[ I_rms = \sqrtI_1^2 + I_2^2 + ... + I_h^2 ] And derating factor:
[ 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 | cable calc formula
For AC circuits:
: 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: [ I_rms = \sqrtI_1^2 + I_2^2 +
[ I = \sqrt\frac\theta_max - \theta_ambR_dc \cdot \left(1 + \alpha(\theta_max - 20)\right) \cdot \left( R_th \right) ] [ R_ac = R_dc \left(1 + y_s +
(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.
[ R_ac = R_dc \left(1 + y_s + y_p\right) ] Where (y_s) (skin) and (y_p) (proximity) depend on frequency and conductor spacing. For longer faults (>0.5s), the heat conducts into insulation. Use IEC 60949’s iterative method, which adds a factor (\epsilon):