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Posted By Topic: Running cables in ceilings

Feb 01 2018 12:54

Hi all

When I trained I was taught never to run cables over trusses. Only over batters. What are current rules / best practice?


Feb 01 2018 16:30


Applying that to an accessible ceiling space, if a cable crosses a joist it needs mechanical protection i.a.w
Note: an RCD is NOT acceptable for this purpose.

Feb 01 2018 19:02

Thank you AlecK

I've had a read and it not as clear as I had hoped.

I agree with your conclusion but I can see why others might disagree.


Feb 01 2018 20:00

A lot of people read and as if they are the same ; but in fact each outlines a slightly different situation and imposes a different set of options for protection.
More correctly, covers ALL wiring that is subject to mechanical damage.
Then following clauses cover ADDITIONAL measures required for particular situations.
The RCD option for wiring fixed in position within 50 mm of wall surface is additional to the general requirement for protection i.a.w
So perhaps misleading to say "RCD not acceptable" for the cable-over-joist; more like "not acceptable by itself".

It's one of many areas that have been amended in new edition to try to make it clearer.

The Q&D solution being to nail / glue a small block of timber beside the cable.
Or maybe do it the way I was trained" run 2x1 batten through the ceiling, nailed to each joist; and clip the cable to the side of the batten.
Or use a bit more cable, and follow a longer route that doesn't create the problem (and may not require any clipping either).

Feb 01 2018 22:56

It's disappointing when I go to some modern houses to install something and when I come time to get access into the roof the cables are draped over the man hole cover and tossed everywhere with no thought to it whatsoever.

Having said that, some houses from the 1960s and 1970s aren't any better - cables that dart through 10 mm2 holes in a top plate, which don't go straight to a light switch or socket outlet; rather through a dogs leg through a stud, cross brace and notched into a dwang, and in Rimu for one hell of a job to fix.

Feb 02 2018 09:58

Simple and cheap fix is 2m lengths of 50x50. Short enough to get up through the manhole and long enough to span several joists. Couple of screws or nails to keep it in place. Cables clipped to the vertical. Easy as.

Feb 02 2018 10:48

or saddles over the timber.

Feb 07 2018 12:34

The cable over the man-hole is not always the sparky's fault. I've been to a couple of places where the ceiling access has been moved after the fact...

Feb 07 2018 14:13

yes the manhole might have been moved. But while "supporting cables" may refer to "within 2m of any access"; 3.9.4 applies throughout an accessible space.
So those cables should have already been protected where they cross joists etc against being stood / knelt on (unless there was previously NO access to the space at all).


Feb 07 2018 14:56

AlecK I don't think it's right to to say "3.9.4 applies throughout an accessible space."

Just because a cable runs over a joist in an accessible space doesn't necessarily mean that the cable "may reasonably be expected to be subject to mechanical damage" per

Consider the apex of the vee formed by a joist and a diagonal brace. A cable passing through this gap is accessible but isn't likely to be stood or knelt on.

So our job is not to decide whether the cable (or space) is accessible, but rather to determine for every cable whether it might be subject to mechanical damage.

Feb 07 2018 16:53

fair comment, each situation on its merits as to risk of damage.
Just pointing out that being >2m from manhole doesn't switch of the rule, as many seem to think.

Feb 08 2018 09:32

Yeah I should've been clearer - I was thinking more of the sections where cables are run loose across the ceiling (under the batts) rather than where they cross a joist.