Note: Altitude affects air cooling but NOT buried cables. The Physics: Non-linear loads (VFDs, computers, rectifiers) generate harmonic currents. Triplen harmonics (3rd, 9th, 15th) add in the neutral conductor, causing overheating even if phase currents are balanced.
12 current-carrying conductors in bundle: $K_group = 0.50$ (NEC 10-20 conductors)
The NEC uses a formula based on conductor temperature rating. cable rating and derating factor
Derate by 1% to 2% per 300 meters above 2,000m.
Use thermal backfill (sand/cement mix) to lower resistivity. 4. Altitude Derating ($K_alt$) The Physics: At high altitudes, air density is lower. Less air means less convective cooling. For cables in air (not buried), above 2,000 meters (6,500 ft), you must derate. Note: Altitude affects air cooling but NOT buried cables
Trying a 240mm² cable (base rating ~ 450A): $450A \times 0.326 = 146.7A$ (Still slightly low).
Running a 90°C XLPE cable through a 70°C boiler room reduces its capacity by 42%. A 100A cable becomes a 58A cable. 2. Grouping / Bundling Derating ($K_group$) The Physics: When cables are tied together in a tray, conduit, or bundle, they heat each other. The inner cables cannot radiate heat outward. 12 current-carrying conductors in bundle: $K_group = 0
While a cable might theoretically carry 100 Amps in a perfect, lab-controlled environment, real-world conditions—heat, sunlight, bundling, and altitude—force us to reduce that capacity. This process is called .