Other tables of current-carrying capacity in Appendix 4 tell a similar story. Also, Table 52.2 and Regulation 523.9 give rating factors for cables totally surrounded by thermal insulation, according to the length of cable surrounded.
For a circuit for which overload current protection is required, the lowest rated conductor used for the circuit should have an effective current-carrying capacity (Iz) of not less than the rated current (In) of the overload protective device of the circuit, or, if the device is a semi-enclosed fuse to BS 3036, not less than In ÷ 0.725 (Regulations 433.1.1 and 433.1.101 refer). The tabulated current-carrying capacity (It) for the size of conductor that meets the above requirement can be found by using Formula (1) below. This value of It (or the next highest value) should be located in the appropriate table of current-carrying capacity in Appendix 4 of BS 7671, and the corresponding cross-sectional area of conductor identified directly.
Requirements of BS 7671 Regulation 523.9 requires that a cable shall preferably not be installed in a location where it is liable to be covered by thermal insulation. Where a cable is installed in such a location, the same regulation requires that the cable shall preferably be fixed in a position where it will not be covered by thermal insulation. Where this is not practicable, the regulation requires the conductors of the cable to be adequately sized so that their effective current-carrying capacity – taking account of (amongst other things) the reduction in current-carrying capacity caused by thermal insulation – meets the requirements of Chapter 43 (Protection against overcurrent). Regulation 523.9 permits the nature of the load (such as cyclic loads) and diversity to be taken into account. However, this is not applicable where the maximum current is sustained for long periods, such as in a circuit supplying a night storage heater. In such cases the cable and protective device ratings must be adequate for the maximum current (see note, right). The implications of Regulation 523.9 as regards the cross-sectional area required for the circuit conductors are discussed below for radial circuits and ring final circuits. Radial circuits For a radial circuit, a useful preliminary step when considering the implications of thermal insulation on the size of conductors required is often to identify whether or not protection against overload current is required. Regulation 433.3.1(ii) allows overload protection to be omitted for circuit conductors which, because of the characteristics of the load, are unlikely to carry overload current. However, the conductors are still required to be protected against fault current (short-circuit and earth fault) in accordance with Section 434. An example of a circuit unlikely to carry overload current is one supplying only luminaires or heating elements, if the conductors are adequately sized to carry the full load current continuously. Socket-outlet circuits, however, invariably require overload protection, as users can easily add load (possibly too much load) by plugging in current-using equipment. www.electricalsafetyregister.com
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Note for formula (1): If the circuit cable is to be buried in the ground, additional factors Cs, Cc and Cd must also be included in Formula (1). Section 5.2 of Appendix 4 of BS 7671 refers.
Note: A cyclic load is one where the current varies in a regular, repetitive, predictable way. An example could be a programmable logic-controlled machine having a punching tool (45 A for 8 s), spot welding operation (80 A for 2 s) and conveyor belt (20 A continuous). Regulation 533.2.1 gives relevant requirements. Specific advice from the switchgear/controlgear assembly manufacturer should be followed regarding circuit protective device selection for a cyclic load.
Formula (1) (For use where overload protection is required). Based on Section 5.2 of Appendix 4 of BS 7671. It ≥
In Ca× Cg× Ci ×Cf
Where: • It is the tabulated current-carrying capacity of the cable in amperes. • In is the rated current of the overload protective device of the circuit in amperes. • Ca and Cg are the rating factors for ambient temperature and grouping, respectively, taken from the relevant tables in Appendix 4 of BS 7671. • Ci is the rating factor for thermal insulation, taken from Table 52.2 of Regulation 523.9. Ci is not used (or its value is set to 1) if the installation Reference Method takes account of thermal insulation, such as in the case of Reference Methods 100 to 103 of Table 4A2 of Appendix 4 of BS 7671, for flat twin-and-earth cables in thermal insulation. • Cf is the factor for the type of overload protective device. Where the overload protective device is a semi-enclosed fuse to BS 3036, Cf = 0.725. Otherwise Cf = 1. For a circuit for which overload current protection is not required, the conductors used should have a tabulated current-carrying capacity (It) not less than the current (Ib) to be carried by the circuit in normal service (Regulation 523.1 and Section 5.2 of Appendix 4 refer). The tabulated current-carrying capacity (It) for the size of conductor that meets the above requirement can be found by using Formula (2). Again, this value of It (or the next highest value) should be located in the appropriate table of current-carrying capacity in Appendix 4 of BS 7671, and the corresponding cross-sectional area of conductor identified directly. NICEIC Connections Autumn 2013
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