AWG conductor wire sizing & voltage drop

$$V_{\text{drop}}=\frac{2\cdot L\cdot I\cdot R_{\text{wire}}}{1000},\quad \%V_{\text{drop}}=\frac{V_{\text{drop}}}{V_{\text{source}}}\times100$$

$R_{wire}$ in Ω/1000 ft (copper), $L$ one-way length in ft. Source: Engineering ToolBox, NEC (NFPA 70).