Cables through Joists

[the yorkshire dalesman]

The IEE On Site Guide shows (fig 7.3.1 page 51) a sheathed cable threaded through holes across joists. It is shown (as is often seen in the flesh) apparently unsupported between joists.

That doesn't seem to comply with the support methods in OSG Appendix 4 items 1-8 (page 97). And even if it dids comply, there would still seem to be a problem as if you consider the joist holes to be the 'cable supports' they would normally be too far apart according to table 4A (page 100).

Eg the commonest joist spacing is 400mm (16in) for 50mm (2in) joist thickness - giving an unsupported length of 350mm when table 4A requires 300mm maximum for the most common twin & earth cables (ie dia

9-15mm = 1.5mm2 to 4mm2 cable).

Looks like to comply with OSG appendix 4 you have to support the cable (eg on a timber noggin) between joists.

Is this correct?

TIA

[Dave Plowman]

I'd say the support distance refers to keeping a surface mounted cable taught to a wall etc so it can't be snagged. There's no earthly other reason - cable has a pretty high mechanical strength.

[BigWallop]

If the cables are under a floor where no one can stand on them, then the extra 50mm you talk about is thrown out of the window. They're going to be covered with flooring and never seen again for many, many years, so just cover them up and no one will be any the wiser. :-)) The support idea comes into play if the cables are to be exposed in air where they may become tagged with someone putting pressure on them, but yours under the floor will (should never) have this type of problem.

[Grunff]

When we bought this house, there was a 25m overhead span of 6mm T&E connecting the (falling down) barn to the house!! It had no wire supporting it, nothing other than the cable itself. It was the old grey PVC with stranded conductors.

[the yorkshire dalesman]

Many thanks for the comments - above is the clue which appears to lead to the definitive answer (meaning I didn't read far enough) - effectively this is overhead slung cable without a catenary wire - ie case 15 in OSG page 99 which allows a max span of 3m for pvc sheathed cable supported only at ends. ie it is allowed.

What was concerning me was possibility of drooping when softened by operating at full load (70deg C).

thanks again.