Underground Cable Derating Calculation

This simple but useful calculation tool will produce underground cable derating factor. Derating factor is complied with IEC 60502 derating table from B11 to B21. Hence cable specification is in line with IEC 60502 thermosetting insulation, i.e. Cu/XLPE/SWA or Cu/EPR/SWA. In case, project condition (for example, the ground temperature is 28 degrees) differs with a predetermined value, the derating factor could be interpolated.

INPUT:

Ground Temperature (degree C):

10 15 20 25 30 35 40 45 50

Depth of Cable Laying (m):

0.5 m 0.6 m 0.8 m 1 m 1.25 m 1.5 m 1.75 m 2 m 2.5 m 3 m

Soil Thermal Resistivity (K*m/W):

0.7 K*m/W 0.8 K*m/W 0.9 K*m/W 1 K*m/W 1.5 K*m/W 2 K*m/W 2.5 K*m/W 3 K*m/W

Multicore or Single core:

Multicores Single core

Cable Size (sqmm):

16 sqmm 25 sqmm 35 sqmm 50 sqmm 70 sqmm 95 sqmm 120 sqmm 150 sqmm 185 sqmm 240 sqmm 300 sqmm 400 sqmm

Number of cable

2 3 4 5 6 7 8 9 10 11 12

Installation method:

Type D1 – In Duct Type D2 – Direct Buried

Spacing Betweeen Cables or Group (mm):

Touching 200 mm 400 mm 600 mm 800 mm

RESULT:

Derating factors	Reference Table	Result
Derating Due to Ground Temperature (k1)	Table B11
Derating Due to Depth of Cable Laying (k2)	Table B12/B13
Derating Due to Soil Thermal Resistivity (k3)	Table B14/B15/B16/B17
Derating Due to Cable Arrangement (k4)	Table B18/B19/B20/B21
Overall Underground Derating (k1 x k2 x k3 x k4)

Cable derating factor has been discussed in Cable Derating Factors article. A short description here will explain the inputs required for underground cable derating calculation using IEC table. Finally, an overall derating will be generated to count for all 4 individual derating factors.

Derating due to Ground Temperature (k1)

Select the ground temperature from the dropdown list. Derating factor k1 will alter accordingly. In case, the ground temperature is 20 degrees, the derating factor is unity (1).

Derating due to Laying Depth (k2)

Select the cable laying depth from the dropdown list and cable data (single-core or multicores, area of conductor). Derating factor k2 will alter accordingly. In case laying depth is 0.8m, the derating factor is unity (1).

Derating due to Soil Thermal Resistivity (k3)

Select the project soil thermal resistivity from the dropdown list. Derating factor k3 will alter accordingly. In case, soil thermal resistivity is 1.5 K*m/W derating factor is unity (1).

Derating due to Cable Arrangement (k4)

Cable arrangement generally defined in Cable Installation. For underground cable, there are 2 method D1 and D2 is applicable.

Overall Underground Cable Derating

This factor is multiply of all above 4 factor viz. k₁ x k₂ x k₃ x k₄