Having just had a partial damp course installed, I am taking advantage
of the pause before the plasterers arrive to replace the rather old
and dubious radial circuit that provides power to downstairs in our
house with ring cicuit. Although I am happy with the details of the
wiring itself, I am looking for advice on the type of pattress box to
use. The choice seems to be plastic boxes that only have punch-outs
for the cables at the rear or metal boxes with punch outs at the rear,
sides, bottom and top. The plastic ones seem preferable as they do
not have to be lined up exactly with the level of the (future) plaster
but they are awkward to install because the cable has to enter from
the back. Is there a reason why plastic boxes with cable entry at the
bottom are not available? Could I just drill holes in the bottom of
Thanks for any help
The plastic boxes are for surface mounting (although metal surface mount are
also available). The metal boxes are for burying in walls. You appear to
want the metal ones.
Eh? I don't understand. Metal flush boxes do not need to be accurately lined
up with the plaster. You'll notice that the accessories will even screw down
until the metal box is well inside the level of the back face. It is
intended that the accessory actually tightens against the plaster wall,
rather than the box.
These sound like dry lining boxes - designed to be fitted in plasterboard,
But dry lined boxes have a lip at the top which is designed to sit against
the surface - anything else would look odd. Normal surface mount plastic
boxes have side entries and I suppose wouldn't look that bad if they stuck
out a bit, but your plasterer won't thank you for them.
The correct way would be to use metal boxes and sink them below the
eventual level of the plaster. Leave the wires sticking out so you can
find them again when the plasterer fills them in. ;-) You might also need
longer screws if the brickwork is very uneven.
*If at first you don't succeed, destroy all evidence that you tried *
Dave Plowman firstname.lastname@example.org London SW 12
If the plaster is off the walls, then go for metal back boxes straight to
the brickwork, and leave the sockets off until the plasterer has done his
thing. Then you can make the socket plates fit flush on top the new plaster
because the spread will have taken the gunk right up to the edges of the new
You might find that the white surface boxes are to deep for the new plaster,
so you'll be left with the sockets sticking out from the wall.
If the radial circuit is using 2.5mm csa' cable, then all you'll need to do
is extend one end of it back to your consumer unit (fuse box). That way you
don't have to remove any of the existing cable, just find the end where the
circuit stops and add on to it.
Buried in plaster you should be using the galv steel boxes. They are set
to be just at or below the finished plaster depth. Good ones will have
one of the face plate fixing lugs adjustable so you can get your sockets
dead level. (I hope I'm not the only one who uses a spirit level when
finally fixing accessories ;-)
Unless your brickwork is rock hard, I'd also recommend the 35mm deep
versions over the 25mm ones. The extra depth makes final wiring a lot
easier. Don't forget to grommet the cable holes.
Depending on your situation, also consider just fitting 20mm oval
conduit for now, your can then run in the cables after the plaster work
has been done. Depends on you arrangment with underfloor access etc.
Don't expect to get more than one 2.5mm T&E in each conduit, so you'll
need two drops from each pattress.
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