Replacing a very old light switch - How?

[TenaciousJohnny]

Hi,

I want to replace a light switch with a dimmer. I was hoping that I would just remove the old switch, wire up the new one & attach, however its not turned out as simple as that.
The existing switch is attached to a very substantial (cast iron?) backbox which in turn has a (cast iron?) pipes running from it at 12 o'clock & 6 o'clock. These contain the wires & presumably connect the back box to the lighting ring. The existing back box is slightly too small (it's deep enough but the area is smaller) for standard dimmer switches, so I thought I'd remove the box & put in a standard size one. This isn't working though, as the backbox is firmly attached to the metal pipes meaning I can't remove it without removing the metal pipes, which I presume run up & down the entire wall. I've just plastered & don't want to do that.
Has anyone any suggestions?
I thought I could add a new standard back box next to the old one, drill through the side of the existing box, & feed the cables through.
Alternatively can you get a dimmer switch that fits a smaller back box?

TIA

Johnny

[andy1249]

Measure the back box , then google that size and see if its still commonly available , if it is , then there will be a dimmer switch available for that size back , which should be a lot easier than reworking for the switch you currently have.

[M2GX]

A, When was the last time the place was rewired, as it sounds like old wiring unless the spark rewired the house through the existing conduit using single cables which is unusual. So what i am saying is how good is your wiring.

B, Not ideal but if you don't mind a surface box you could by a standard box cut a 20m hole in the middle to allow the wires to come through and drill 2 holes that would line up with the existing holes and gently tighten up the box to the original if you get what i mean. Or a twin box and screw it direct to the wall a bit of a bodge job but would work.

C, Search for a small dimmer however i doubt the screws on the plate will line up with the existing ones.

D, I am sure MK used to make an adapter plate/surface box for this but doubt if they still due.

E, Hit the box with a good cold chisel to crack it, hit it just in front of each pipe and in the center, you should get it out with out making to much mess although not sure how this would affect the earthing as not sure if separate earth wires are also running through the box.

Knowing the dimensions of the box would be helpful and did you say it was square or round. Also someone else who is still in the electrical game may be able to offer better advice.

[TenaciousJohnny]

Thanks for the advice guys. Couldn't find any back boxes with "non-standard" dimensions on google.
I have ended up installing a new standard 25 mm backbox next to the old one & drilling a hole thru the cast iron to run the cable thru.