Hell the cord that comes with most ballasts is like 15 foot long and only 16 or 14 gauge and generally cheap as fuck. If you have a cord that is even the least bit warm to the touch anywhere, replace it immediately.
Yeah they are often 16 gauge, but even a 1000 watt ballast running at 120 volts is pulling say 9 amps, thats well within the tolerance of a 16 gauge wire (I wanna say thats 10 amps on the conservative side 13 on the high side) that is dedicated to that single load. Now with extension cords, folks tend to plug in more ballasts and there is when you get into issues. The simple way to move more power is to increase the voltage. This is why I prefer to run all my lighting loads at 240 volts, this not only drops the amperage by 50%, it balances the loads between phases, keep that current off that neutral whenever possible I say.
I remember back in the day when we would rent grow houses and run them for a cycle or two and move. We would find circuits that were only receptacles (no ceiling fans or lighting or appliances) pull those neutrals and put that circuit on a double pole breaker making all those receptacles 240 volts. When we left we just switched it back. That really saved a lot of extension cords. "Freedom boards" were big then, just a dryer cord wired into an intermatic double pole timer and some receptacles for ballasts.