Buried Cable Derating

Underground cable installations rely on the surrounding soil to conduct heat away from the cable surface. Soil thermal resistivity — measured in K·m/W — varies enormously depending on soil type, moisture content, and compaction. Moist clay has a resistivity around 0.7–1.0 K·m/W, while dry sand can exceed 3.0 K·m/W. Standard ampacity tables assume a reference soil resistivity of 2.5 K·m/W and a ground temperature of 20°C. Correction factors adjust the cable's current-carrying capacity when actual conditions differ. The ground temperature correction is analogous to the ambient air temperature correction but uses the soil temperature at burial depth, which varies less than air temperature. Burial depth also affects ratings — deeper cables have better thermal stability but higher thermal resistance to the surface. For cable routes through areas with varying soil conditions, the worst section determines the cable rating for the entire route. Thermal backfill — specifically selected sand or cement-bound material with low thermal resistivity placed around the cable — can improve ratings in poor soil conditions. For directly buried cables in ducts, additional derating applies because the air gap inside the duct adds thermal resistance.