Circuitry and Related Parts of BS 7671: 2008
First select an appropriate protective rating, larger than the design current (step 1). Next the initial, tabulated cable size (It) is obtained by using the protective device rating and correcting for ambient temperature, grouping factor and, where applicable, correction factors for thermal insulation and rewirable fuses (step 2). It =
In Ca Cg Ci Cc
(Equation 1)
where: It is the tabulated current-carrying capacity from Appendix 4 of BS 7671 C denotes a correction factor as follows: Ca is ambient temperature; see Appendix C Table 4B1 (cables in air) or 4B2/4B3 (cables in ground) Cg is grouping; see Appendix C Table 4C1 to 4C5 Ci is for conductors totally surrounded by thermal insulation. Although not given in Appendix 4 this factor is 0.5 (523.6.6). It should be noted that some 17th Edition tables include a certain amount of thermal insulation and where these are used no further correction should be made. For example, new Table 4D5, included due to ECA suggestion, includes thermal insulation for cables in loft-roof spaces, and the Ci correction should not be used as it is already in the It value. Cc is for semi-enclosed fuses to BS 3036. A cable with tabulated current-carrying capacity (Iz) is then selected such to exceed the It (step 3). The procedure can be depicted as in Figure C 4.2.
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C