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IEC 60287-3-3: Cable Current Rating — Why Buried Cables Carry 30% More Current Than Air-Installed
Cable Ampacity Is Fundamentally Thermal Circuit Calculation: Soil Resistivity, Air Convection, and Mutual Heating
IEC 60287-3-3:2007 is the core part of the cable current rating calculation standard — addressing thermal correction for different installation conditions. Cable ampacity is not fixed — the same 400 mm² XLPE cable carries ~600 A in free air, ~780 A direct-buried (+30%), but if the soil dries out, the advantage disappears.
| Installation | Thermal Environment | 400mm² Cu/XLPE Ampacity |
|---|---|---|
| Free air (tray) | Air convection, ~0.026W/(m·K) | ~600A (40°C ambient) |
| Direct-buried | Soil 1.0–2.5W/(m·K) | ~780A (soil resistivity 1.5) |
| In duct | Duct wall + air gap additional resistance | ~520A (150mm duct) |
Ampacity correction (simplified):
I = I₀ × k₁ × k₂ × k₃
k₁=ambient temp (40→30°C: k₁≈1.15)
k₂=soil resistivity (1.0→1.5 K·m/W: k₂≈0.85)
k₃=grouping (3 cables touching: k₃≈0.70)
→ I = I₀ × 0.68TN Lab — Cable ampacity is thermal circuit calculation. Soil thermal resistivity is the most overlooked critical parameter.
