A = (I × t) / (K × ρ)
| Fault current (kA) | Time (s) | Insulation | Min Cu size (mm²) | Selected std size (mm²) | |--------------------|----------|------------|-------------------|--------------------------| | 5 | 0.2 | PVC | 19.4 | 25 | | 10 | 0.2 | PVC | 38.9 | 50 | | 20 | 0.2 | PVC | 77.8 | 95 | | 25 | 0.3 | XLPE | 95.8 | 120 | | 40 | 1.0 | Bare Cu | 177 | 185 | earthing cable sizing
Suppose we need to size an earthing cable for a electrical installation with a fault current of 10 kA and a fault duration of 1 second. The soil resistivity is 100 Ωm, and the ambient temperature is 20°C. Using the formula above and assuming a copper cable with a K value of 226, we get: A = (I × t) / (K ×