How Deep Should I Channel Out

[terrysdelight]

Hi

I've come to an arrangement with electrician on a full rewire - basically to save costs, he said if I channel out for him etc.....

For electric sockets/switches he is using 38mm deep boxes. I did ask why he is using 38 as if he used 25 I wouldn't have to channel out so far - just said this is his preference.

With the run up to the boxes from floorboards or in case of light switches, down from ceiling - I've channelled out 30mm - this is the depth of my cutter. Electrician said this isn't deep enough. It's only twin and earth and I'm wondering if this chap is having a laugh at me or if this is genuinely the way the regs are now?

Grateful for clarification on how deep I should go.

Many thanks.

 

[ricicle]

30mm is more than enough unless your electrician is trying to avoid the 'cables buried less than 50mm need 30mA RCD protection' reg.
Channelling out to 50mm might cause structural problems. Ask your electrician why he wants it that way and why you won't be doing it that way. If he is difficult find another one.

 

[EFLImpudence]

You cannot chase any deeper if brick or block.

One third the leaf thickness vertically and one sixth horizontally.

 

[terrysdelight]

Thanks for replies, much appreciated.

I know nothing about electrics and don't want to upset the guy if he is saying this for a genuine reason. Can you give me replies in 'Laymens' as I don't understand about electrics.

The knocking out is causing problems structurally.

The electrician is saying this is the Regulations? When the wires are in the wall, they will have steel or plastic caps over them.

Surely there is no regulation that says steel boxes have to be 38mm when they are selling the smaller 25mm?

Grateful for answers/clarifications. Thanks again.

 

[terrysdelight]

30mm is more than enough unless your electrician is trying to avoid the 'cables buried less than 50mm need 30mA RCD protection' reg.
Channelling out to 50mm might cause structural problems. Ask your electrician why he wants it that way and why you won't be doing it that way. If he is difficult find another one.

If Electrician is trying to avoid 'cables buried less than.........' What would be the benefit? He doesn't seem to like it when I ask questions so I would appreciate the facts when trying to reason with him.

 

[JohnW2]

If Electrician is trying to avoid 'cables buried less than.........' What would be the benefit? He doesn't seem to like it when I ask questions so I would appreciate the facts when trying to reason with him.

It's probably too late to be thinking about this, but are you sure you've chosen the right electrician? I personally would not be happy with any tradesman (or, indeed, professional) who didn't like me asking questions!

As for your above question, it would, IMO, be crazy to try to bury cables >50mm deep (which is very often impossible, per structural regs) just so as to avoid fitting an RCD - which is the only real 'benefit'. If, as extremely likley, you are going to have RCD protection anyway, then there's really no 'benefit' at all. As for the back boxes, it is a lot easier to work with ones deeper than 25mm (more room for cables).

Kind Regards, John

 

[EFLImpudence]

When the wires are in the wall, they will have steel or plastic caps over them.

Not a requirement but it is done to keep the cable in place.

Surely there is no regulation that says steel boxes have to be 38mm when they are selling the smaller 25mm?

Correct - although larger cables need deeper boxes.

If Electrician is trying to avoid 'cables buried less than.........' What would be the benefit?

That would remove the need for RCD protection.

However, as I wrote, it is not possible in brick or block built walls because of the Building Regulations (re: sixth/third).

He doesn't seem to like it when I ask questions so I would appreciate the facts when trying to reason with him.

Perhaps he doesn't know the answers.

 

[terrysdelight]

Thank you.

I can see that this guy is taking the mickey. We did as he said and went back deeper but basically right into the bricks and then we had bricks falling out of the wall.

I've told the guy we are using 25mm boxes and we are not going back any deeper than what is already done.

I now have to get someone in to sort out all the bricks - render them up, so they can be skimmed up.

You live and learn! Thanks for all your help people and I won't be using this electrician again!!

 

[Taylortwocities]

25mm boxes are OK if you are going to use plain old plastic sockets etc and there isn't much else to go in the box.

BUT

if you are going to use fancy plat plate sockets then there will not be enough room for cables behind the guts of the socket.

Also, its tight in a 25mm box if you are running spurs. But you wouldn't need to in a 'decent' rewire.

 

[terrysdelight]

Thanks - there won't be any spurring off as its a full rewire and sockets going where I want them to go.

The real problem has been the depth of the channelling for the trunking he's made us go back over 40 mm and this has really eaten into the bricks.

Also with the light switch boxes - going back deeper to set the 38mm has made the bricks collapse. I did want Chrome or polished steel sockets, but I guess I'll have to have white plastic.

 

[sparkwright]

Usually in a house you only have to cut into the plaster. If the plaster is shallow you may just have to break the surface of the bricks.

Around 15 mm deep is plenty.

If using capping or conduit you only need about 4 mm of plaster over it. Any less may cause the plaster to crack.

I can only think there may be some confusion about the depth and the WIDTH of the chase. Capping can be around 40 mm WIDE.

As mentioned, there are some rules about cables deeper than 50 mm don't require RCD protection, but as so many circuits will need RCD protection for other reasons it's hardly an issue.

Can only imagine the electrician is very inexperienced or an idiot.

Nowadays would favour 35 mm deep boxes to allow a choice of fancy sockets, but regular 25 mm deep boxes would do if you're not bothered.

 

[xr4x4]

....also, 38mm back box for a light switch is bonkers...!!

As in most cases you can get away with 16mm.

 

[flameport]

As in most cases you can get away with 16mm.

Although such things should never be fitted, because the very next thing is the customer wants a dimmer, or other changes to the lighting layout requires some evil thing like a double intermediate switch to be fitted there.

 

[SimonH2]

Although such things should never be fitted, because the very next thing is the customer wants a dimmer ...

Indeed.
Personally I think most electricians use boxes that are too shallow - I assume because they want to minimise the work of sinking them. One on flat I looked at, the electrician was working in there at the time - one box (by the front door) has 6 (yes, SIX) cables going into a single box, and a shallow one at that. I cannot conceive of any way that switch could possibly be wired and fitted safely.

Doing my mates bungalow, I've used deep boxes wherever I've aded or changed one. As you say, when he got the dimmers, it was a very tight squeeze for a double Varilight screwless dimmer with 4 cables in the back of it. I think that was 32mm, or might have been 25 and set deep in the wall). His varilight sockets are "fairly deep" as well compared to some of the slim plastic sockets.