No one actually knows how to lculate what speaker cable they need

I needed a 24-foot run from my Bryston 4BSST2 amplifier to my Thiel 2.7 speakers. (4 ohm,  down to 2.6 ohm @ 160 cps.)

I chose Belden 5T00UP 10 gage cables. (1.03 ohm resistance /1000  feet; 26pF capacitance per foot)

Blue Jeans Cable made them up for me, with ultrasonic welded spades for termination. 

Couldn't be happier. Excellent bass, midrange and treble for my classical music CDs. 

Disclaimer: I'm a retired electrical engineer. 

Remember guys, don't feed the Trolls.......

Any MULTI strand 99.5% pure copper (NOT clad) wire will work.
The thicker the better.  12 gauge and thicker will easily 
handle up to 1000 watts/channel rms with no problems.

Simple rule, the thicker the better, the purer the copper the better.
12/10 gauge & thicker will easily handle up to 1000 watts rms amps 
with no problems.

Simple math. Take into account the power demands of the speakers to achieve the desired SPL. Then the power output of your amplifier, and subtract the two. This gives you the margin that can be lost to resistance in the speaker cable, P=I^2*R. You know the power needed to drive your speakers, and you know the speaker average impedance, which gives you the amount of current by plugging in the power and approximating resistance with average impedance. Take the power margin from above, and thus current value and use the same equation to solve for resistance of the cable. Divide this resistance by the distance in feet to obtain maximum ohms per foot of cable, and go shopping!