13mm back box: which grommet?

[Stephen]

Hello again,

Sorry, I should have appended this to my other post about back boxes. In addition to the 35mm boxes I was asking about, I have also bought some of the ultra slim 13mm boxes. What size grommet fits the cable entry on these? I have tried squashing a 20mm grommet but it seems to be too big. I've had a search on line but the smallest I can find is

20mm? Do I need to try harder?

Thanks.

[Dave Plowman (News)]

They usually come with 'oval' grommets. If you need the odd extra one cut down a 20mm open one.

[js.b1]

Consider 25mm backboxes for light switches. Makes wiring simpler, yes it means a bit of chiselling but worth it.

I always use 25mm for switch, 35mm if any chance of a dimmer.

[1501]

Squashing or searching?

[Stephen]

Both ;)

To answer the other reply, I do try to use 35mm for everything where possible, following the advice on this group.

Thanks again for your replies.

[Andrew Gabriel]

or they come with the hole edge rounded into a lip, so gromet not required (and can't be fitted).

[Stephen]

I think this is what I have here: one hole is already punched out and it does have a slight lip. On some boxes the lip is more obvious than on others.

Thanks.

On a different topic, I notice some of the 35mm boxes have both 20 and

25mm knock outs. It's a bit of a pain if the 25mm one is where you want to run a 20mm!