Since you’re running this amp from the car power system (which is usually around 13.5V when running), you actually won’t get much power output from the amp, unfortunately. I have owned this amp for over a year now and have done extensive testing on it, and have found that the actual power output from each channel (when driving a 4 ohm resistive load) for several different power supply voltages is: - 30V supply: 60W/channel - 24V supply: 39W/channel - 18V supply: 23W/channel - 12V supply: 13W/channel
I know that this is quite disappointing since the marketing material for this amp claims 100W/channel, but that’s just the way it goes. This amplifier has no power regulator built into it, so the supply voltage will limit the actual output power, as we see above. So, even though the description for this amp says that it can be powered by supply voltages of 10V - 30V, you will get very different power outputs from those ranges of supply voltages. Also note that even at its highest rated supply voltage of 30V, it only puts out 60W into a 4 ohm load, rather than the 100W that it claims. I have found that this is for two reasons: 1. Internal inefficiencies of the amp limit the current output at high output levels; 2. There is a diode on the board on the supply voltage line that prevents you from damaging the amp if the supply voltage is hooked up in the wrong polarity. But, like all diodes, it has a forward voltage drop, and when this amp is being driven hard this voltage drop is around 1V, so even though the supply voltage is 30V, the amp itself is only seeing about 29V. So if that diode weren’t there and if there were no internal inefficiencies in the amp, then I could see it putting out 100W/channel (to 4 ohm loads) with a 30V supply, but unfortunately that’s not physically possible.
(On a side note, if you’re wondering, most car amps (such as your 500W amp) have built-in power regulators, which allows them to produce very high power outputs with just the 12V supply from the car. They do this by implementing a large DC-to-DC converter to bring the 12V supply up to something higher. This Sure amp (as well as all the other Sure amps) does not have any kind of DC-to-DC up-converter, hence the low power outputs at low voltages.)
As such, this amp is going to be drawing very little power, even at full volume. With your 12V car power supply, you’ll get 13W x 4 = 52W total output power. Factor in the small inefficiencies of the amp, and you’ll find that this amp will NEVER draw more than about 60W or 70W when powered by a 12V power supply, and that’s assuming that all channels are outputting pure sinewaves. In a real-world situation, where the amp is outputting typical music, which has lots of quiet sections and rarely hits peak output power, you could expect the amp to typically draw 30W, with peaks up to about 40W or 50W (at full volume, that is).
So, which wire gauge should you use to power this amp? As you can see, it barely matters. A 50W peak will only be about 4A at 12V, so that 8AWG wire you’re using for the sub amp will have absolutely no problem powering this amp as well.