For example - Installation Methods 1 and 2 of Table 4A2 (non-sheathed cables and multicore cables in conduit in a thermally insulated wall).

The wall consists of an outer weatherproof skin, thermal insulation and an inner skin of wood or wood-like material having a thermal conductance of at least 10W/m²K. The conduit is fixed such that it is close to, but not necessarily touching, the inner skin. Heat from cables is assumed to escape through the inner skin only. The conduit can be metal or plastic.

For example - Installation Method 4 of Table 4A2 (non-sheathed cables in conduit mounted on a wooden or masonry wall) and Installation Method 5 of Table 4A2 (multicore cable in conduit on a wooden or masonry wall).

The conduit is mounted on a wooden wall such that the gap between the conduit and the surface is less than 0.3 times the conduit diameter. The conduit can be metal or plastic. Where the conduit is fixed to a masonry wall the current-carrying capacity of the non-sheathed or sheathed cable may be higher.

For example - Installation method 20 of Table 4A2 (single-core or multicore cables on a wooden or masonry wall).

Cable mounted on a wooden wall so that the gap between the cable and the surface is less than 0.3 times the cable diameter.  Where the cable is fixed to or embedded in a masonry wall the current-carrying capacity may be higher.

Note: The term 'masonry' is taken to include brickwork, concrete, plaster and similar (but excluding thermally insulating materials).

For example - Installation method 70 of Table 4A2 (multicore armoured cable in conduit or in cable ducting in the ground).

The cable is drawn into a 100mm diameter plastic, earthenware or metallic duct laid in direct contact with soil having thermal resistivity of 2.5K.m/W and at a depth of 0.7m. The values given for this method are those stated in Appendix 4 of BS 7671 and are based on conservative installation parameters. If the specific installation parameters are known (thermal resistance of the ground, ground ambient temperature, cable depth), reference can be made to the cable manufacturer or the ERA 69-30 series of publications, which may result in a smaller cable size being selected.

Note: The current carrying capacity for cables laid in direct contact with soil having a thermal resistivity of 2.5K.m/W and at a depth of 0.7m is approximately 10% higher than the values tabulated for Reference Method D.

For example - Installation methods 31-35 of Table 4A (single-core or multicore cables in free air).

The cable is supported such that the total heat dissipation is not impeded. Heating due to solar radiation and other sources is to be taken into account. Care is to be taken that natural air convection is not impeded. In practice, a clearance between a cable and any adjacent surface of at least 0.3 times the cable external diameter for multicore cables or 1.0 times the cable diameter for single-core cables is sufficient to permit the use of current-carrying capacities appropriate to free-air conditions.

Installation methods for flat twin and earth cable clipped direct to a wooden joist, or touching the plasterboard ceiling surface, above a plasterboard ceiling with thermal insulation not exceeding 100mm in thickness having a minimum U value of 0.1 W/m²K.

Installation methods for flat twin and earth cable clipped direct to a wooden joist, or touching the plasterboard ceiling surface, above a plasterboard ceiling with thermal insulation exceeding 100mm in thickness having a minimum U value of 0.1 W/m²K.

Installation methods for flat twin and earth cable in a stud wall with thermal insulation with a minimum U value of 0.1 W/m²K with the cable touching the inner wall surface, or touching the plasterboard ceiling surface, and the inner skin having a minimum U value of 10 W/m²K.

Installation methods for flat twin and earth cable in a stud wall with thermal insulation with a minimum U value of 0.1 W/m²K with the cable not touching the inner wall surface.

Wherever practicable, a cable is to be fixed in a position such that it will not be covered with thermal insulation. Regulation 523.9, BS 5803-5: Appendix C: Avoidance of overheating of electric cables, Building Regulations Approved document B and Thermal Insulation avoiding risks, BR 262, BRE, 2001 refer.

All information is based on BS 7671:2018: As amended, Requirements for Electrical Installations Appendix 4. For further information, consult BS 7671 Appendix 4 directly.