Regulation for rewring back box?

Chris Pearson: 
 

This is getting very complicated for a simple question!

As BOD said at the start, the pattress box shown is a surface mounted one. Alternatives are galvanised steel ones, which are set into the wall, and plastic ones for stud walls, which sit just proud of the plasterboard, which not only holds them in place, but also disappears if the wall is skimmed.

The spacing for the screws (M3.5) is the same for all of them.

So, does the O.P. want to use surface pattresses or metal sunk boxes?

Z.

Thank you all for these amazing insights. After some digestion and further inspection of the current back box. Here is my update and I think is on the brighter side.

The back box is type “dry lining” mounted in a stud wall. After removing the back box, there is about 55 mm before interfering with the back box on the other side of the wall. So I think all I need to do is buy and fit a "1 gang, dry lining, 45mm back box" 

In term of the current wiring, there is 2 wire in the common, 1 wire in the other side. Seems like "loop at switch" to me? And all earth and neutral are connected. 

To wire the new switch, I think I should:
- connect the two common wires to the common port
- connect the other brown to the L1 port (light switch output)
- connect all the neutral wires to the neutral port
- leave earth untouched

Furthermore, I'm thinking to use "wago" to connect the earth wires. It seems to take less space than terminal blocks. Is there any disadvantage to this approach?

PS: upon inspection, I noticed insulation of the earth wire were cut or maybe just sleeved in? Want to check if this is standard practice? 

Cutting off earths is not good practice, but sadly far from unknown at the shallow end of the profession…

The small wagos are very good if you get the right ones. the lever ones are best for folk who may change their minds.

Personally I like the wago 221s,  and have a couple that I keep for testing as well.

I think  a 45mm box is the deepest you will commonly find..

Mike.

benforone: 
 

Hi, I'm currently holding an MEng in EEE, but not a certified electrician otherwise. 

I want to install smart light switches in my “dwelling”. I have encountered issues where the current back box is standard 74x74x25, but I would need extend it to 86x86x38 for the new light switch. 

I want check do I need part P certificate, or planning permission? How to be compliant while minimise the cost? (I have no experience in dwelling electrical work)

PS: The dwelling is a new build (2019) 
PPS: I might consider obtain required certificate(s) myself if is not too expensive. Since I like do these kind of work, so it might be cost effective in the long term. 

A couple of three day courses and you could be doing landlords EICRs.

I had better go to bed ???

Does 2391 now take 3 days Andy, It used to take 12 Weeks? A person could not become “experienced and competent” with 6 days of training!

davezawadi (David Stone): 
 

Does 2391 now take 3 days Andy, It used to take 12 Weeks? A person could not become “experienced and competent” with 6 days of training!

2391?  It should take no more than a day to teach somebody what numbers to write down on the EICR for it to be a “pass”.

I underestimated it six days for the testing course and three for the wiring regulations, so a couple of weeks.

benforone: 
Thank you all for these amazing insights. After some digestion and further inspection of the current back box. Here is my update and I think is on the brighter side.

The back box is type “dry lining” mounted in a stud wall. After removing the back box, there is about 55 mm before interfering with the back box on the other side of the wall. So I think all I need to do is buy and fit a "1 gang, dry lining, 45mm back box" 

In term of the current wiring, there is 2 wire in the common, 1 wire in the other side. Seems like "loop at switch" to me? And all earth and neutral are connected. 

To wire the new switch, I think I should:
- connect the two common wires to the common port
- connect the other brown to the L1 port (light switch output)
- connect all the neutral wires to the neutral port
- leave earth untouched

Furthermore, I'm thinking to use "wago" to connect the earth wires. It seems to take less space than terminal blocks. Is there any disadvantage to this approach?

PS: upon inspection, I noticed insulation of the earth wire were cut or maybe just sleeved in? Want to check if this is standard practice? 

The workmanship is a little rough, but everything looks OK. I think that you have the right plan of action.

PS: upon inspection, I noticed insulation of the earth wire were cut or maybe just sleeved in? Want to check if this is standard practice? 

Yes, that's normal. Flat “T&E” cables have a bare earth wire - which then needs to be sleeved with green/yellow sleeving where it is exposed.

   - Andy.

I'm still researching this topic. Find it quite insightful in general.

I'm thinking to buy “IET Wiring Regulations: Electric Wiring for Domestic Installers” by Scaddan Brian. (Which I assume would be sufficient for what I'm doing?)

I noticed some light switches are 2-way or 3-way. I'm planning to use one wired switch, with the rest replaced by some battery power switches. If the wiring is like the image below (although I have not checked), would it be legal for me to terminate the live and neutral wire at the topmost box? Such the live and neutral wires to the latter two junction boxes will be abandoned. I assume it would be reasonable to leave the earth connected to other back boxes? 

I assume I need some labelling to indicate the abandoned neutral and live wires, are there any conversions? I will put a blank plate on the latter two back boxes with abandoned wires, and just glue the battery-power switch on top, so you can't tell from a distance.

Based on 3.13 of the Part P, I need somehow get the “non-notifiable work” inspected and tested. What is the most economic way of achieving it? (I will of course keep a record of all the modifications I made so it can be referenced later)