We often treat cable sizing as a simple lookup: "10 amps? Use 1.5mm²." But in the real world—where ambient temperatures hit 50°C in a rooftop conduit, where harmonic currents distort neutrals, and where voltage drop starves a motor 400 meters away—that naive approach fails.
[ A_min = \sqrt\fracI_sc^2 \times tk ]
That means a cable rated for 100A in free air can only carry in this installation. This is why we oversize. The Grouping Trap (k₂) The most dangerous oversight. According to Neher-McGrath (the foundation of both IEC and NEC ampacity tables), the derating for (n) equally loaded cables is severe: cable selection calculation
[ A_min = \sqrt\frac10000^2 \times 0.4115 \approx \sqrt\frac40 \times 10^6115 \approx \sqrt347826 \approx 590 \text mm^2 ] We often treat cable sizing as a simple lookup: "10 amps
The condition:
