| Altitude (m) | Approx. Factor | | :--- | :--- | | 0 – 2,000 | 1.00 | | 2,000 – 3,000 | 0.96 | | 3,000 – 4,000 | 0.92 | | > 4,000 | Engineering judgment |
At the heart of cable selection lies two critical concepts: (Ampacity) and Derating Factors . cable rating and derating factor
Correction factors for buried cables: | Soil Resistivity (K·m/W) | Correction Factor | | :--- | :--- | | 0.7 (Wet, marsh) | 1.05 – 1.10 | | 1.0 (Moist loam) | 1.00 | | 1.5 (Dry sand) | 0.85 | | 2.0 (Very dry gravel) | 0.70 | | 3.0 (Thermal backfill required) | 0.50 | | Altitude (m) | Approx
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. The Physics: Non-linear loads (VFDs
$I_actual = 185A \times 0.71 \times 0.50 \times 0.92$ $I_actual = 185A \times 0.326$ $I_actual = 60.3A$
The NEC uses a formula based on conductor temperature rating.