Back box depths


Postby cheesman » Tue Aug 07, 2007 5:14 pm

Hi there.

I would like to know what rules apply to the different depths of metal back boxes for sockets. I would like to replace a surface switch with a flush mounted one on a wall that will be plastered. The deep boxes seem to involve chasing into the brickwork whereas the thinner boxes (25mm I think) will sit flush with standard plaster work.

Any help would be appreciated.
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Postby BLAKEY1963 » Tue Aug 07, 2007 6:40 pm

[quote="cheesman"]Hi there.

I would like to know what rules apply to the different depths of metal back boxes for sockets. I would like to replace a surface switch with a flush mounted one on a wall that will be plastered. The deep boxes seem to involve chasing into the brickwork whereas the thinner boxes (25mm I think) will sit flush with standard plaster work.

Any help would be appreciated.[/quote]

CHEESMAN
16 MM2 WOULD BE OKAY, BUT 25 MM2 WOULD ACCOMADATE
SWITCHES SUCH AS DIMMERS.
GENERALLY HORIZONTIALLY CHASED THE DEPTH SHOULD BE 1 SIXTH ( ALLOWABLE AND CALLED SAFE ZONE )
OF THE WALL THICK NESS AND VERTICALLY, IF SWITCH
WIRE NO MORE THAN 1 THIRD OF THE THICKNESS OF THE
WALL.
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Postby Reynardine » Wed Aug 08, 2007 7:48 am

Thank you for the guidance on depth and height when chasing into brick and plaster. With the new SDS box sinkers and an average hammer drill it is all to easy to chew too large a hole into masonry.

I would add that the depth of the back box that you need is largely determined by your choice of flush fitting socket. I have found that the crabtree 'screwless' range of sockets won't fit in a shallow 16mm box. I have found they need at least a 25mm deep back box. However, the MK range can fit in a shallow box.

Additionally, the more cables that come into your socket, the harder it is to cram all these into a shallow box.
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Postby cheesman » Thu Aug 09, 2007 1:10 pm

Thanks for the advice.

However I didn't make it clear :oops: that I was talking about power sockets. Can I screw a 'thin' backbox directly to the brick without aditional chasing in?

I don't want the plasterer to hace to do the whole wall to a greater depth than he needs to.

Cheers

Cliff
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Postby Reynardine » Fri Aug 10, 2007 6:27 am

Cliff, For your power socket, it sounds like you are choosing the wall mounted route instead of the flush mounted route.

This approach involve buying a different fitting to your metal back box. You buy a wall mounted plastic box fitting ( pattress ? ) and attach this directly to the brick/plaster and then routing the cable into the fitting with plastic conduit.

IMO you can't really screw a metal back box directly to the wall surface. The live wires would be too accessible. You have to chase out a recess and attach to the back of that.

Why not go flush mounted ? When filling in a 16mm, 25mm or 40mm hole it's all the same to the plasterer ? I doubt he/she will charge you by the millimeter!
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