Page 221 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 205

Table 9.20. Land based Installations. 6350/11,000 V. Cu/XLPE/PVC/AWA/PVC sin-
gle core Cu/XLPE/PVC/SWA/PVC three cores
Nominal Single core in trefoil 3 cores
conductor
2
area (mm ) Resistance Reactance at Resistance at Reactance at
◦
◦
at 90 C 50 Hz 90 C 50 Hz
(ohm/km) (ohm/km) (ohm/km) (ohm/km)
16 1.47 0.137
25 0.927 0.129
35 0.668 0.121
50 0.494 0.138 0.494 0.115
70 0.343 0.130 0.343 0.108
95 0.248 0.123 0.248 0.102
120 0.196 0.118 0.196 0.0988
150 0.159 0.117 0.159 0.0962
185 0.128 0.112 0.128 0.0931
240 0.098 0.109 0.098 0.0900
300 0.080 0.105 0.080 0.0874
400 0.064 0.101 0.064 0.0849

Table 9.21. Land based Installations. 8700/15,000 V. Cu/XLPE/PVC/AWA/PVC sin-
gle core Cu/XLPE/PVC/SWA/PVC three cores
Nominal Single core in trefoil 3 cores
conductor
2
area (mm ) Resistance Reactance at Resistance at Reactance at
◦
◦
at 90 C 50 Hz 90 C 50 Hz
(ohm/km) (ohm/km) (ohm/km) (ohm/km)
16 1.47 0.147
25 0.927 0.138
35 0.668 0.129
50 0.494 0.144 0.494 0.123
70 0.343 0.135 0.343 0.115
95 0.248 0.129 0.248 0.109
120 0.196 0.124 0.196 0.105
150 0.159 0.121 0.159 0.102
185 0.128 0.117 0.128 0.0986
240 0.098 0.113 0.098 0.0952
300 0.080 0.108 0.080 0.0922
400 0.064 0.107 0.064 0.0893

9.4.2.3 Derating factor due to thermal resistivity of the ground

Cables that are laid in the ground need to dissipate heat into their surroundings. The thermal con-
ductivity of soil varies considerably from 0.7 km/W for wet soil e.g. near lakes, coastal areas, high
water tables, to 3.0 km/W for very dry soil such as desert sand, see Reference 7.
The higher the value of thermal resistivity the more difficult it becomes to remove the heat from
the cable. A value of 1.2 km/W is frequently used as the base value in data given by a manufacturer.