Cramped surface mount "back box"

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I want to feed a 45A dp switch with two (feed and load) 6mm2 T&E cables fed from behind through a 25mmx25mm hole in a single skinned brick wall, using a plastic surface mount box.

I've tried to use a 25mm back box, but it feels very tight when I try to close it, and makes some complaining noises as I do so. I've not fully closed it yet.

Is it futile/stupid to stick with a 25mm box? I was wondering if I removed more of the outer insulation to the live, neutral and earth on the cables so these sat in the hole in the wall, was bad practice?

Any dos and don'ts with 6mm2 cable much appreciated.
 
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Is it futile/stupid to stick with a 25mm box?

yes

the bigger ones are sold near the "cooker connection" units, which are the common thing needing them.

If your wall is grey thermal concrete blocks, you may be able to scrape it out with your bolster. If it is a single-leaf wall you may otherwise go through it. If so, I like to patch from the other side with sand and cement mortar to repair the wall, and pack round the box on both sides, with reasonable strength, as you will not be able to rely on fixing screws. Some people might just use filler or plaster.

just recess it slightly below the finished surface so you can tidy with plaster for your decorated surface. 6mm will do.
 
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