Cables under concrete floor

[Martin Pentreath]

I need to do some rewiring in my kitchen. It has a concrete floor, and the easiest way to route the twin & earth would in places be under the floor. Once everything is done I'm intending to put Wickes electric floor heating on top followed by ceramic tiles.

So, what I'd like to do is to put my SDS drill to good use to gun up some channels in the floor, lay the cables and fill in the channels with mortar.

Is it OK to do this? Must I use conduit (in which case I'd go for

)? Are there rules about the course the cables must take (no diagonals, etc)?

While I'm at it I intend to put some cat5 and coaxial cables in as well (although not to close to the electrics), anyone foresee any problems with that idea? I'm a bit worried that the underfloor heating might bugger it up.

Thanks for any ideas,

Martin

[Martin Angove]

Thought someone else would have answered this one by now... don't have much time, but here goes anyway.

1: PVC cable is unsuitable for embedding directly in concrete according to the On Site Guide, table 3A (p94) so some sort of conduit is called for. 2: If you use metal conduit (watch out for its earthing) then depth and route shouldn't be an issue - the usual rule is 50mm below any surface is "safe" or if less than 50mm, must be "suitably protected". The only caveat I'd see to this is that some people have a habit of firing nails into concrete floors in order to fix partition battens down (etc.) and any such nail is unlikely to find a bit of, even metal, conduit a problem. However, if you are laying electric UFH on top then anyone who wants to do this is probably a bit silly. 3: The UFH will certainly affect the capacity of the cable - at what temperature does it (the UFH) operate, and how well insulated is it (the UFH) to below? Again, the OSG has advice in the forms of tables 2A (p93) and 9B (p159) however, if you can be sure that the ambient temperature of the cable will not get above 30C you can assume that 2.5mmsq PVC cables in conduit will be able to carry at least 20A, and are hence suitable for their "standard" circuit configurations. This information is in table 6F of the OSG (p126).

HTH.

Hwyl!

M.

[Rick Hughes]

If PVC cable is unsuitable for this - why is it suitable for direct bury in roughging when used on blockwork walls ?? it's the same material in contact ?

Rick

[Martin Angove]

Hmmm... come to think of it, what the OSG probably means is not to lay cable and then pour concrete all over it. It is possible that something in the unset/setting mix will react with the PVC. I really don't know, and someone else here can probably advise better. When putting cable in channel, the stuff is already set and so presumably more-or-less inert? They're not worried about wet plaster - good job too!

Hwyl!

M.