What size wire do I need to connect a portable generator inlet to my main panel?
As has been discussed many times in this forum, back feeding a generator into a double or even a single pole breaker without a transfer switch is risky business. If the main  breaker is not disabled while generator is online, you will be back feeding power to the grid and pose a safety hazard to utility workers.  

With that said, look at the following link, http://www.southwire.com/support/voltage-drop-calculator.htm   The wire size for an 80 run at 30 amps/120vac is 8 AWG.  Use this calculator to determine wire size or determine voltage drop. 3% drop is usually considered max for 120/240 runs. Once you start to draw amperage, the drop can become greater as you tax the voltage source with a fixed or max amp output.