Back box with central oval knockout

[Grumps]

Hi, You know those metal back boxes with oval knockouts, like

you still need to grommet the top hole as it says it has been deburred?

[Tricky Dicky]

I doubt you would get a grommet on the top entry as the edge has been piened over so providing a smooth deburred edge. So the answer is - no!

Richard

[John Rumm]

You should still use a grommet - a normal 20mm round one will fit.

[John Rumm]

A normal 20mm round grommet will still fit. About the only time I would leave one out is if running oval trunking straight into the top...

[alan_m]

Not answering the question but if you can fit a deeper backbox you may have future options for dimmer or smart switches which may/will not fit in a 16mm backbox.

[Grumps]

The hole in the wall will take a 25mm box. I haven't found a 25mm with an oval hole. But then, I'm crap at Google.

[John Rumm]

Is there any reason you need an oval hole? (you only normally see them on slim boxes because there is not enough depth for a 20mm one, but they need to get the total area up to something similar)

[Animal]

Mount your 16mm backbox recessed by 9mm and you've effectively got a 25mm box. OK if the wall material is nonflammable.

[Grumps]

I'm replacing a back box with one that has an earth connection. The existing box has been removed (and destroyed in the process), so please don't say I could've added an earth to that box. The current wiring is central to the existing box, and without doing any more chiselling of the brickwork, I can't get the existing wires to go to standard round knockout positions.

So, I don't necessarily need an oval holed one, but I haven't found any with a centrally located round hole.

[John Rumm]

If you can go deeper, then many of the 47mm deep boxes have a centre hole (probably 25mm diameter)

[Grumps]

That's a thought, thanks.

I'll need to bash out another 20mm into the wall. The wall is thick enough, but I don't want to redecorate afterwards.

[alan_m]

The 16mm one you list hasn't got a centrally located hole for wires. Depending on which backbox you buy you may find one with a 25mm round knockout in the same location as the oval knockout in th one you list.

[Fredxx]

Do you have access to a Dremel and a carbide bit? Make your own hole by enlarging the existing mounting hole?

[Grumps]

Oh, OK. What is the centrally located oval hole for?

[alan_m]

Are you referring to the very small oval holes for the fixing screws? You will not find a back-box grommet to fit those holes.

The holes for the wires are those knock-out positions where you remove the metal and insert a grommet to protect the wire being cut by the metal.

What you could possible do is drill out the centre hole to 20mm with a step drill to enable a standard grommet to be fitted. Knockouts come in

20mm and 25mm with grommets readily available for both sizes.

BTW when fitting a back box it is recommended that the terminal for the earth wire is at the bottom. Possible not too important for a light back-box but with a socket back-box nearer the floor it can be a complete PITA to install the earth wire as you cannot see the terminal unless you are physically low down, especially with deep back-boxes.

[Grumps]

These holes:

Recommended to have earth terminal at bottom? Is that a regs thing or just accessibility?

[John Rumm]

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Oh yes it does (look at the angled photo)

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[John Rumm]

I was not aware of any recommendation as to its location.

Note that an earth terminal on a metal back box is not actually required unless both screw lugs for the accessory are adjustable. Typically one is fixed, and thus you can rely on the screw connecting to the accessory to connect the box to earth.

[ARW]

That's not going to work if the OP has a plastic light switch.