Whilst the current-carrying capacity of a cable is dependent on a number of factors, this article looks at the impact of thermally insulating materials on the current-carrying capacity of flat twin and earth cables installed in domestic premises.
A cable that is run in a space where thermal insulation is likely to be applied, such as in the loft of a dwelling, should preferably be installed so that contact with the thermal insulation is avoided. Otherwise, the operating temperature of the conductors may be exceeded (70˚C for thermoplastic (pvc) insulated conductors), possibly leading to a reduction in the service life of the cable or damage to the insulation of the conductors or adjacent material (Regulation 523.9 of BS 7671 refers).
However, it is common practice for thermally insulating materials to be installed in the walls and ceilings of domestic and similar premises to satisfy the energy efficiency and noise requirements of Building Regulations. Consequently, as shown in Fig 1, contact with thermal insulation is generally unavoidable for most twin and earth cables installed in domestic and similar premises.
So that cables installed in such locations will remain sufficient to meet the requirements of BS 7671, the cross-sectional area of conductors may need to be increased to compensate for any reduction in the current-carrying capacity due to thermal insulation.
Two different cases that need to be considered are:
- cables surrounded by thermal insulation, and
- cables routed in a thermally insulating wall or above a thermally insulating ceiling, the cable(s) being in contact with a thermally conductive surface on one side.
For a single cable likely to be totally surrounded by thermally insulating material for a length of 0.5 m or more, the current-carrying capacity must be taken as 0.5 times the current-carrying capacity for that cable clipped direct to a surface (Reference Method C).
Where a cable is to be totally surrounded by thermal insulation for less than 0.5 m, the current-carrying capacity of the cable must be reduced appropriately, depending on the size of cable, length in the insulation and thermal properties of the insulation. The rating factors (Ci) shown in Table 52.2 of BS 7671 should be applied, as appropriate, for conductor sizes up to 10 mm2 in thermal insulation having a thermal conductivity (λ) greater than 0.04 Wm-1 K-1.
For cables installed in a thermally insulating wall or above a thermally insulating ceiling – the cable being in contact with a thermally conductive surface on one side – it is not necessary to apply a rating factor for thermal insulation from Table 52.2.
For such purposes, tabulated currentcarrying capacities are given in Appendix 4 of BS 7671 based on the Installation Method used. The electrical installation designer must decide which of the methods listed in Table 4A2 of Appendix 4 is the most appropriate.
Installation Methods 100 – 103 (typically associated with new domestic and similar premises) are specifically for flat twin and earth cable in thermal insulation. Where cables are installed using these Installation Methods, the current-carrying capacity of the cables should be determined using Table 4D5 of Appendix 4 (70ºC thermoplastic insulated and sheathed cable with protective conductor).
In Fig 2, the current-carrying capacity of a 2.5 mm2 flat twin and earth cable is shown in brackets for each of these Installation Methods.
The values shown illustrate the impact of these Installation Methods on the current-carrying capacity of the cable. Particularly, considering a 2.5 mm2 conductor clipped direct (Reference Method C) has current-carrying capacity of 27 A (Table 4D5 of Appendix 4 refers).
It should be noted that where more than one Installation Method is applied for a circuit route, the method which provides the lowest current-carrying capacity (worst case scenario) should be used to determine the cable size.
For Installation Methods 100 – 103, Table 4A2 does not attribute a letter to indicate the Reference Method. Therefore, when compiling the electrical certificate, the Installation Method number should be recorded in the box entitled ‘Reference Method’ in the Schedule of Circuit Details (SOCD) for the particular circuit.
For more information on installation practices refer to the following publication;
