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

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!

@b4icu said:

For the 3x0 AWG suggestion: My suggestion was for 3 AWG at 8’.
You require 20’. That increases the cable resistance by 3 and if you go to the guage table it calls for a 3 times 0 AWG to keep it the same as a 3 AWG of 8’ long.
That’s Ohms law.

@b4icu, you are sadly mistaken. This quote demonstrates your ignorance of the electrical “engineering” subject matter that you profess expertise in.

For your information, Ohm’s Law states that resistance (in Ohms) is equal to the potential difference (in Volts) measured across a conductor divided by the current (in Amperes) flowing through it.