There are many cases in which wiring can be installed in PVC /PVC without further protection . For example , this may be done in any voids in a building such as false ceilings and wooden floors . WHen the cable runs parallel to joists in a wooden floor , it can be clopped to the sides of the joists . WHen it has to run across them it is better to thread it through holes drilled in the neutral axis of the joists than to notch the top of the joists . Holes drilled on the neutral axis weaken the joist less than notches cut in the top , and because the cable is further from the floorboards on top of the joists , it is safer from nails driven into the floor . BS 7671 states that a cable passing through a timber joist must be 50mm from the top or botoom of the joist , or be mechanically protected .
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