Originally Posted by meierlde
... for lamps, clocks, up to 7 amps total up to 25 feet 18 guage is ok. 16 for 50 feet etc.
Yes. The major consideration for cords is voltage drop, which is linear with specifid resistance, linear with current and linear with length. Incandescent light bulbs just get dim with low voltage, but trying to operate a motor below its rated voltage can be problematic. Specific resistance goes down with increasing wire diameter, which allows length to go up.
There is actually no such thing as an "amp rating" for a piece of wire. Carrying capacity depends on its environment (free or packed in with other wires, for example), type of insulation (heat resistance), and ambient temperature. The code sort of wraps all of these issues together to develop a simple-to-use concept called "ampacity," but it is really a one-size-fits-none kind of concept.
Interesting Factoid: The conductors in high-voltage distribution systems (aka "the grid") actually heat up enough that wire sagging limits current carrying capacity in the summer.