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.
In the world of electrical engineering and power distribution, selecting the correct cable size is rarely as simple as looking up a current rating in a manufacturer’s table. Those tables—often printed in neat, optimistic columns—assume a perfect world. They assume an ambient temperature of 30°C, a solitary cable in free air, and soil with ideal thermal resistivity. cable derating factors
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%. A 95mm² copper cable with XLPE insulation (90°C
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. They assume an ambient temperature of 30°C, a
Leave space. Use ventilated trays. Derate less if cables are flat-spaced rather than trefoil (triangular) packed. 3. Soil Thermal Resistivity (Buried Cables) Burying cables solves aesthetic and mechanical problems but introduces a complex thermal variable: the soil's ability to conduct heat away from the cable.