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
area (mm^2 ) horizontally trefoil^33
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 2530354045505560
Rating factorKairfor cables laid in air 1.0 0.96 0.92 0.88 0.84 0.79 0.73 0.68
Ground temperature◦C 10152025303540
Rating factorKgrdfor 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

area (mm^2 ) Resistance

at 90◦C

(ohm/km)

Reactance at

50 Hz

(ohm/km)

Resistance

at 90◦C

(ohm/km)

Reactance

at 50 Hz

(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 (Kgrd) based on the chosen
ground temperature, see the above tables, and Chapter 8 of Reference 4.