MV & LV Cable Derating Factors: Depth of Laying & Temperature
Multiply the catalogue rating by these factors for your actual depth, ground temperature and air temperature.
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Catalogue current ratings assume standard conditions: 900 mm laying depth, 30°C ground, 40°C air. Real installations differ, so the rating must be multiplied by correction factors. These are the factors used for APAR MV XLPE cables (3.3 kV to 33 kV); LV practice is similar.
Depth of laying (direct in ground)
Depth (mm)
3.3–11 kV single core ≤185
3.3–11 kV single core >185
3.3–11 kV three core
22/33 kV single core ≤185
22/33 kV single core >185
22/33 kV three core
900
1.00
1.00
1.00
—
—
—
1050
0.99
0.98
0.98
1.00
1.00
1.00
1200
0.97
0.96
0.97
0.99
0.98
0.99
1500
0.95
0.94
0.95
0.97
0.96
0.97
Ground temperature (90°C conductor)
Ground temp (°C)
15
20
25
30
35
40
45
50
Factor (direct or in duct)
1.12
1.08
1.04
1.00
0.96
0.91
0.87
0.82
Ambient air temperature (90°C conductor)
Air temp (°C)
25
30
35
40
45
50
55
60
Factor
1.14
1.10
1.05
1.00
0.95
0.89
0.84
0.77
Note what the air row implies: at a 50°C summer ambient a conventional 90°C cable keeps only 89% of its rating, and at 55°C only 84%. This is exactly the problem 105°C high-ampacity XLPE is designed for — raising the conductor limit recovers the lost margin. Factors for soil resistivity and cable grouping are available from our technical team on request.
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