Automotive wiring operates at 12V DC (or 24V for heavy trucks), which means higher current draw than household circuits for the same wattage. Wire sizing is critical because voltage drop at 12V is much more significant — a 0.5V drop is over 4% at 12V but less than 0.5% at 120V.
Instrument panel lights and indicators: 18-20 AWG. Interior lights, turn signals: 16 AWG. Headlights, fog lights: 14 AWG. Horns, windshield wipers, fuel pumps: 12 AWG. High-output stereo amplifiers: 8-4 AWG. Winches and high-draw accessories: 4-2 AWG. Battery cables and ground straps: 4 AWG to 1/0 AWG.
At 12V DC, a 120W headlight draws 10A (vs 1A at 120V AC). This is why automotive wire for even modest loads is thicker than you'd expect from household wiring. Always calculate wire size based on current draw, not wattage alone.
Use automotive-grade wire (SAE J1128 GPT or SXL) — it's designed for vibration, temperature extremes, and oil/fuel resistance. Household NM-B wire should never be used in vehicles. Marine-grade tinned copper wire (SAE J1127) is preferred for boats and harsh environments.
Every automotive circuit must be fused. The fuse protects the wire, not the device — size the fuse to the wire's ampacity, not the load. A 14 AWG wire should use a 15A fuse maximum, regardless of what it's powering.
Disclaimer: This guide is for informational purposes only. Always consult a licensed electrician and follow local building codes for actual installations. Wire sizing depends on specific conditions including ambient temperature, conduit fill, and derating factors.