Let’s break down the primary derating factors, the physics behind them, and how to apply them in practice. Heat is the enemy of insulation. Every cable has a maximum continuous operating temperature (e.g., 70°C for PVC, 90°C for XLPE, 105°C for EPR). The cable generates heat due to resistive losses ($I^2R$). The surrounding environment also imposes its own heat.
A cable at 0.5m depth dissipates heat better than at 1.5m depth. Derating factors for depth are typically small (0.95–0.98 per 0.5m increase) but become significant for long, high-current runs. 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. Let’s break down the primary derating factors, the
The cable can only carry current if it can dissipate its self-generated heat plus the ambient heat without exceeding its rated temperature. The cable generates heat due to resistive losses ($I^2R$)
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.
Soil thermal resistivity ($\rho$, in K·m/W) measures how effectively soil transfers heat. Dry sand or gravel is a terrible conductor (high resistivity). Moist clay or loam is excellent (low resistivity).