The rated current of the protective device (Breaker/Fuse) must be greater than $I_b$.
| Mistake | Consequence | |---------|--------------| | Ignoring ambient temp/grouping | Overheating, insulation failure | | Using DC resistance for AC | Underestimates voltage drop (skin effect) | | Forgetting neutral current (harmonics) | Overloaded neutral in 3-phase 4-wire | | No short-circuit check | Cable vaporizes under fault | | Wrong (k) factor (e.g., PVC vs XLPE) | Undersized for fault duration |
The most commonly used formula for cable sizing is:
[ I_b = \fracP\sqrt3 \cdot V_L \cdot \cos\phi \quad \text(3-phase) ] [ I_b = \fracPV \cdot \cos\phi \quad \text(single-phase) ] Where:
This is the first filter. The cable must handle the steady-state load current ($I_b$) without exceeding its maximum operating temperature (typically 70°C for PVC, 90°C for XLPE).
