16mm boxes: what use are they?

[AdamW]

I've just been confronted with a stupid and undoubtedly common mistake. I just replaced two two-way switches with a two-way switch and a two-way dimmer. The switch went in great, tested it and no problems. Shut off the power again, started on the dimmer. I had it all wired up, was about to push it into the box to screw it in... then I noticed that it was a 16mm box So, tomorrow when there is more light, I will be whipping out the bolster and installing a 35mm box.

But then I was thinking: what use is a 16mm box? A simple switch barely fits in one, and it is no more hassle to install a 25 or 35mm box. Why are they still used when so many people now (and for the last 30 years or so) replace switches with dimmers?

 

[plugwash]

penny pinching i imagine

also depening on the plaster depth i'd imagine you'd get situations where a 16mm will fit without attacking rock hard engineering brick but a 25mm won't

 

[mapj1]

Though in the age of the cheap SDS drill, this problem has mostly dissapeared.
However, plug is right, the point is that 16mm is roughly the depth of a wall finished in sand cement skim and then a thin topping of plaster - the tradional wall finish before plater board.
So very little or maybe even no chasing at all at first fix. Time is money, so thin boxes it is.
If time is no object however, and for the DIY-er its not soo pressing as the paid by the box contractor, I'd suggest that the deeper boxes are much more future proof, and easier to wire up. (otherwise we'd all use minitrunking and surface mount pattreses, and we don't beccause cheap is not always good looking is it)

 

[AdamW]

Ah, that would make sense: I have uncovered wiring in this place which is simply capped barely below the plaster finish (even plastic capping on non-reg dog-leg runs... not good, but possibly legal in 1984 when it was built). On such runs there is no chasing into the breeze block substrate, just plastic capping and plastered up to level. I have replace this with chasing, especially since finding a rawlplug that was miraculously right to the left of the T&E, but through the capping.

So, the 16mm boxes in question are almost certainly screwed straight onto the breeze blocks. Will require some drilling and bolstering to fit man-sized boxes for man-sized accessories...

Which brings up another question: when chiselling breezeblocks, I have noticed a tendency for the blocks to move back towards the cavity, which results in the screed and plaster dipping back behind the level of the rest of the finished wall. I have a sharp bolster but I am generally pretty lunk-fisted (popeye arms and spade hands ), is it me or do blocks usually move when thwacked with a chisel? I am using a 4lb hammer. The making-good is taking me longer than the wiring!!!

 

[ban-all-sheds]

If the brickie did his job properly they should not move - they should be held firmly together by some stuff called mortar.

 

[Big_Spark]

16mm Boxes used to be very common as they are switch boxes, used for the original plate switches introduced by the likes of MK, Crabtree, Doorman Smith etc back in the 1960's.

However they have laregly fallen out of favour as many people like to fit dimmer switches, which do not fit on 16mm boxes, and even many multi gang units will not fit on the box without damage to cables behind being trapped on the box.

It is only cheating for time if you do rewires, first fixes of new installs are more economic if you keep material types as similar as possible, so 25mm boxes for all single gang instances are preferred.

 

[mapj1]

If you are doing all the chasing by 'knock out' then you will be using a fair amount of muscle power - on delicate stuff, I'd much rather drill a chain of holes the outside with the SDS drill (I only have a cheap 30 quid one - for years I worked without, and if it broke now I'd be so lost !) and chisel more or less sideways rather than straight back, breaking off triangles of brick, into the holes already drilled, not trying to drive the chisel in at right angels to the wall, like a nail. Without seeing it it sounds like you are going at it too hard (or as BAS suggests, you have a sand only mortar mix between the bricks...)
With care it is possible to do a 25mm or even 35mm deep back box hole in about 5- 10 mins, with edges good enough that there is no need for mixing plaster to make good (on a good day anyway ) a couple of table spoons of poly filla is enough.
hope that helps.
The other thing - when did you last sharpen your masonry chisel - is it really blunt, this increases the cutting force enormously?