You cant..... cos they can Providing.... all other things are complied with Protection against flora and fauna, damage, bridging, and so on
Do a search on wall cavities on this formum, it's come up twice before. It's not a good idea due to, mainly, that cables cannot be supported in the cavity unless they are built in before the outer skin goes on plus various other reasons.
Cables in Cavity Walls Question: Can cables be run in external walls with cavities? Answer: Cables such as pvc/pvc insulated and sheathed cables should not be run in cavity walls. Mineral insulated copper sheathed cables may be judged by the installation designer to be suitable for such a location. Explanation: Electrical installation designers often have to consider whether it is permissible to install thermoplastic (pvc) insulated and sheathed cables in external cavity walls. The NICEIC considers that installing such cables in this way is generally an undesirable practice, as it is unlikely that all of the applicable requirements of BS 7671, the Building Regulations and the NHBC requirements listed below will be fully met. Damage to cables during installation. Obstructions in a cavity wall, such as metal wall ties or mortar projections, create a risk of unseen damage occurring to the cable sheath and conductor insulation during installation. (Regulation 522-08-01 refers). Strain on cables lacking support. Long unsupported vertical drops may place undue strain on the conductors, leading to damage (Regulation 522-08-04 refers). Materials liable to cause mutual or individual deterioration. Expanded polystyrene sheets, granules or foam may be used in buildings for thermal insulation purposes. If this material comes into contact with thermoplastic (pvc) cable sheathing, plasticiser can migrate from the thermoplastic to the polystyrene. The thermoplastic sheathing then becomes less flexible and the polystyrene becomes soft and tacky. Such contact should be avoided. (Regulation 522-05-03 refers). Cables in thermal insulation. Thermal insulation is often installed in cavity walls during or after construction. Cables in the cavity may not then be able to carry the load current without overheating due to their current-carrying capacity being reduced. For example, where a cable is surrounded by thermally insulating material over a length of 500 mm or more, the cable current- carrying capacity is to be taken as half the ‘clipped direct’ Method 1 rating unless more precise information is available (Regulation 523-04-01). Cables should be installed where they will not come into contact with thermal insulation (present or reasonably to be expected in the future) unless the current-carrying capacity is adequately maintained by, for example, increasing the cross-sectional area of the conductors at the design stage. Presence of flora and fauna. Designers and installers may not necessarily detect or predict the presence of flora or fauna in a cavity wall. The wiring system should be selected to withstand all the external influences expected, or damage to cables may occur from, for example, mould or rodents (Regulations 522-09-01 and 522-10-01 refer). Building Regulations. Building Regulations 2000, Approved Document C, 1992 Edition, Section 4: Walls, paragraph 4.12 b Cavity external walls, states: ‘A cavity external wall may be built with the cavity at least 50 mm wide. The cavity is to be bridged only by wall ties or by damp-proof trays provided to prevent moisture being carried to the inner leaf.’ A cavity is intended to provide a gap to prevent water penetration. Cables could bridge this protection if they touch both the inner and outer leaves of a cavity wall. Furthermore, the cables could provide a route for water to drain directly into accessories, with potentially dangerous results. NHBC Requirements. It is noteworthy that the National House-Building Council states, in Section 8.1 - S2 of NHBC Standards, that no cables other than electricity meter tails are to be located in the cavity of an external wall. Where meter tails do have to pass through the cavity, however, this does not preclude the requirements of BS 7671 referred to in this article having to be met.
Cables in Cavity Walls Question: Can cables be run in external walls with cavities? Yes Answer: Cables such as pvc/pvc insulated and sheathed cables should not be run in cavity walls. Mineral insulated copper sheathed cables may be judged by the installation designer to be suitable for such a location. Protect the cable then ! Explanation: Electrical installation designers often have to consider whether it is permissible to install thermoplastic (pvc) insulated and sheathed cables in external cavity walls. The NICEIC considers that installing such cables in this way is generally an undesirable practice, as it is unlikely that all of the applicable requirements of BS 7671, the Building Regulations and the NHBC requirements listed below will be fully met. Undesirable.... maybe, but sometimes the only route available. Damage to cables during installation. Obstructions in a cavity wall, such as metal wall ties or mortar projections, create a risk of unseen damage occurring to the cable sheath and conductor insulation during installation. (Regulation 522-08-01 refers). Protect it then... Strain on cables lacking support. Long unsupported vertical drops may place undue strain on the conductors, leading to damage (Regulation 522-08-04 refers). We are talking about generic houses here... not a 100' multi floor factory Materials liable to cause mutual or individual deterioration. Expanded polystyrene sheets, granules or foam may be used in buildings for thermal insulation purposes. If this material comes into contact with thermoplastic (pvc) cable sheathing, plasticiser can migrate from the thermoplastic to the polystyrene. The thermoplastic sheathing then becomes less flexible and the polystyrene becomes soft and tacky. Such contact should be avoided. (Regulation 522-05-03 refers). Protect the cable then Cables in thermal insulation. Thermal insulation is often installed in cavity walls during or after construction. Cables in the cavity may not then be able to carry the load current without overheating due to their current-carrying capacity being reduced. For example, where a cable is surrounded by thermally insulating material over a length of 500 mm or more, the cable current- carrying capacity is to be taken as half the ‘clipped direct’ Method 1 rating unless more precise information is available (Regulation 523-04-01). Cables should be installed where they will not come into contact with thermal insulation (present or reasonably to be expected in the future) unless the current-carrying capacity is adequately maintained by, for example, increasing the cross-sectional area of the conductors at the design stage. Fair point... do the maths. Presence of flora and fauna. Designers and installers may not necessarily detect or predict the presence of flora or fauna in a cavity wall. The wiring system should be selected to withstand all the external influences expected, or damage to cables may occur from, for example, mould or rodents (Regulations 522-09-01 and 522-10-01 refer). Protect the cable then Building Regulations. Building Regulations 2000, Approved Document C, 1992 Edition, Section 4: Walls, paragraph 4.12 b Cavity external walls, states: ‘A cavity external wall may be built with the cavity at least 50 mm wide. The cavity is to be bridged only by wall ties or by damp-proof trays provided to prevent moisture being carried to the inner leaf.’ A cavity is intended to provide a gap to prevent water penetration. Cables could bridge this protection if they touch both the inner and outer leaves of a cavity wall. Furthermore, the cables could provide a route for water to drain directly into accessories, with potentially dangerous results. We are not talking about bridging the cavity. NHBC Requirements. It is noteworthy that the National House-Building Council states, in Section 8.1 - S2 of NHBC Standards, that no cables other than electricity meter tails are to be located in the cavity of an external wall. Where meter tails do have to pass through the cavity, however, this does not preclude the requirements of BS 7671 referred to in this article having to be met. 'that no cables'.... yeah, shure.. when they stop doing it, I'll listen.
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