6A is not a lot of juice. Most three-prong extension cords are rated for 13A or 15A. Even a 16 gauge extension cord 100 ft long should be able to handle 10A safely.

However, If you have a real long cord, it's good to uncoil it before drawing any significant power through it though - I once melted the insulation off a cord by running a computer & projector (Probably around 6-7A) on a (fat enough) cord on a spool (for 10 hours, though).