Deeper burial means more soil above the cable, which acts as thermal insulation. The earth’s temperature also increases with depth due to geothermal gradient, though for shallow power cables (<2m), ambient ground temperature dominates.
At high altitudes, air density drops. Less dense air means fewer molecules to carry away heat via convection. cable derating factors
For cables buried directly in the ground or in ducts, the ability of the soil to conduct heat away affects ampacity. Deeper burial means more soil above the cable,
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. Less dense air means fewer molecules to carry