The exact DC/AC formula for single-phase:
Where each (k) is <1 (or rarely >1). A real-world example:
[ Z_loop \times I_a \leq U_0 ]
For three-phase:
The correct size is the smallest cross-section that satisfies checks simultaneously – not the largest, and certainly not the first guess from a table. cable selection calculation
Next time you see a burnt cable in a panel, ask: was it undersized, or was it sized for 30°C free air and installed in a 50°C bundled tray? The answer is rarely just "too much current."
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: The exact DC/AC formula for single-phase: Where each
The condition: