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.
– Number of current-carrying conductors in a raceway or cable: | Number of Conductors | Derating Factor | | :--- | :--- | | 1 – 3 | 1.00 | | 4 – 6 | 0.80 | | 7 – 9 | 0.70 | | 10 – 20 | 0.50 | | 21 – 30 | 0.45 | | > 30 | 0.40 | cable rating and derating factor
12 current-carrying conductors in bundle: $K_group = 0.50$ (NEC 10-20 conductors) Use thermal backfill (sand/cement mix) to lower resistivity
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. Less air means less convective cooling
We must select a larger base cable. Iterate until $I_actual > 150A$.
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 .