Cable Selection Calculation May 2026

| Number of circuits | k₂ (touching, in air) | |-------------------|------------------------| | 1 | 1.0 | | 2 | 0.80 | | 3 | 0.70 | | 4 | 0.65 | | 5 | 0.60 | | 6 | 0.57 | | 8 | 0.52 |

Yes – a 10kA fault requires a cable just to survive 0.4 seconds. In practice, faster protection (e.g., current-limiting fuses with 0.01s clearing) reduces this dramatically. Earth Fault Loop Impedance: Ensuring the Fuse Blows This is the most commonly skipped calculation, and it's a killer. The cable's earth conductor (or armour, or separate PE) must have low enough impedance that a phase-to-earth fault draws enough current to trip the protective device within the required time (0.4s for final circuits per IEC 60364). cable selection calculation

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: | Number of circuits | k₂ (touching, in

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." The cable's earth conductor (or armour, or separate