I have one of these (or very similar)
I too have cinder block walls and tried one that I borrowed from an ex-electrician. It removed the material quickly and when used with the box shaped sinker it produced a good flat bottomed box, it just did it rather too well completely distorting the guide hole rendering it useless for plugging.
I tend to use a multi tool with a carbide blade and have been a lot happier with the results even though slower. Using the multi tool I managed to get three double boxes side by side with faceplates touching each other in a horizontal row and they did not need any filling such was the neatness of the fit.
Richard
One of the problems with the big cutters and the box shaped sinkers is that they produce an awful lot of dust. They turn the entire hole into dust. Times 20 is an awful lot of dust. Will the OP be living in the house during this work?
Thanks. No, he won't.
Which carbide blade did you use, and how many double boxes do you think it could do before wearing out?
An angle grinder sure is quicker. A core drill would also be quicker. Since it's nothingcrete, one of those square shaped cutters might also work.
60's house might well be breeze block which, depending on the source of the slag/clinker, can be a bloody sight harder than modern medium density blocks which are themselves much harder than lightweight blocks (assuming that is what you mean by nothingcrete).
Hammer and bolster chisel is it is cinder blocks.
Bugger! What about SDS with chisel bit?
When chasing the slot for the ring cables, is there a specific min depth they should be (RCD protected)? I think I read that they should have a cable widths separation. Can both ring cables enter the backbox using one grommeted hole?
They make phenomenal amounts of dust and don't work well in anything other than lightweight block. Easiest way is to stitch drill (with a depth stop) and then use an SDS or bolster to break-out the bits, and a wide SDS bit to "plane" the bottom of the hole. Always use a deeper box than you first think of, and always make the hole a bit bigger than you need - both save a lot of faffing-about later.
If carefully done that would always be method of installation. Just don't knock the cinder blocks into the adjacent room. Some cinder blocks are more delicate than others.
No minimum depth or separation width is required in this case.
Yes. But I would not recommend it.
I usually use the middle two entrance points to the backbox and put one cable through each. If just using one hole then use the left or right hole (a 2g back box usually has 4 holes on the top and bottom).
The reason for this is nothing to do with grouping of cables but simply to make it easier to second fix and make the socket fit the backbox when you dress the cables.
Thanks Adam. Your advice is always welcome.
I have this (excerpt) IET On-Site Guide BS 7671:2018 which has this (about derating):
7.2.1 Grouping of circuit cables For cables of household or similar installations, if the following rules are followed, derating for grouping is not necessary: (b) Cables clipped direct (including in cement or plaster) are clipped side by side in one layer and separated by at least one cable diameter.That's where I got the bit about leaving a separation.
And, if you have two dbl-skts side by side, would you route (from under the floor in this case) the ring up to the 1st skt, then across to the
2nd, and then back down from the 2nd to under-floor? Or could you route to the 1st, then to the 2nd, then route back through the 1st box and down to under-floor?And, if you were re-wiring a light switch in a plasterboard wall, using T&E (currently the lighting circuit has no CPC), how do you clip it at
400mm spacing? Does that only apply if the cable run is accessible?
Useful tip, thanks.
both are fine
it goes through holes in the uprights rather than using clips.
Adam emailed me to say he was having some eternal september posting problems, and asked me to post the following for him - Adam's words in "quotes":
"I would not worry about it unless you expect cables to be fully loaded for hours on end. "
"Either or, both are OK"
"You cannot do 400mm spacing. It will not hurt anything."
That is usually my go to. I run round the perimeter of the cutout using a 20mm chisel - sinking to depth with a series of plunge cuts. The break out the waster using a 40mm wide chisel, working out toward the cut edges, deepening with successive passes, and also making the angle more shallow, until the last past the back of the cutout is basically flat.
Same technique as I used for a concealed shower valve here:
There is a specific depth (>= 50mm - so often impractical on a 100mm deep wall since that needs to be >= 50mm from both sides!) if you want to *omit* RCD protection.
Not usually an issue for cables from the same circuit. Also unless heavily loaded for continuos periods, not something to worry too much about IMHO.
They can - although that is not always the easiest way to wire - especially if you are short on backbox depth (say modern fancy flat plate sockets).
A great many years ago, my BiL was installing a cooker box on the party wall of a 1930s house in Rugby. As he hit the chisel quite hard, the brick shot away and the nextdoor neighbours said "Would you take your brick out of our bath"
Thanks John, and Adam. Most helpful (as always).
I think I have told this tale before, but your brings to mind a similar case where my cousin was redoing his kitchen, and was attempting to shift a lump of bath stone that appeared to be sticking out of the wall and in the way of a new cabinet. Slowly he prised it side to side, edging it out, growing ever more surprised that it was deeper than he expected. Eventually it came free, allowing a view of his neighbour sat at his breakfast table, puffing on his pipe watching a bit of his kitchen wall slowly recede! He says (in rich Somerset accent) "Morning David. Know what you are doing does you?". Dave responds quick as a flash, yup don't worry, I will be round to plaster and make good your side later!
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