The real world, however, is far less forgiving.
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. cable derating factors
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 real world, however, is far less forgiving
$$ I_{eff} = I_{nom} \times k_{temp} \times k_{group} \times k_{soil} \times k_{depth} \times k_{altitude} \times k_{harmonics} \times ... $$ Derating factors are not bureaucratic red tape
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.
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).
A derating factor (often denoted as a multiplier, k, between 0 and 1) adjusts the nominal current-carrying capacity of a cable to reflect actual installation conditions. Instead of asking, "How much current can this cable carry in a lab?" we ask, "How much current can this cable safely carry in my specific environment?"