: Select ( I_n = 100A ) (circuit breaker).
[ I_z = I_ref \times k_temp \times k_group \times k_soil \times k_depth \times k_therm ] how to calculate cable size
: Voltage drop. For 50 mm², AC resistance at 70°C ≈ 0.494 Ω/km, reactance ≈ 0.088 Ω/km. Three-phase drop: [ V_d = \sqrt3 \times 90.3 \times (0.494 \times 0.85 + 0.088 \times \sin(\cos^-10.85)) \times 0.120 ] sinϕ = 0.527. R term = 0.494×0.85 = 0.4199 X term = 0.088×0.527 = 0.0464 Sum = 0.4663 Ω/km per phase. ( V_d = 1.732 \times 90.3 \times 0.4663 \times 0.120 = 8.74V ) Percent = 8.74/400×100 = 2.18% < 3% OK. : Select ( I_n = 100A ) (circuit breaker)
: Short-circuit withstand. ( k=115 ). ( S_min = \sqrt10000^2 \times 0.4/115 = \sqrt40\times 10^6/115 = 6324/115 = 55 mm^2 ). Required minimum 70 mm². Our 50 mm² fails. Therefore increase to 70 mm². Three-phase drop: [ V_d = \sqrt3 \times 90
For a three-phase circuit (line-to-line): [ V_d = \sqrt3 \times I_b \times (R \cos\phi + X \sin\phi) \times L ]