Increase to 4 mm² (( R_ac ≈ 5.62 \text Ω/km )). Drop = ( 2 \times 40 \times 21.74 \times 5.62 / 1000 = 9.77V ) → 4.25% (still over). Increase to 6 mm² (( R_ac ≈ 3.71 )). Drop = ( 2 \times 40 \times 21.74 \times 3.71 / 1000 = 6.45V ) → 2.8% (acceptable).
[ I_b \leq I_n \leq I_z ]
Voltage drop. For 2.5 mm² copper, ( R_ac ≈ 8.91 \text Ω/km ). Single phase drop = ( \frac2 \times 40 \times 21.74 \times 8.911000 = 15.5V ). 15.5V / 230V = 6.7% → exceeds 3% limit (6.9V max). how to calculate cable sizes
References: IEC 60364-5-52, BS 7671 (UK), NEC Article 310 (US), AS/NZS 3008 (Australia). Increase to 4 mm² (( R_ac ≈ 5
Where ( I_z ) = derated cable ampacity. Load: 5 kW heater, 230V AC, power factor = 1. ( I_b = 5000 / 230 = 21.74 A ) Drop = ( 2 \times 40 \times 21