Thank you, JMS.
A personal opinion: The longer the copper-based cable run, the heavier the gauge. Try to keep the round-trip resistance of the wire near or below 3% of your speaker's nominal impedance. For 50’ of cable, a total of 100’ of wire, that means 14 or 12-gauge wire for an 8-ohm loudspeaker. For a longer run and/or lower impedance loudspeakers, this might mean dropping to 12 or 10-gauge wire, or doubling up on thinner gauges.
When using genuine tri- or bi-amplification, a higher frequency driver may have a higher nominal impedance allowing for a slightly lighter gauge wire to be used.
When the cable contains significant amounts of metals other than copper, the gauge needed may be lighter if the resistance of the cable is significantly lower than copper.
Again, this is a personal preference.
[This message has been edited by bestbang4thebuck (edited April 30, 2004).]