Page 220 - Handbook of Electrical Engineering

204 HANDBOOK OF ELECTRICAL ENGINEERING

Table 9.18. Land based Installations. 3800/6600 V. 6350/11,000 V. 8700/15,000 V. Cu/XLPE/PVC/AWA/
PVC single core
Nominal Air Air Ducts Direct buried Notes
conductor trefoil 3 trefoil
2
area (mm ) horizontally trefoil 3 3
spaced horizontally horizontally
touching spaced
50 250 300 220 220 220 230 Thermal resistivity
70 310 370 260 270 270 280 of soil is 1.2 C
◦
m/W
95 375 460 305 320 317 335 Ambient air temp. is
◦
120 430 530 340 360 355 380 25 C
150 490 600 375 410 403 430 Depth of laying
185 550 690 410 455 445 485 cables is 0.5 m
240 650 820 470 520 505 560 Standard ground
◦
300 740 940 500 580 560 640 temp. is 15 C
400 840 1100 530 650 620 730 Conductor surface
◦
temp. is 90 C.
◦
Ambient air temperature C 25 30 35 40 45 50 55 60
Rating factor K air for cables laid in air 1.0 0.96 0.92 0.88 0.84 0.79 0.73 0.68
◦
Ground temperature C 10 15 20 25 30 35 40
Rating factor K grd for cables laid in the ground 1.03 1.0 0.97 0.93 0.89 0.86 0.82
Table 9.19. Land based Installations. 3800/6600 V. Cu/XLPE/PVC/AWA/PVC
single core Cu/XLPE/PVC/SWA/PVC three cores
Nominal Single core in trefoil 3 cores
conductor
2
area (mm ) Resistance Reactance at Resistance Reactance
◦
◦
at 90 C 50 Hz at 90 C at 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 0.134 0.494 0.115
70 0.343 0.125 0.343 0.108
95 0.248 0.119 0.248 0.102
120 0.196 0.114 0.196 0.0988
150 0.159 0.111 0.159 0.0962
185 0.128 0.109 0.128 0.0931
240 0.098 0.105 0.098 0.0900
300 0.080 0.103 0.080 0.0874
400 0.064 0.100 0.064 0.0849

9.4.2.2 Derating factor due to ground temperature

Manufacturers quote current ratings of their cables laid in the ground at a particular ambient temper-
◦
◦
ature e.g., 15 C and 20 C. They also provide tables of derating factors (K grd ) based on the chosen
ground temperature, see the above tables, and Chapter 8 of Reference 4.