Back Boxes

[Mascard]

Why do all 25mm back boxes come with round knock outs ?

I want my sockets flush so I will need to chop out the wall for the back box but i don't want to chop out the wall for 20mm round conduit so i'm going to use oval conduit so that I can bury it in the plaster.

wouldn't it make sense to make 25 mm back boxes with oval knock outs ?

 

[RobFurn]

Why bother with oval conduit. Just chase the wall for the cable and place metal or plastic capping over the top. Then plaster over the capping.

If you decide to use conduit you dont need it to go inside the metal box just flush to it. Remember your only protecting the cable form the plasterers trowel.
BTW make sure your cables are in safe zones. Oval conduit does not mechanically protect your cable.

 

[FredFlintstone]

I don't think I've ever seen a flush mounting box with anything other than round knockouts and I ain't ever seen different shaped grommets either.

Personally, from experience though, I'd never use 25mm back boxes again.

I added a number of sockets to my new (old) house and used 25mm back boxes because I figured I'd save time knocking out the recess.

Did I save time? No doubt I did - I hate the process of knocking out the recess, especially in my house where many of the bricks are rock solid. However, when it comes to wiring, the restricted space in a 25mm back box makes the task of wiring much more difficult and I now only ever use 35mm boxes. The extra time spent knocking out an extra 10mm brickwork is saved when it comes to wiring. In addition, the chances of trapping a cable and splitting the insulation is greatly reduced.

I've used oval conduit too for burying cables (I've also used round and that rectangular stuff too) and providing you butt the conduit right up to the round knock out there isn't really a problem. Just try not to push the plaster into the small gap that's left.

Regards

Fred.

Regards

Fred.

 

[JohnD]

It is possible to poke oval conduit through the round holes. You can use grommets as well but I can't see that they're neccessary.

It is easiest to do this before fixing, and you will have to put a bit of a bend on the conduit to reach the hole.

 

[Chri5]

25mm boxes are a PITA when you use any sockets from a metal clad 'flush' fitting range.

Due to the sockets being flush the amount of 'innards' (physical switch and terminating points) are deeper, thus causing really issues with wire placement and space in the back box.

25mm boxes are just daft, future proofing with deeper boxes is better.

 

[breezer]

I don't think I've ever seen a flush mounting box with anything other than round knockouts and I ain't ever seen different shaped grommets either.

you havent lived, i dont know if still available but years ago you could get a metal back box with oval knockout.

and as for oval grommets, you use round ones the oval hole is 20mm "squashed a bit"