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

Mr. kijank

I didn’t defined what DF is. https://en.wikipedia.org/wiki/Damping_factor
"Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive".
Loudspeaker impedance is given (when DF is kindly provided) for 8 ohms, and at a frequency of 1kHz. Just saying.
Speaker’s impedance is not just the coil. It has some RCL network in the crossover circuit, and mostly very poor wiring.
But that is a given. It sounds as you hear it on demo and buy it as a sealed box (black box).
Thats to cover your "lack of understanding of basic principles".
When analyzing such a circuitry, doing it with all 3:
Amp’s output, cables, speaker,
You get nowhere.
In electronics, DF, or output resistance is practically looking from the Amp’s output terminals into the Amp, and figure what is it’s output resistance. I extended it to include the speaker cables. This is my way to unlock the puzzle. You may go your way. However, none of you, including you, haven’t came with a say: what should your speaker cable be like?
I did.
For the sake f this endless arguing between me and those who sit on the bench and criticize, go give it a try and then lets talk. Till then, you better spend this precious time and energy, preying in the house of god. It will make you feel better and you will be pissing of one less guy.

Mr.  ebm

Thanks. 
All you have to say on the subject, is that I had typo?
It's not much from your side to bring to this table.
Just for the record, some others already paid attention and made this remark B4.
Are you the guy who mock those who stutter or have a limp?
You must be very proud of yourself.

Mr. dill

I've heard that there are people, that all there purpose in life, is to be a bad example. You fit quite well into that category.
Your place must be in politics. Audio is kind of small for you. 

Amp DF = 50
4 ohm speaker
6 ft length

Wire gauge?

Mr.  eichlerera

For DF=50 and length of 6 feet (2 m), you need a #16 AWG.

1. No need for speakers impedance. They are not in this game (speaker cable).
2. DF=5 is relatively low. It was common (40~) at Japanese amps and receivers of the 70's.