How it's calculated

Single-phase: Vd = 2 × I × L × ρ ÷ A Three-phase: Vd = √3 × I × L × ρ ÷ A ρ(T) = ρ20 × (1 + α(T − 20))

This uses conductor resistance only, which is accurate for small and medium cables. Above about 25 mm², cable reactance adds noticeably to the drop, so use manufacturer mV/A/m tables for large cables.

70 °C is the maximum operating temperature of PVC cable, a conservative default. Use 90 °C for XLPE/SWA at full load.
UK (BS 7671 Appendix 4): typical limits are 3% for lighting and 5% for other uses, from the origin of the installation.
US (NEC informational notes): 3% on a branch circuit, 5% total.

Voltage drop is only one part of cable selection. Current-carrying capacity, installation method, grouping and fault protection must also be checked.