I recently bought a 25-foot long "lightweight" extension cord (which I assume is likely 16 gauge) for stuffing into the frame tube. Anything heavier and I would not be able to fold the cord up and I wanted the longest cord I could carry on the bike without taking up any space should I ever have a charging emergency and couldn't get too close to the outlet. I measured 122 volts at the wall and 118 volts at the end of the cord, when connected to my charger. The cord near the plug got slightly warm, but it was not very obvious. In any case, the cord was rated for 15 amps, which is a lot higher than the 8 amps that my charger pulls. Having said that, 14 and 12 gauge extension cords are readily available at any hardware store in 10 or 12 foot lengths for not too much money. I believe the cord that you get with your bike from Zero is 14 gauge, at least that is what I got with my 2012 model.
The Zero owner's manual recommends a 12-gauge extension cord not longer than 25-feet for charging my 2012 S.