CSA of the cable. Nothing wrong with Prysmian branded cabe.

You need to do some research into circuit design and construction and the legal requirements associated with this task.

 

[DonutHole]

Thanks guys for all the great advice
I asked about the 2.5mm2 cable because I found a single 8m length of 4mm2 cable coming out of my consumer unit before it reduced down to 2.5mm2 that supplied my garage ring circuit from the house. Not dangerous of course, but enough for me to ask for clarification. I've already taken out some sockets in brick and concrete block and after SDS drilling found an SDS 75mm tile lifting chisel worked really well at chasing out sockets, easier than a hammer and bolster.

The extension is a 'Work by Notice' with building control, so I think I should email the building control officer to see what he says about what I can and can't do myself. Point taken 'Taylortwocities' that I can't expect an electrician to approve my work, and yes in my little experience I have found the 35mm deep boxes easier than the 25mm ones. As for face plates I was planning to use standard white plastic MK switches and sockets.

I wasn't sure how thick the plasterboard adhesive would be but I just measured some 2.5mm2 cable with my vernier calipers, and it measured 5.5mm's thick so probably about 7mm for the adhesive, with the 12mm plasterboard 'Taylortwocities' advise to leave the back boxes 20mm proud of the block sounds about right. Also would you normally protect the cable with conduit or capping, or is that not necessary in this situation? So with my 35mm boxes that would mean a recess/chase out of 15mm into the Thermalite, presumably with a small bit of mortar to bed them in.

Also I've heard that now all circuits should be RCD protected, but only my sockets and shower are currently protected, while the water heater and lights are not, so that also needs to be rectified.

Obviously trying to keep to a tight budget, spent £17,000 so far and still going :v Thanks again everyone for all the useful feedback, Andy.

 

[Taylortwocities]

For your work notice, the LABC will expect the work to be done and notified by a registered electrician, unless you told them you would be doing it yourself. You need to take their advice before you paint yourself into a corner.

They would charge extra for that privilege and (per the above) expect you to install everything in accordance with BS7671 and produce an installation certificate. You'll need special calibrated equipment to provide the test results and convince them of your competence.

BS7671 now dictates RCD protection on ALL circuits that are concealed and less than 50mm from the surface, so that is all of the new wiring. Sockets, lighting, cooker, everything.

 

[mfarrow]

Also I've heard that now all circuits should be RCD protected, but only my sockets and shower are currently protected, while the water heater and lights are not, so that also needs to be rectified.

The rule is 30mA RCD protection unless mechanically protected or covered with earthed sheath/screen. Also you need one if the wall's metallic construction (i.e. metal studs).

So your options are RCD protection, earthed sheath/screen (SWA, ali-tube, metal conduit, pyro) or mechanical protection over the cables (3mm steel plate is good for that).

I've used 50x3mm steel plate in the past and it's fairly cheap compared to RCBOs. If you have other lights/items on the same circuit designed to old standards (i.e. no cover/RCBO) however then you may consider the RCBO an 'investment' that makes the installation safer. That's assuming your board is new/big enough to take an RCBO currently. It's generally mandatory if you have sockets of course.

 

[DonutHole]

Also I've heard that now all circuits should be RCD protected, but only my sockets and shower are currently protected, while the water heater and lights are not, so that also needs to be rectified.

The rule is 30mA RCD protection unless mechanically protected or covered with earthed sheath/screen. Also you need one if the wall's metallic construction (i.e. metal studs).

So your options are RCD protection, earthed sheath/screen (SWA, ali-tube, metal conduit, pyro) or mechanical protection over the cables (3mm steel plate is good for that).

I've used 50x3mm steel plate in the past and it's fairly cheap compared to RCBOs. If you have other lights/items on the same circuit designed to old standards (i.e. no cover/RCBO) however then you may consider the RCBO an 'investment' that makes the installation safer. That's assuming your board is new/big enough to take an RCBO currently. It's generally mandatory if you have sockets of course.

Thanks for the advice mfarrow,

It does sound like upgrading my consumer unit is probably a better solution than armored cabling. The shower and sockets are already 30mA RCD protected, can't things just be reshuffled inside the existing consumer unit, so that everything is RCD protected? Probably take out the 2 Bus Bars and replace it with a single Bus Bar? Would I need RCBO's if all the existing MCB's are rearranged and run through the existing RCD? It does seem daft that every circuit wasn't run through the RCD when the consumer unit was installed 5 to 10 years ago.

Consumer Unit

• DonutHole
• 2 Mar 2014
• 2

MK, Split load, Single RCD.

 

[Taylortwocities]

You should not do that because the whole point of a consumer unit is to provide differentiation between circuits. This means that if there is a fault on one circuit then the rest still work.
If all circuits are controlled by one single RCD then an earth fault on ANY circuit completely kills your entire house. Not a good plan, I hope that you agree.?
To get around this, the consumer units "17th edition" have multiple RCDs. Or consumer units can have one RCBO per circuit.

You comply with BS7671(2008) if all of the circuits are RCD protected and cables are run in safe zones (see the WIKI).

 

[xr4x4]

Also I've heard that now all circuits should be RCD protected, but only my sockets and shower are currently protected, while the water heater and lights are not, so that also needs to be rectified.

The rule is 30mA RCD protection unless mechanically protected or covered with earthed sheath/screen. Also you need one if the wall's metallic construction (i.e. metal studs).

So your options are RCD protection, earthed sheath/screen (SWA, ali-tube, metal conduit, pyro) or mechanical protection over the cables (3mm steel plate is good for that).

I've used 50x3mm steel plate in the past and it's fairly cheap compared to RCBOs. If you have other lights/items on the same circuit designed to old standards (i.e. no cover/RCBO) however then you may consider the RCBO an 'investment' that makes the installation safer. That's assuming your board is new/big enough to take an RCBO currently. It's generally mandatory if you have sockets of course.

Thanks for the advice mfarrow,

It does sound like upgrading my consumer unit is probably a better solution than armored cabling. The shower and sockets are already 30mA RCD protected, can't things just be reshuffled inside the existing consumer unit, so that everything is RCD protected? Probably take out the 2 Bus Bars and replace it with a single Bus Bar? Would I need RCBO's if all the existing MCB's are rearranged and run through the existing RCD? It does seem daft that every circuit wasn't run through the RCD when the consumer unit was installed 5 to 10 years ago.

Consumer Unit

• DonutHole
• 2 Mar 2014
• 2

MK, Split load, Single RCD.

You could install a second rcd in that board to rcd protect the left hand side.

That would comply with the 17th ed.

 

[mfarrow]

You could install a second rcd in that board to rcd protect the left hand side.

That's assuming you can procure and install a second neutral bar.

I would put the lights on RCBOs and then evaluate the water heater. Is it just an immersion? If so is the cable route buried in plaster?

12ft length of 3mm steel capping £12, each RCBO £20+.

 

[JohnW2]

It does sound like upgrading my consumer unit is probably a better solution than armored cabling. The shower and sockets are already 30mA RCD protected, can't things just be reshuffled inside the existing consumer unit, so that everything is RCD protected? Probably take out the 2 Bus Bars and replace it with a single Bus Bar? Would I need RCBO's if all the existing MCB's are rearranged and run through the existing RCD? It does seem daft that every circuit wasn't run through the RCD when the consumer unit was installed 5 to 10 years ago.

As has been explaind, having all circuits protected by a single RCD would not be a very good idea (and most would say is non-compliant with current regs.).

However, I'm getting a little confused by the discussion here, and wonder whether it is not perhaps ranging a little more widely than may be strictly necessary for you.

Although it is obviously desirable that your (or anyone's) entire electrical installation should be brought into compliance with current regulations, there is no requirement for that, and BC's only interest in the electrics should relate to the electrics in, and/or associated with, your new extension. From what you say, that probably amounts to just the new sockets ring circuit and an extension to an existing lighting circuit.

Your CU appears to have two unlabelled RCD-protected ways - are they 'spare'? If so, one of those could be used to provide an RCD-protected supply for the new ring (which, being a sockets circuit, now must have RCD protection, regardless of how the cable is routed and protected). As for the extension's lighting, any requirements for RCD protection (which would depend upon cable routing etc.) could, as has been said, be satisfied by changing the MCB protecting that (currently not RCD-protected) circuit to an RCBO.

Am I missing something?

Kind Regards, John

 

[OwainDIYer]

you might as well put the cable in for an electric cooker point now; you or someone in the future might want an electric cooker. The extra cost is marginal - if you're tight on MCB ways just leave the cable spare near the CU for now.

 

[DonutHole]

You could install a second rcd in that board to rcd protect the left hand side.

That's assuming you can procure and install a second neutral bar.

I would put the lights on RCBOs and then evaluate the water heater. Is it just an immersion? If so is the cable route buried in plaster?

12ft length of 3mm steel capping £12, each RCBO £20+.

Thanks mfarrow,
Yes it is just an immersion heater, no the cable is surface mounted not buried, and I hope to get gas central heating once my new kitchen extension is finished, which should do away with the need for an electric immersion heater.

 

[mfarrow]

Do that then, immersions don't need an RCD solely owing to the point of use, usually only circuits in bathrooms and sockets.

John is right to point out you don't need to do anything for now, just concentrate on the alterations you're making, which is why I recommended mechanical protection of lighting drops if you're on a budget and/or don't want to mess around inside the CU. Do that and board either side (5mm cable +2mm clip +3mm steel = 10mm, take 19 for dabs and board = 9, easily bonded then skimmed).