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IET - Technical Article - Changes to Appendix 4 of Amendment 1 of BS 7671:2008
IET

IET - Technical Article - Changes to Appendix 4 of Amendment 1 of BS 7671:2008

IET

IET - Technical Article - Changes to Appendix 4 of Amendment 1 of BS 7671:2008

Published: 12 July 2013 · Category: Technical articles

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WiringFeature #44 Changes to Appendix 4 of Amendment 1 of BS 7671:2008 By Geoff Cronshaw Amendment 1 of the 17th edition of the Wiring Regulations brought changes to Appendix 4: selection of conductor cross sectional area (article excludes voltage drop and fault current calculations) The current-carrying capacity of a cable corresponds to the maximum current that can be carried in specified conditions without the conductors exceeding the permissible limit of steady-state temperature for the type of insulation concerned. The values of current tabulated represent the effective current-carrying capacity only where no rating factor is applicable. 16 | Autumn 2012 | IET wiring matters | www.theiet.org/wm A rating factor has to be applied where the installation conditions differ from those for which values of current-carrying capacity are tabulated in Tables 4D1A to Tables 4J4A of Appendix 4. The first significant change introduced in the 17th edition, compared with the 16th edition, is the large number of rating factors related to cables buried in the ground. Selection of conductor cross-sectional area with overload protection The required tabulated current rating (It) of a cable where overload protection is to be provided is given by: In It ≥ –––––––––––––––– CgCaCiCfCsCcCd Where: It is the tabulated current-carrying capacity of a cable found in Appendix 4 of BS 7671; In is the rated current or current setting of the overcurrent device; Cg is rating factor for grouping; Ca is...

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