Taken from a pro amplifier owner's manual:
PL /ft DCR/ft
10 0.0191 0.00204
12 0.0303 0.00324
14 0.0482 0.00515
16 0.0767 0.00819
18 0.1218 0.01302
Power Loss with an 8 ohm Load:
To calculate the total power loss in the speaker cable, multiply the power loss per foot of the cable gauge selected in the above table, by the length of the cable. For example, suppose 160 feet of 10 GA cable with the 8 ohm speaker impedance. The total power loss in the cable is:
0.0191 watts/foot x 160 feet = 3 watts
Does this mean that whenever the amplifier produces 75 watts of output power, 72 watts will be delivered to the 8 ohm speaker? NO! The actual load impedance is 8 ohms plus the resistance of the cable (0.00204 ohms/foot times 160 feet) for a total load impedance of 8.3264 ohms. At the 8 ohm output, the output voltage is 24.5 V rms. Therefore, the amplifier produces 72 watts and the actual power delivered to the speaker is only 69 watts (72 w minus 3 w). This was calculated by squaring the voltage and dividing by the load impedance.
Now, had 16 GA wire been used in the above example, the loss would have been 12.27 watts!
To calculate power loss with 4 ohm loads, multiply the loss in the above table by 3. In the above example, the 10 GA cable would consume 9 watts of power while the 18 GA wire would waste 58.5 watts, more than half of the amplifier power output.
These examples illustrate the importance of using the proper wire size.
