Cable Derating | Factors

$$ I_{eff} = I_{nom} \times k_{temp} \times k_{group} \times k_{soil} \times k_{depth} \times k_{altitude} \times k_{harmonics} \times ... $$

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).

The real world, however, is far less forgiving. cable derating factors

The professional engineer who ignores derating factors builds a ticking thermal time bomb. The wise engineer uses them as a design tool—optimizing spacing, choosing cool locations, improving thermal backfill, and selecting appropriate cable types.

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. $$ I_{eff} = I_{nom} \times k_{temp} \times k_{group}

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.

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. Moist clay or loam is excellent (low resistivity)

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).