This means that for a lighting circuit, the smallest permitted conductor size of 1.0 mm2 (Table 52.3 of BS 7671) has a de-rated current-carrying capacity above that which is necessary for the rating of the protective device.įor example, using Table 4D5 column 6, a 1.0 mm2 cable has a tabulated current-carrying capacity of 16 A. Where the installation method ‘clipped direct’ applies (Table 4A2 and 4D5 in Appendix 4) and the cable is subsequently covered by thermal insulation for a length of at least 0.5 m, the current-carrying capacity of the cable must be reduced by half (Regulation 523.9). To ensure cables meet the requirements for protection against overcurrent (Chapter 43 of BS 7671) the cross-sectional area of the conductors may need to be increased to compensate for any reduction in the current carrying capacity due to the additional insulation.Ĭircuits having an overcurrent protective device not exceeding 6 AĪs with many domestic circuits, and most typically lighting circuits having an overcurrent protective device rated at 6 A, the subsequent addition of thermal insulation is unlikely to have any impact on the circuit design. However, the actual risk would depend upon the type of cable, the type and thickness of the thermal insulation and the current demand for the circuit. Any increase in temperature may lead to a reduction in the life expectancy of the cable or, in extreme cases, cause damage to the insulation, sheath or surrounding area.
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