Wire Gauge / Ampacity Calculator
Find the minimum wire gauge that satisfies both current capacity and voltage drop limits for any automotive circuit.
System voltage:
Max voltage drop:
Minimum Recommended Gauge
8 AWG
Max Ampacity
73A
Voltage Drop
0.251V (2.09%)
Ω/1000 ft
0.628
AWG Reference Table — 20A, 10ft, 12V
| Gauge | Max A | VDrop | VD% | Typical use |
|---|---|---|---|---|
| 20 AWG | 11 | 4.060V | 33.83% | Sensors, low-draw signals |
| 18 AWG | 16 | 2.554V | 21.28% | Gauges, interior lighting |
| 16 AWG | 22 | 1.606V | 13.39% | Relays, small accessories |
| 14 AWG | 32 | 1.010V | 8.42% | Headlights, horns, fans (small) |
| 12 AWG | 41 | 0.635V | 5.29% | Large fans, audio amps |
| 10 AWG | 55 | 0.400V | 3.33% | High-current accessories, EFI feeds |
| 8 AWG ✓ | 73 | 0.251V | 2.09% | Starter secondary, large audio |
| 6 AWG | 101 | 0.158V | 1.32% | Alternator output, starter motor |
| 4 AWG | 135 | 0.100V | 0.83% | Battery cables (short), 100A+ systems |
| 2 AWG | 181 | 0.062V | 0.52% | Battery cables, high-output alternators |
| 1 AWG | 211 | 0.050V | 0.41% | Heavy battery/ground cables |
| 1/0 AWG | 245 | 0.039V | 0.33% | Main power distribution, audio mains |
| 2/0 AWG | 283 | 0.031V | 0.26% | Dual battery systems, serious audio |
| 3/0 AWG | 328 | 0.025V | 0.21% | Race car main power harness |
| 4/0 AWG | 380 | 0.020V | 0.16% | Maximum automotive use; welder-level current |
Formula: VD = (2 × L × I × R) / 1000
• The 2× factor accounts for both the positive run and the ground/return path (same gauge assumed for both).
• 2% max drop for critical circuits (starter, fuel pump, EFI). 3% acceptable for most accessories.
• Ampacity is for 60°C rated automotive primary wire in open air. Bundled runs in conduit require derating.
• Always fuse at or below the wire's ampacity — the fuse protects the wire, not the component.