If 14 gauge wire can supply 120 volts to three 60 watt light bulbs
Then why do some vehicles require 3 power wires for one light bulb ( 2 for HID lighting for the ballast and one for the parking light ) with 2 grounds, couldn't they just use one ground and one supply that could hold the current, then split at the connector for the internals ?
DC current flows from negative to positive. The starter only needs one large gauge red wire from battery positive, ground is via battery negative to chassis and engine block. The alternator only needs a fusible link wire for power, fused to blow around 25 amps or more That's input power from ground. Output power is thru frame ground to supply all the electrical needs of a vehicle while recharging the battery, anywhere from 125 amps to 250 amps plus for heavy duty trucks and police vehicles. Ground is where current flows. HID lights requires two wires; the ballast starting current can jump to 10 amps+ momentarily then drop back to running current, approximately 3 amps or so. A a separate power wire to parking lights still uses a separate ground to the chassis so lighting won't dim when hid lights startup. DC circuitry can be complicated and I'm still learning about it myself.
