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).
$$I = \fracPV \times \textPower Factor$$ how to calculate cable sizes
Let's assume standard installation conditions (no derating). Looking at a standard Copper/PVC table: Increase to 4 mm² (( R_ac ≈ 5