[ I_rms = \sqrtI_1^2 + I_2^2 + ... + I_h^2 ] And derating factor:

: [ S_min = \frac25,000 \cdot \sqrt0.2143 \approx \frac25,000 \cdot 0.447143 \approx 78 , mm^2 ] 185 mm² >> 78 mm² — thermal withstand OK.

[ V_d = \sqrt3 \cdot I \cdot L \cdot (R \cos\phi + X \sin\phi) \quad \text(3-phase) ]

[ 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):

[ V_d = 2 \cdot I \cdot L \cdot (R \cos\phi + X \sin\phi) \quad \text(single-phase) ]

[ \boxedS = \max\left( S_ampacity, S_V_d, S_short-circuit \right) ]