Volt drop is the voltage lost as current flows through the resistance of the line conductor — it does not involve the earth or CPC conductor, which only carries current during a fault. BS 7671 limits volt drop to 3% of supply voltage for lighting circuits and 5% for power circuits, measured from the installation origin to the furthest point. A result within 1V of the limit is flagged as a caution — the cable passes, but the margin is too thin for comfort and the next size up is worth specifying.
For single-phase circuits, current travels out via the line and returns via the neutral, so the effective resistance is doubled over the cable length. For three-phase balanced circuits, the neutral currents cancel and only the line conductor resistance counts. Resistance values are from BS 7671 Table B1 and manufacturer data for copper conductors at 20°C. The temperature correction factor adjusts for ambient conditions — copper resistance rises with temperature, increasing volt drop proportionally.