soory to sound numb but why do you need such a heavy duty cable for an item running 200 watts from 12v? Domestic electricity (eg hoover!) runs at 1000w on 240 volts on smaller diameter cable??
Never mind I've just worked it out!

A domestic hoover at 1200 watts (240v) draws 5 amps!
a 200 watt amp at only 12volts draw 16 amps, hence the need for bigger cables!!!
No wonder you need bigger batteries and alternators to run everything!!!