Don't confuse household voltage(120/240V AC) with car voltage (12.5-14V DC). Its like comparing apples to oranges. With your car running, say you have 14V DC. Voltage can be related to the "pressure" pushing the current flow(like pressure pushing water out of a hose). 14 Volts of pressure isn't that much...car audio requires lots of current...add to that the fact that with DC voltage, the electrons have to flow from the battery...thru the load(stereo components, lights, etc)...then back to battery. There is a lot of resistance in the wiring. So, those are the reasons why you need humongo cables for high current draw accessories in your car(also why you need huge fuses). Now, in your house............120V AC....you have a lot more "pressure" to push the current(electrons)...also AC voltage is back and forth(sine wave)...Therefore, smaller wires and circuit breakers can be used.
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