In high-resistivity soil, depth derating is more severe because the already-poor thermal path becomes longer. 5. Altitude (For Cables in Air) At high altitudes, air density drops. Less dense air means fewer molecules to carry away heat via convection.
The cable can only carry current if it can dissipate its self-generated heat plus the ambient heat without exceeding its rated temperature. cable derating factors
Heat transfer from the center cables is blocked by the outer cables. The hottest cable in a dense bundle can run 20-30°C hotter than an isolated cable carrying the same current. In high-resistivity soil, depth derating is more severe
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). Derating factors are not bureaucratic red tape. They are the mathematical expression of thermodynamic reality. Every degree of temperature, every adjacent cable, every grain of sand around a buried conductor extracts a price in current-carrying capacity. Less dense air means fewer molecules to carry
Cables are often bundled in trays, buried in hot sand, routed through sun-scorched attics, or installed next to harmonic-generating drives. When these real-world conditions deviate from the "ideal," the cable’s ability to dissipate heat diminishes. If we ignore this, the cable overheats, insulation degrades, voltage drop increases, and ultimately, system reliability collapses.
If a cable carries 100% load for 5 minutes then rests for 55 minutes, the average heat is far lower than a continuous 100% load. Derating factors for cyclic loads can increase allowable current (up-rating) but require careful analysis of the thermal time constant of the cable (typically 10-30 minutes for medium cables).