The maximum amps for power transmission for a 20-gauge American Wire Gauge (AWG) wire is 1.5. This is based on a guideline from the Handbook of Electronic Tables and Formulas for the ampacity or the copper wire current carrying capacity for American Wire Gauge sizes.
The maximum current transmission capacity uses 700 mils per amp rule, which means that the estimate is very, very conservative. The rating should only be taken as a rule of thumb and not as an exact measurement. There are several factors that determine the actual amp capacity of any AWG and these factors include thermal conductivity, insulation temperature limit, voltage drop, air convection and temperature.