Cable - Calc Formula

(IEC 60364-5-52): Base cable 120 mm² Cu XLPE → 380 A in free air. Derate: (k_amb = 0.87), (k_group = 0.8) → (380 \times 0.87 \times 0.8 = 264 A) → too low. Try 185 mm² → base 500 A → derated = (500 \times 0.696 = 348 A) — acceptable.

And always, always derate for grouping.

(185 mm² Cu, R=0.106 Ω/km, X=0.078 Ω/km): [ V_d = \sqrt3 \cdot 340 \cdot 0.250 \cdot (0.106\cdot0.85 + 0.078\cdot0.527) \approx 12.9V ] Drop % = (12.9/400 = 3.2%) — within typical 5% limit. cable calc formula

For AC circuits:

…but that’s only the beginning. The steady-state ampacity of a cable is derived from the heat balance equation: (IEC 60364-5-52): Base cable 120 mm² Cu XLPE

[ S = \fracI_sc \cdot \sqrtt\sqrt\epsilon \cdot \frac1k ] In non-linear loads, harmonic currents cause extra (I^2R) losses. The effective RMS current is:

[ F_harmonic = \frac1\sqrt1 + \sum_h=3,5,7... \left(\fracI_hI_1\right)^2 \cdot h^0.5 ] | Standard | Approach | Key Features | |----------|----------|---------------| | NEC (USA) | Table-based | Ampacity tables, 60/75/90°C columns, adjustment factors | | IEC 60287 | Calculation | Explicit thermal resistance model (Rth), for complex installations | | BS 7671 | Mixed | Simplified tables + voltage drop formula | | IEEE 835 | Calculation | For large power cables, includes soil drying effects | And always, always derate for grouping

: 185 mm² Cu XLPE. 6. Advanced Nuances 6.1 Skin & Proximity Effect For large conductors (>240 mm²), AC resistance exceeds DC resistance. The ratio (R_ac/R_dc) is given by: