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

Mr. cakyol
All cooper for electrical use is 99.5% pure. Try to get one with less!
Most cables are using wires as thick as 12-14 AWG because that’s the thicker they can fit into their banana plug (Not a very sintific one).
All qualities they say they give the cables: Cooper purity, use of silver, cryogenic treatment, Bi Wire, Burn In etc’ - has nothing to do with the spec. of a speaker cable. It’s all snake oil mambo jumbo, to cover their and customers ignorance.

Mr.  sleepwalker65
The answer is with the Damping Factor (DF) of your amplifier. There are some exceptions, if he speakers are not coil loaded (ribbon, electrostatic etc'.) or if the Amp. is a class-D type. But most, use a A-B class amp with ordinary speakers.
DF is defined as the ratio between the Amp's. output resistance (Ro, in Ohms) vs. an 8 ohms speaker, @ 1kHz.
As so, an Amp. with a DF of 500 has a Ro of 8/500=0.016 ohms.
The DF is given and you get it as you buy your Amp. It is used to be the higher the DF, the better is the Amp. Some use very stiff feedback to achive it - Not so good.
Others, may have tube amps. that have very low DF as low as 8! 
The DF is actually the way the Amp. is getting control over the speaker's coil. A coil, especially a moving one in a magnetic field, generates an electric current that is equal to the one made it move, but in the opposite direction. It is called the "Lentz" law.
Speaker cables are in a way two resistors (Rc) that connect the power Amp. to the speaker. Why two? becuse the cables go to the speaker (red) and goes back (black) to the amplifier. 
As so, the speaker's impedace has no significance in this electrical circuit.
The speaker cables resistance, just need to be significantly lower than the DF.
Other wise, the DF would decrease. So if the Rc = DF you actually have a 1/2 of the DF you paid for. This guy with his  Bryston 4BSST2, with a DF of 500 is now enjoying an Amplifier equals to a poor reciever of the 70's with a DF of about 40!
In other words, he is maybe listening to less than 10% of his equipment's full potential.
The long (24') that would count as 2xRc, is actually equals to the resistance of a 48' long cable. A few more feet and you need transportation to go visit them! :-) Why so long?
A thick cable (4 AWG and thicker) is hard to make and would cost a lot. Rather than go that long with cables, it is better to get the amplifier closer, maybe use two monoblocks and guid a blanced interconnect (XLR) that can run up to 100m with no sound degradation, while cost's little. 

0 guage (AWG) wires are difficult to work with. They are thick. Too thick to solder, as by the time the tin melts, the isolator also gets burned. It is no longer as good for isloating, it exposes the cooper to oxidize and look bad. The cable itself can not fit into any banana plug on the market. An agent must be connected between the two.
A 0 AWG cable is more costly than a 14 AWG, more heavy and less flexible. However, all can be overcome with some technology. Even up to 4 times 0 AWG!
Most cables on the market (regardless of price factor) are not getting thicker than 14 AWG. For this reason.
Some would look thick, but hey are looking so because a thick isolating tube over a regular 12-14 AWG cooper core.
Some make thick cable, but charge a lot for them. This might not be a problem, for others who spend that much on a power cable, that really makes no sense at all.
The market, as for what it is, is tricky. None tels you what their cable is good for (DF, Rc, or other), a lot of urban legends are sticked to them (cryogenic, cooper purity, high end banana plugs!) and a nylone sleve that looks nice, to cover the poor materials beneath.
Price: What would you think a 0 AWG, 2.5m (8’) long speaker cable set should cost? (if guage goes thicker or cable longer, it would naturally cost more).

Did you make it to the ICU?

DeKay

"When are you gonna come down
 When are you going to land"