Running cable through a switch back box

[charlie76]

I would like to run a cable through a galv back box (at a light switch) which is not terminating in that back box. Is there a reg which forbids this?

Reason being the switch back box is already tubed from the switch to the loft. I am chasing from the floor up to the switch to get a 3c+e from ground floor to the loft for a smoke detector interlink. Trying to keep damage down to a minimum and therefore chasing up to the switch allows me to chase in a permitted zone and then from the switch up the tube to the loft. Not pretty I know but practical.

 

[telectrix]

can't see any problems. esp. if the new 3 core is on the same circuit as the light, i.e. on same MCB?

 

[Wilko]

Hi - cables are permitted to run in a bundle, laced, in trays, in ducting and conduits. So this sounds fine. It all comes unstuck if the cable is damaged or the 2 ccts mix, so I would like to see the box with grommets and earthed to the original cct if I was inspecting.

 

[davesparks]

I was taught that this is very bad practice if the cable passing through is on a different circuit.
I was also taught to avoid it if at all possible even if the cable passing through is on the same circuit, but that was more for surface mounted boxes which may get damaged and need to be replaced

 

[Wilko]

Me too and I wondered why. So, hot from my 17th ed reg study I got out the Yellow Book. I found 521.6 where specific approval given (I think) as long as 528 (insulation all ok) and if terminated then 521.8.3 (jb to BS60670-22). So that's why I thought it should be ok, unlike when I was teached in the '70s (yikes!!!). Cheers, David.

 

[charlie76]

I was taught that this is very bad practice if the cable passing through is on a different circuit.
I was also taught to avoid it if at all possible even if the cable passing through is on the same circuit, but that was more for surface mounted boxes which may get damaged and need to be replaced

Yes it is on a different circuit. The box being run through is the upstairs lighting circuit, the cable to run through it is the smoke alarm interlink powered from the ground floor lighting circuit.

I'm not a big fan of it to be honest as I think it could be confusing for someone taking the switch off. I suppose it could be labelled as it passes through the box to identify it.

 

[Andy78]

Have you considered the option of wireless interlinks ?

 

[charlie76]

Have you considered the option of wireless interlinks ?

I will do next time

 

[Andy78]

Is there an option to run the cable that passes through behind the box instead of through it ? Standing the box off with spacers may allow cable space yet keep the cable within the prescribed zone.

 

[charlie76]

Is there an option to run the cable that passes through behind the box instead of through it ? Standing the box off with spacers may allow cable space yet keep the cable within the prescribed zone.

Not really got much depth. Single brick and i'm installing 35mm boxes and the walls are cr@p, bricks are crumbling as it is.

 

[Peter Monfort]

I tend not to do this. The cable takes up space in the back box and can be awkward when fitting your face plate. There is also a risk that t when screwing in the face plate the small threaded screws can pierce the cable. It would also not likely be able to move if the screw did make contact. If you are chiselling out why not run the cable up neatly along the side of the back box. Some might argue that the cable is not in the prescribed zone but it is unlikely anyone would drill right next to a fixture. The cable can be dressed straight back into place so it runs vertically above the light switch.

 

[Peter Monfort]

I did I digital search of the GN1 and can't find anything that says it isn't permitted either. But that doesn't make it sensible.

 

[Risteard]

Some might argue that the cable is not in the prescribed zone but it is unlikely anyone would drill right next to a fixture.

Just because it's unlikely doesn't make it permissible.

 

[HandySparks]

Just because it's unlikely doesn't make it permissible.

I managed to chop right through a piece of 2.5mm² T&E run horizontally, chased into the wall just below a socket. I was intending to run a spur off the ring in a downwards direction from the socket. The only cables visible in the backbox were the two connected to the socket, one from each side.

It was a right faff to fix. Luckily, due to the location of the socket, the owner didn't mind me sinking another box just below it to join the two ends of cable with a blanking plate to cover.

 

[charlie76]

I've run it through the box and kept it central at the back so the screws cant hit it.