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).
For PVC, derating starts to bite above 30°C. For XLPE, above 40°C. Every 10°C above the baseline typically reduces ampacity by 10-15%. 2. Grouping (or Proximity) Derating When cables are bundled together, they form a thermal blanket. Each cable heats its neighbors. This is one of the most frequently underestimated factors, especially in cable trays, conduits, and ladder racks. cable derating factors
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
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. For PVC, derating starts to bite above 30°C
A 95mm² copper cable with XLPE insulation (90°C rating) in air has a base current of 350A at 30°C. If the ambient air temperature in a boiler room is 55°C, the correction factor from IEC 60364-5-52 might be 0.75. The derated capacity = $350 \times 0.75 = 262.5A$. Ignoring this could cause premature insulation failure within months.
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).