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Wire Gauge & Voltage Drop
Undersized cable is the cause of most “my lights are dim / my pump is weak” faults. Enter the load and run length — get the smallest correct gauge and a fuse to match.
6 AWG
Recommended cable
9.72 mm²
Minimum copper area
0.27 V
Actual drop (2.3%)
35 A
Recommended fuse (125%)
| Gauge | Area | Drop @ 4 m | Drop % |
|---|---|---|---|
| 18 AWG | 0.82 mm² | 4.34 V | 36.2% |
| 16 AWG | 1.31 mm² | 2.74 V | 22.8% |
| 14 AWG | 2.08 mm² | 1.72 V | 14.4% |
| 12 AWG | 3.31 mm² | 1.09 V | 9.1% |
| 10 AWG | 5.26 mm² | 0.68 V | 5.7% |
| 8 AWG | 8.37 mm² | 0.43 V | 3.6% |
| 6 AWG ← | 13.3 mm² | 0.27 V | 2.3% |
| 4 AWG | 21.2 mm² | 0.17 V | 1.4% |
| 2 AWG | 33.6 mm² | 0.11 V | 0.9% |
| 1 AWG | 42.4 mm² | 0.08 V | 0.7% |
| 1/0 AWG | 53.5 mm² | 0.07 V | 0.6% |
| 2/0 AWG | 67.4 mm² | 0.05 V | 0.4% |
How this works: the cable must drop no more than 0.36 V (25 A × 1.35 Ω/km × 2 × run length ÷ 1000).
Assumes a solid return path (chassis ground is fine for short runs; long runs should add a matching ground cable and use its length too).
Good to know
- 3% total voltage drop is the usual target for general 12 V loads; sensitive electronics prefer 2%.
- Fuses protect the WIRE, not the device — always size the fuse to the cable and use 125% of the load as a starting point.
- For charging circuits (alternator → battery), keep runs short and gauges generous — every 0.1 V matters there.
- Always use proper automotive cable with heat-resistant insulation, and crimp — don’t twist-and-tape.