Installing Cable Passages Safely
It is often necessary to drill notches and holes in floors and roof joists in order to create cable passages. However, great care needs to be taken with cable passages in roofs and under floors, as detailed in BS 7671 and the Building Regulations.
BS 7671 states that wiring systems must not penetrate any element of building construction which is designed for load bearing, unless its integrity can be guaranteed afterwards.
Meanwhile Building Regulations dictate that any notches and holes drilled in joists must not weaken the building structure, and must be no bigger than necessary.
Single family homes with 3 storeys or less, which are simply supported, i.e. supported at each end of a floor or roof joist, must adhere to the following:
Notches:
- Must be no deeper than 1.25 times the depth of a joist.
- Should be no closer to the support than 0.07 of the span.
- Should be no further away from the support than 0.25 times the span.
Holes:
- Must be no greater in diameter than 0.25 times the depth of the joist.
- Should be drilled at the netural axis, i.e. where the joist is under the least tension, which is usually at the centre line of the joist depth.
- Should be 3 diameters or more apart, centre to centre.
- Must be located between 0.25 and 0.4 times the span from the support.
Further information is available in the Electricians Guide to the Building Regulations (Approved Document P: Electrical Safety in Dwellings).
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