Electrical Installation Calculations: For Compliance with BS 7671:2008, 4th Edition

About the authors vii

Preface ix

Acknowledgements xi

Symbols xiii

Definitions xv

1 Calculation of the cross-sectional areas of circuit live conductors 1

General circuits 4

Circuits in thermally insulating walls 5

Circuits totally surrounded by thermally insulating material 6

Circuits in varying external influences and installation conditions 6

Circuits in ventilated trenches 8

Circuits using mineral-insulated cables 9

Circuits on perforated metal cable trays 10

Circuits in enclosed trenches 11

Circuits buried in the ground 14

Grouped circuits not liable to simultaneous overload 18

Circuits in low ambient temperatures 24

Grouped ring circuits 26

Motor circuits subject to frequent stopping and starting 27

Circuits for star-delta starting of motors 29

Change of parameters of already installed circuits 30

Admixtures of cable sizes in enclosures 33

Grouping of cables having different insulation 39

2 Calculation of voltage drop under normal load conditions 40

The simple approach 40

The more accurate approach taking account of conductor operating temperature 43

The more accurate approach taking account of load power factor 55

The more accurate approach taking account of both conductor operating temperature and load power factor 58

Voltage drop in ring circuits 59

Voltage drop in ELV circuits 62

3 Calculation of earth fault loop impedance 65

The simple approach 70

The more accurate approach taking account of conductor temperature 75

Calculations taking account of transformer impedance 81

Calculations concerning circuits fed from sub-distribution boards 82

Calculations where conduit or trunking is used as the protective conductor 87

Calculations where cable armouring is used as the protective conductor 94

4 Calculations concerning protective conductor cross-sectional area 101

Calculations when the protective device is a fuse 104

Calculations when an external cpc is in parallel with the armour 111

Calculations when the protective device is an mcb 113

Calculations when the protective device is an RCD or RCBO 119

5 Calculations related to short circuit conditions 126

a.c. single-phase circuits 127

The more rigorous method for a.c. single-phase circuits 135

a.c. three-phase circuits 141

6 Combined examples 153

Appendix: The touch voltage concept 175

Index 189