Cable routing under tricky floor

[iep]

I need to run mains cable across a room (both parallel and then perpendicular to the joists). Normally I'd simply drill holes >50mm below the top of the joist when running perp and then clip to the joists below 50mm when running parallel to them.

However, this floor comprises a concrete slab with 5cmx5cm joists set out across it (shimmed here and there to help level them). So, I can't drill holes 50mm down the joist and I can't run cables under the joists (when running perp to them) and, though I can clip to the side of them when running parallel, I'm still running closer to the boards above than I'd like.

In this situation do I need to use some form of metal conduit to protect the cables in order to be able to run them closer to the floorboards?

Alternatively, if I run the cables right up against the wall, does this place them in a safe zone and so mean the usual 50mm rule does not apply?

Cheers,

iep

 

[Chri5]

The 50mm rule is referenced to non rcd installations. Does the site have rcd or rcbo protection?

Are you aware of the building regs regarding routes under floor and the position of holes in joists?

//www.diynot.com/forums/viewtopic.php?t=136759

 

[iep]

All circuits on their own rcbo. Have to admit that I've always stuck to the 50mm rule but perhaps that is overkill with an rcd.

Thanks for the link to the details on holes in joists but it's worth noting that our joists are not structural, they are very small at 5cm x 5cm and are shimmed every 30cm or so for strength. really they just lie flat on the concrete slab and provide something to nail the floorboards to.

So, it sounds like I can notch out the joists and run the cables below the floorboards without further protection? Seems a bit too easy but maybe those RCBOs I shelled out for have decided to pay for themselves after all!.

Cheers,

iep

 

[Lectrician]

No.

The RCD / RCBO protection refers to cables in a wall buried at a depth of less than 50mm in safe zones - nothing to do with cables below a floor.

522.6.5
A cable installed under a floor or above a ceiling shall be run in such a position that it is not liable to be damaged by contact with the floor or the ceiling or their fixings. A cable passing through a joist within a floor or ceiling construction or through a ceiling support (e.g. under floorboards), shall:
(i) be at least 50 mm measured vertically from the top, or bottom as appropriate, of the joist or batten, or
(ii) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective conductor of the circuit concerned, the cable complying with BS 5467, BS 6346, BS 6724, BS 7846, BS EN 60702-1 or BS 8436, or
(iii) be enclosed in earthed conduit complying with BS EN 61386 and satisfying the requirements of these Regulations for a protective conductor, or
(iv) be enclosed in earthed trunking or ducting complying with BS EN 50085 and satisfying the requirements of these Regulations for a protective conductor, or
(v) be mechanically protected against damage sufficient to prevent penetration of the cable by nails, screws andthe like.

A shame you didn't screed, as the regulations are then a tad more relaxed:

522.6.4
A wiring system buried in a floor shall be sufficiently protected to prevent damage caused by the intended use of the floor.

In this case, generally, cables installed in PVC or Galv conduit are considered fine.

 

[pjcomp]

If you decide to notch the top of the joists, you can get steel plates to pin across the notches to provide nail protection.

http://www.tlc-direct.co.uk/Main_Index/Necessities_Index/Necessities~Cable_Protection/index.html

PJ

 

[iep]

That makes sense to me (or at least correlates with my previous understanding).

Only thing that puzzles me is how a PVC conduit can help since the points ii, iii, and iv all referred to an earthed protection of some type?

Cheers,

iep

 

[Lectrician]

PVC conduit is considered OK in a SCREEDED floor (ref my last comment about screeded floors).

 

[iep]

pjcomp, would using this approach allow me to meet the current regs? I have four cables to run so this looks a lot less hassle than running 4 lots of conduit.

Cheers,

iep

 

[iep]

Lectrician, yes, sorry, noticed that as soon as I posted my response. Makes sense.

Cheers,

iep

 

[Lectrician]

Safe plates have been a bone of contention.

They do provide the required 'nail proof' mechanical protection at the joists, but not the span in-between. The NICEIC have always mentioned this, and taken the easy way of saying "this doesn't comply".

 

[iep]

I guess my other option is to notch the bottom of the joists where they meet the concrete. Wedge a couple of bits of 6mm ply either side of the notch and the cable will sit exactly 50mm under the top of the joist (albeit lying on the concrete slab). Bit of a fiddle but definitely doable. That gets round any ambiguity regarding use of safe plates.

iep

 

[ban-all-sheds]

If the boards aren't in place, notch them where required, and get busy bending steel conduit.

 

[sparkwright]

Getting the cables under the joists certainly seems the best (or practical) solution.

Are there any places where there is a gap between the concrete and the joists? Sometimes if the concrete is not quite level there may be a gap.

 

[sparkwright]

Have a look at the ends of the joists in case they don't meet the walls - there may be a gap here.

 

[iep]

Ta for the further posts. The joists do meet the walls (in fact they prop the wall framing up). There are gaps under the joists here and there (hence the need for shims) however they are usually quite a small gaps which might mean the cable might get a little squashed every time someone walks over it.

I think I'll try to take one of these slight gaps and enlarge it to let the cables run underneath. Maybe try to pin the cables down to the concrete somehow too to stop them moving around (and keep things tidy).

Cheers,

iep