Wire Gauge Calculator — AWG by Amps & Length for Truck Horn
The right gauge for your compressor or solenoid circuit. 12V/24V voltage drop, ABYC-compliant ampacity, fuse sizing — all in one calculator.
Quick pick — common train horn loads
Continuous amp draw of the device
Battery to device (not including return)
3% is industry standard; 5% is OK for low-priority
Max allowed drop: 0.36 V at 3% of 12V.
Minimum wire gauge
6AWG
At this gauge, 30A over a 10 ft run drops 0.24V (2.0%) — within your 3% limit.
Recommended fuse: 40A (at 125% of continuous load).
All gauges compared
| AWG | V drop | % of 12V | Ampacity | Verdict |
|---|---|---|---|---|
| 20 | 6.09V | 50.7% | 7.5A | Drop high |
| 18 | 3.83V | 31.9% | 10A | Drop high |
| 16 | 2.41V | 20.1% | 15A | Drop high |
| 14 | 1.51V | 12.6% | 20A | Drop high |
| 12 | 0.95V | 7.9% | 25A | Drop high |
| 10 | 0.60V | 5.0% | 35A | Drop high |
| 8 | 0.38V | 3.1% | 50A | Drop high |
| 6 | 0.24V | 2.0% | 70A | Recommended |
| 4 | 0.15V | 1.2% | 95A | OK |
| 2 | 0.09V | 0.8% | 130A | OK |
| 1/0 | 0.06V | 0.5% | 170A | OK |
| 2/0 | 0.05V | 0.4% | 195A | OK |
| 4/0 | 0.03V | 0.2% | 260A | OK |
Ampacity values from ABYC E-11, bundled engine-bay wire @ 30 °C ambient, 105 °C insulation. Single-run chassis wire can safely carry more (up to 30% higher).
The math
- Voltage drop formula
- V = 2 × I × L × R / 1000
- Round-trip conductor length
- 20 ft
- Copper resistance @ recommended gauge
- 0.395 Ω / 1000 ft
- Recommended fuse (1.25× load)
- 40 A
The formula doubles the one-way run length because current flows through both the supply and ground conductors. For chassis-ground installs, the return path is still steel with non-trivial resistance — running a dedicated ground wire the same gauge as the supply cures most mystery voltage-drop complaints on a compressor install. Resistance rises roughly 0.4% per °C above 20 °C, so in a 140 °F engine bay add another ~15% on the drop number.
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