Loudspeaker Cable Choice
Damping Factor
The damping factor refers to the ratio of nominal loudspeaker impedance to the total impedance driving the loudspeaker. This total impedance is the sum of the amplifier and the speaker cable. The damping is the ability of the amplifier to control the loudspeaker's motion after the signal has stopped. It is the ability of the amplifier to control the loudspeaker's load.
The longer a loudspeaker moves uncontrolled after the signal has stopped the more this is audible as "overhang" or "one note bass". The 'overhang' frequency is normally the system's resonance frequency. With a high damping factor the impedance of the amplifier can absorb the electricity generated by the speaker coil motion and minimize the speaker's 'overhang' vibration.
Highly damped loudspeakers sound tighter in the low end. The effect of a lower damping factor usually is less controlled bass and unpredictable interaction in areas where the crossover is complex.
The contact resistance of bad loudspeaker cable connections can easily exceed the loudspeaker cable’s low resistance and hamper the damping factor.
Operation of amplifiers at 2 Ohms reduces the sound quality and the system efficiency dramatically. The loss on the loudspeaker cable reduces the damping factor at 8 Ohms by about 70% when the amplifier operates at 2 Ohms! Almost all amplifiers on the market operating at 2 Ohms use use destructive protections to keep the components safe, that reduce the sound quality.
Even with a very low output impedance of the amplifier, practical damping factors are mostly not better than 50, due to speaker cable and contact resistances.
Loudspeakers should generally not be wired in series for sound reinforcement applications.
The shorter the loudspeaker cable, the more intimate is the connection between amplifier and loudspeaker. Subtle differences between different cables become more dramatic with increasing length and shrink toward zero as the cable gets shorter.
Damping factor, for instance:
A loudspeaker has an impedance of 4 Ohms,
the used wire has a resistance of 0,5 Ohms,
the power amplifier has an output impedance of 0.05 ohms.
The damping factor would be 4 / 0.55 = 7.27
Max. Length of Loudspeaker Cables for a Damping Factor larger than 50
Source: Marty McCann
AWG

Square mm

Diameter mm

Square Inch

Meter / Ohm

Feet / Ohm

max. Power

max. cable length with damping factor larger 50

18

0.82

1.02

0.001276

46.8

153.6

150 W

3 m

10 ft

16

1.31

1.29

0.002028

74.5

244.3

280 W

4.6 m

15 ft

14

2.08

1.63

0.003226

118.4

388.4

400 W

7.6 m

25 ft

12

3.31

2.05

0.005129

188.3

617.7

800 W

12 m

40 ft

10

5.26

2.59

0.008155

299.5

982.3

2000 W

20 m

65 ft

Speaker Cable Suggested Conductor Area 
Power Amp  Cable Run 
Watts 
5 meters 
10 meters 
20 meters 
1040 
1 mm2 
1 mm2 
1.5 mm2 
50100 
1.5 mm2 
2 mm2 
2.5 mm2 
100500 
2.5 mm2 
4 mm2 
4 mm2 
5001000 
4 mm2 
6 mm2 
8 mm2 
Damping Factor, by 'Community'
Another way to select the proper speaker cable dimensions
Calculate 75% of the minimum impedance of the speaker system.
(8 Ohm speaker, 75% == 6)
Take the length of the speaker cable and multiply it by 200.
(100 feet times 200 == 20.000)
Divide the 75% impedance by the number just calculated.
(6 / 20.000 = 0.0003)
This is the desired resistance per foot (Ohms) of the cable.
The following table gives the proper wire size:
(Pick a wire whose resistance per foot is less than the number just calculated)
American Wire Gauge
American Wire Gauge 
Resistance per Feet (Ohms) 
Cable Diameter (mm) 
4 
0.000292 
5.18 
6 
0.000465 
4.11 
8 
0.000739 
3.25 
10 
0.00118 
2.59 
12 
0.00187 
2.06 
14 
0.00297 
1.63 
16 
0.00473 
1.30 
18 
0.00751 
1.02 
20 
0.0119 
0.81 
22 
0 .0190 
0.64 
24 
0.0302 
0.51 
26 
0.0480 
0.41 
28 
0 .0480 
0.33 