I'm supplying electric to a remote building with no other power source. The generator supplies 240v 30 amp. How far away can I keep the generator outside with a 10 gauge line connecting to a service panel? What if I use 8 gauge? The building is used infrequently and I run an air compressor a few lights and occasional shop tool.

How far away? Is that a trick question? I'd keep less than 60 feet for the 10 gauge... and that's for occasional use. If you go the 8 gauge, 100 feet would be about the max... same load situation. The draw of the air compressor motor while starting would be the biggest issue. The closer you can put the generator to the building is the best way to go, in cost and efficiency. Is this a hard wired project or are you using a cord? 4 wire cords get a bit pricy.

For 30 amps

#10 wire = 40'
#8 wire = 64'
#6 wire =101.7

3% drop is the most you should have in your run. Here's a calculator I have used to help with the calculation. 404page.html

Personally, I would run 4ga copper. It's expensive but is rated at 100 amps and would not offer much voltage drop. If you don't want to use copper because of the expense, run 2/0 aluminum.