Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning installing cable in a new attic, is answered by BASEC:
Answer: Cabling in roofspaces such as lofts and attics needs to take account of several items:
1. The thermal insulation used - the electrician should assess the cable sizes based on current loading and mounting method as set out in the Wiring Regulations, which set out several generic mounting methods. These take account of the effectiveness of various thicknesses and types of mounting arrangements and thermal insulation. This leads to de-rating factors being applied to the cables in question to avoid overheating.
2. Mechanical protection - cables should be routed to avoid potential damage from external influences such as floorboards or moving items around, and from vermin (mice, birds, etc.).
3.Future expansions and use - sufficient short slack lengths of cable should be incorporated so that routing and accessories (e.g. light fittings) can be modified in the future.
4. Fire protection - consult Building Control. Low smoke halogen free (LSHF) cabling might be required to be used instead of PVC. In certain cases, mineral insulated cable may be appropriate (e.g., thatched roofs). If light fittings (e.g., downlighters) make sizeable penetrations in the ceiling, then fire hoods or fire rated lights may be needed. Insulation would need to be removed from around hot light fittings.
Cables are normally clipped to the side of roof joists etc., to avoid mechanical damage, which will place them in the thermal insulation. Cables can overheat in thermal insulation, so a larger sized cable may be needed (which will run at a lower temperature).
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