Cable Derating Factors - Hot!

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).

Remember: The cable’s rating in a catalog is a promise made in a laboratory. Derating factors are the fine print of physics. Read them. Apply them. Your cables—and your safety record—will thank you. cable derating factors

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. If a cable carries 100% load for 5

A cable buried in dry, sandy soil can reach its thermal limit at 50% of its rated current, whereas the same cable in moist clay might achieve 90%. Derating factors are the fine print of physics

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).