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Regulation for rewring back box?

Former Community Member
Former Community Member

Hi, I'm currently holding an MEng in EEE, but not a certified electrician otherwise. 

I want to install smart light switches in my “dwelling”. I have encountered issues where the current back box is standard 74x74x25, but I would need extend it to 86x86x38 for the new light switch. 

I want check do I need part P certificate, or planning permission? How to be compliant while minimise the cost? (I have no experience in dwelling electrical work)

PS: The dwelling is a new build (2019) 
PPS: I might consider obtain required certificate(s) myself if is not too expensive. Since I like do these kind of work, so it might be cost effective in the long term. 

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    For the practical solution, do you mean because both the 74 mm back box and 86 mm back box have 60mm screw fixing, this problem is effectively extending the depth of the standard back box and cut a bigger hole in the wall to be 86 mm?

    I suspect it's more of a case of 74mm and 86mm being different ways of measuring the same thing (almost). 86mm is about the size of a normal plastic surface mounting box - which has relatively thick walls. Steel back boxes intended to be sunk into a wall are around 74mm on the outside but having thinner walls have about the same internal size as the 86mm plastic box.  The exact sizes vary slightly with brand - they're usually just referred to as 1 gang (or for the wider one 2-gang) boxes … according to the number of outlets the same size socket would have.

    It sounds like you just need a deeper box of the same width/height.

        - Andy.

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  • 0

    For the practical solution, do you mean because both the 74 mm back box and 86 mm back box have 60mm screw fixing, this problem is effectively extending the depth of the standard back box and cut a bigger hole in the wall to be 86 mm?

    I suspect it's more of a case of 74mm and 86mm being different ways of measuring the same thing (almost). 86mm is about the size of a normal plastic surface mounting box - which has relatively thick walls. Steel back boxes intended to be sunk into a wall are around 74mm on the outside but having thinner walls have about the same internal size as the 86mm plastic box.  The exact sizes vary slightly with brand - they're usually just referred to as 1 gang (or for the wider one 2-gang) boxes … according to the number of outlets the same size socket would have.

    It sounds like you just need a deeper box of the same width/height.

        - Andy.

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