Creating Neutral in 2-Gang Switch from Conventional T&E Wiring

[Pal Dhupar]

Hello all,

New to this site but have been doing DIY electrics for about a year now.

I'm looking to upgrade the lighting in my living room to incorporate a smart, 2-gang dimmer switch, which requires Neutral. I know this is a problem that many people have posted and discussed online before and the only way to get Neutral to a switch is to have run a 3-core or a spare single core alongside my T&E to the switch in the first place.

However, here is my scenario and potentially stupid/genius/obvious proposal...

I have a ceiling light and wall light, controlled independently from each other by a 2-gang switch. I have used the conventional loop-in method for my lighting circuit so Line and Neutral goes to the ceiling rose and the wall light and from each of these fittings, Line and Switched Line goes to the 2-gang switch. So, in my switch, I have 4 cores (let's ignore Earth for simplicity) - Line x2 and Switched Line x2.



My question is this...

Is it ever ok to share ONE of the Line cores in the switch between both gangs? Thus, rendering the other Line core redundant and available to use as Neutral, by terminating as Neutral at the light fitting?



If this is ok/allowed (not that anybody would need/want to do that in reality), surely it opens up the possibility for me to do this in a smart switch, which has 4 terminals (L, N, L1 and L2)...



The risks I've considered are as follows:

• MCBO shouldn't kick off as the 'sharing' of the line is on the same circuit (even in the same room) so the current on the Line core into the room should be the same as the current leaving the room on the Neutral core, irrespective of the slight deviations it's taken within there. right? Does the length of the cores have a significant bearing on this, as far as a 30mA MCBO is concerned?
• Would either of the Line or Neutral cores become potentially overloaded in this setup? The smart switch itself is 1W and the LED bulbs on the ceiling light fitting would be around 30W and wall lights around 36W.

Is there anything I've completely misunderstood? Any help and advice would be greatly appreciated.

Thanks

 

[telectrix]

so presumably you'd fit a link in the switches (common-common) to reinstate the perm L to the switch from which you have pinched the L . at first glance, i can't see a problem as long as cores are sleeved according to their porpoise.

 

[Pal Dhupar]

Thanks for responding Telectrix.

I wouldn't actually be doing that, but yes:

1. If I were to do that, I'd ensure the cores are sleeved/identifiable as what they are in any given instance;
2. The link between COMs (pinching L so that N could be brought to switch through the other cable) was a hypothetical question. My concerns were around overloading any given piece of wire. However, since that hypothetical scenario sounds acceptable, I guess my real-world solution is ok too?

These are the terminals on the back of the smart switch that I like:

 

[tlangdon12]

The RCBO won't be affected by the change at all, unless you fit additional smart switches and they leak earth current. (You have not shows the CPCs in your diagrams, which was probably the best for clarity, but I assume that the smart switches will also be connected to the CPC). If you have a lot of smart switches on one lighting circuit and they all leak a couple of mA to earth, you might have the RCBO trip.

The length of the conductor doesn't affect the operation of the RCBO, but you should check that the new route doesn't result in a voltage drop that is above that allowed in the regs. (3%). I doubt it will be a problem because the new route to the final ceiling rose won't be very different to the old route (in terms of length).

 

[Silly Sausage]

so presumably you'd fit a link in the switches (common-common) to reinstate the perm L to the switch from which you have pinched the L . at first glance, i can't see a problem as long as cores are sleeved according to their porpoise.

That sounds fishy to me!

 

[Risteard]

Why on earth were two permanent phase conductors run to the switch in the first place, presuming that both luminaires are on the one circuit?

 

[Pal Dhupar]

The RCBO won't be affected by the change at all, unless you fit additional smart switches and they leak earth current. (You have not shows the CPCs in your diagrams, which was probably the best for clarity, but I assume that the smart switches will also be connected to the CPC). If you have a lot of smart switches on one lighting circuit and they all leak a couple of mA to earth, you might have the RCBO trip.

This is the concern I have with Smart switches that don't require Neutral - surely, they're either (a) never truly OFF; or (B) using Earth to trickle away what should be going back to Neutral. Neither sounds great.

 

[Pal Dhupar]

Why on earth were two permanent phase conductors run to the switch in the first place, presuming that both luminaires are on the one circuit?

Why not?

 

[Pal Dhupar]

Thanks all for your responses. Just spoken to my electrician who has confirmed most of what you've all replied with above, except Archy's reply to Telectrix about linking the COM terminals:

That sounds fishy to me!

FYI Archy, what Telectrix said (and what Risteard was more colourfully getting at) was correct - you only need 1 Line conductor in a switch, regardless of the number of gangs.

The length of the conductor doesn't affect the operation of the RCBO, but you should check that the new route doesn't result in a voltage drop that is above that allowed in the regs. (3%). I doubt it will be a problem because the new route to the final ceiling rose won't be very different to the old route (in terms of length).

You're right - it won't be a problem.

Why on earth were two permanent phase conductors run to the switch in the first place, presuming that both luminaires are on the one circuit?

There's nothing wrong with having done this - it was just never necessary. However, having done it, it's turned out pretty lucky that I have a spare conductor in the back box that I can now use as Neutral

 

[Risteard]

Why not?

Because it's silly and wasteful.

I can understand deliberately running a spare core, but it smacks of a lack of understanding for two conductors serving the same function to be run like that. I would have run a twin brown and a single brown. That's all that's needed.

 

[Pal Dhupar]

Because it's silly and wasteful.

I can understand deliberately running a spare core, but it smacks of a lack of understanding for two conductors serving the same function to be run like that. I would have run a twin brown and a single brown. That's all that's needed.

I don't mean to patronise but you do realise this is the DIY section, right? I think there's a separate section for you to discuss your ideas for best practice with fellow professional electricians.

What I've done isn't technically wrong and your nitpicking isn't of any value - your first post didn't address my original question and neither has your last post. You're just making pointless observations after the fact, instead of offering solutions.

Since you raised it anyway (and again, this is a DIY area), you should be saying that people who have limited-scope wiring projects to do (i.e. those who are not trade electricians) should just buy a reel of 1.5mm blue-brown T&E and a length of brown sleeving for Switched Line wires. That would be much more sensible and resourceful than doing what you'd do, for the vast majority of DIY electrical projects.

 

[Risteard]

Firstly I didn't realise that this was in the DIY section. Secondly, as I had no reason to believe that you had wired the switch I presumed that it was done by whoever wired the installation.

I'm sorry if you found my observations unhelpful, but they were merely my observations based on the facts presented.

Peace & Love etc.

Your humble servant,
Risteard

 

[Octopus]

I don't mean to patronise but you do realise this is the DIY section, right? I think there's a separate section for you to discuss your ideas for best practice with fellow professional electricians.

Are you a professional electrician?

 

[Pal Dhupar]

Firstly I didn't realise that this was in the DIY section. Secondly, as I had no reason to believe that you had wired the switch I presumed that it was done by whoever wired the installation.

I'm sorry if you found my observations unhelpful, but they were merely my observations based on the facts presented.

I appreciate that. I made my original post as clear as I could but I apologise if it was lacking in this regard. I thought my specific area of uncertainty was well defined.

Are you a professional electrician?

No, I'm not. That's why I came here asking for advice.

Anyway, my enquiry has been answered now so that's the end of that and I'm grateful to everyone for their replies.

 

[rolyberkin]

Another satisfied customer!