crossing buried cables

[very_pointy_ears]

Hi,

What is the general opinion of crossing cables (on different circuits) buried shallowly into a solid wall?

The current situation is this:

[code:1] |
|
wall light |
| |
spurred off C | | ring
| |
+---+ +-+-+ +---+
| A | want more sockets here | B | | C |
+-+-+ +-+-+ +-+-+
| | |
| | |
----+-------------------------------------+--------------+-----
concrete floor
[/code:1]

A is a socket spurred from C. The cable goes down from C into the concrete floor, across and then back up to A.
B is a silly socket (that I'm going to remove) wired into the lighting circuit
C is a socket on the ring

The wall is an outside brick wall and the floor is a solid concrete floor.

I want to add some more sockets where I've indicated on the diagram. As socket A is a spur, the only option I can see is to extend the ring from socket C. However, this means crossing the path of the lighting cable at B.

As explained earlier, B currently has a single unswitched socket faceplate wired into the lighting circuit (this was fitted by the former owner). It has a 35mm deep single gang metal pattress/backbox sunk into the wall in which all the gubbins resides. There are currently 3 lighting circuit cables coming into this pattress - the feed, the supply to the above wall light and the supply to another wall light (so I discovered today). I plan to replace this socket B with a blank faceplate.

Now, if I want to add more sockets as per the diagram, I see that I have 2 options.

1. extend the ring across from the socket at C, through the pattress at B that has lighting cabling in it and on to the desired area.

2. I tap into the ring as it drops down from the ceiling towards socket C and carry it across in the safe zone just below the ceiling before dropping down into one of the new socket positions between A and B on my diagram.

Going down into the floor is awkward because it's a concrete floor and I haven't a clue where the madman who wired up the wall lights buried the cables.

I don't really like option 2 because it just doesn't feel right. But then again option 1 doesn't feel all that great either.

The floor above the ceiling is our master bedroom and is nicely decorated with fitted carpet. Lifting the floor up is not an option for the sake of my wife's sanity (and mine).

The ring main is 30mA RCD protected but the lighting circuits are not RCD protected.

Any help will be much appreciaed.

 

[electronicsuk]

As long as you remain within the safe zones created by the accessories on the wall then I see no reason that you cannot cross two power cables at right angles, even if it may be a little inelegant.

Also, the cable in the floor should have suitable mechanical protection or an earthed metallic covering, i.e. T+E in galvanised conduit or SWA.

 

[fattony]

removed

 

[very_pointy_ears]

As long as you remain within the safe zones created by the accessories on the wall then I see no reason that you cannot cross two power cables at right angles, even if it may be a little inelegant.

Also, the cable in the floor should have suitable mechanical protection or an earthed metallic covering, i.e. T+E in galvanised conduit or SWA.

Thanks. I have no idea what protection the in-floor cabling has as that was done before I bought the house. I would be surprised if they bothered to give it any sort of protection at all to be honest, judging by what else I've found. At least it's RCD protected now that I've replaced the CU.

I don't feel inclined to dig up the floor of a 25 x 14 foot room just to find out, though!

 

[ban-all-sheds]

Bear in mind that you may not have horizontal chases deeper than ¹/6th of the thickness of the wall.

Where does the ring cable go to get to and from [C]?

How do you propose to join the old and new cables at [C] where you break into the ring?

If is on the lighting circuit why do you show it connected to the socket circuit?

sounds busy, by the time you've got the choc-block in there (I assume that's what you're using) will there be room for 2 x 2.5mm² cables to pass through it?

Where does the spur cable supplying [A] go to as it heads off to the left?

 

[seasickstevie]

Just to clarify - I assume the dotted horizontal line along the bottom simply indicated the concrete floor, and not a circuit.

Am I correct?

 

[very_pointy_ears]

Just to clarify - I assume the dotted horizontal line along the bottom simply indicated the concrete floor, and not a circuit.

Am I correct?

Yes you are.

 

[very_pointy_ears]

Bear in mind that you may not have horizontal chases deeper than ¹/6th of the thickness of the wall.

Thanks, I didn't know that but I can see why such a rule exists for the sake of the strength of the wall.

In the case of a cavity wall, is this thickness limit ¹/6th of the thickness of the inner blocks that I'm chasing into? ie I presume I can't count the thickness of the entire cavity wall and take ¹/6th of that.

Would I also be right to assume that this ¹/6th starts when I start chasing into block, and that if there's deep plaster on the block, this doesn't count towards the measurement? Ie if I've got 20mm of plaster on a 100mm thick block, is it allowed to chase out the whole of the 20mm plaster and then ¹/6th of the 100mm as well?

Where does the ring cable go to get to and from [C]?

It drops down from the ceiling and then goes back up again. The spur goes downwards from [C] and then (I'm presuming) into the floor and across. It comes up to [A] from underneath.

How do you propose to join the old and new cables at [C] where you break into the ring?

I was presuming I'd have to use a 30A choc connector or similar.

If is on the lighting circuit why do you show it connected to the socket circuit?

It isn't. Sorry, the diagram wasn't very clear. As SNM wrote, the horizontal line at the bottom is merely to indicate the concrete floor.

sounds busy, by the time you've got the choc-block in there (I assume that's what you're using) will there be room for 2 x 2.5mm² cables to pass through it?

It will get moderately busy. At the moment the socket is being the connector but yes, I'll have to put something else in there. It might, I admit, be a bit snug in there with two other cables passing through, though probably not too bad as it's got a 35mm back box. I could replace this with a 47mm one if that helps matters, as long as I'm allowed to chase that deeply into the (thermolite) block.

Where does the spur cable supplying [A] go to as it heads off to the left?

It doesn't, that's just the floor.