You're likely asking about for a solid conductor (single solid metal wire), as opposed to stranded wire.
Example: 12 AWG solid copper, (R \approx 1.588\ \Omega/1000ft) at 75°C, 100 ft, 20 A: [ V_d = 2 \times 100 \times 20 \times 1.588 / 1000 = 6.35\ V \ (\approx 5.3%\ \texton 120V) ] Aim for < 3%. For very high frequencies (kHz to MHz) → skin effect forces you to use AC resistance: [ R_ac = R_dc \times \frack \cdot r2 ] …but at 60 Hz, for solid wire up to ~4 AWG, (R_ac \approx R_dc). cable calculations
A simplified thermal model:
[ I = \sqrt\fracT_max - T_ambR_dc \cdot R_th ] You're likely asking about for a solid conductor