Steve Nelson asks this question, which concerns the running of cable above ceilings having narrow joists, and the drillings for cable in them. It has been answered by the NICEIC (Theme - Installation Tips):
Question: My query concerns cables run above ceilings where the joists are only 100mm thick.
From my interpretation of the regulations, holes in ceilings should be drilled through the centre of the joist and at least 50mm above or below the floor or ceiling - and in the required zone. The maximum size hole that can be drilled into a 100mm joist is 25mm (0.25 x depth).
The ceiling is low to start with so cannot be battened down. I can take the ceiling down and use a safeplate on the underside where the hole is to be drilled, but then it does not comply with the floor above. If I drill the hole off-centre to give me the 50mm clearance above, this could weaken the joist. Clearly I cannot be expected to take up a tiled floor above to fit safeplates on the floor joists.
The use of cables with earthed armour is fraught with problems concerning termination, as these are screwed connections that would need to be accessible. They are also impractical.
Short of lining the hole through the joist to give some form of mechanical protection, or using micc / BS 8436 cable, which is not cheap and requires special tools, is there a commonsense approach to this problem?
Answer: This is indeed a problem that requires a commonsense approach - if these are existing holes and you are not making them.
If you were drilling joists today, you would have to comply with the Part A 'Structures' of the Building Regulations. However, if you use existing holes, then commonsense and reducing the risk of mechanical damage would be prudent.
You cannot control what he gone on before, but you should not make the installation any worse with your scopes of work.
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