A cable rated for 100A at 30°C ambient might only carry 82A at 45°C ambient. Conversely, in a cold environment (e.g., 10°C), you might actually increase the rating (up-rating), though conservative design often avoids this.
Cables have a base current-carrying capacity (often denoted as $I_0$ or $I_z$) determined by the maximum operating temperature of the insulation (e.g., 70°C for PVC, 90°C for XLPE). cable derating factors
The cable’s safe capacity is just 36% of its nominal rating. To carry the desired 350A load, the engineer would need to upsize to ~300mm² or redesign the installation completely (separate trays, improve soil, reduce ambient). A cable rated for 100A at 30°C ambient
Deeper burial means more soil above the cable, which acts as thermal insulation. The earth’s temperature also increases with depth due to geothermal gradient, though for shallow power cables (<2m), ambient ground temperature dominates. The cable’s safe capacity is just 36% of