What size wire for 40 amps at 12V?
40 A over 10 ft one way at 12 V with a 3% drop limit needs 6 AWG copper. By current rating alone 8 AWG would do (50 A), so longer runs are decided by voltage drop and need thicker wire. The table gives every length from 5 to 50 ft.
Use 6 AWG (13.3 mm²) copper or larger.
Voltage drop sets the size. 8 AWG would drop 4.19%, over your 3% limit. Ampacity alone would allow 8 AWG.
Check: 6 AWG (13.3 mm²)
| Load current | 40 A |
| Wire in the circuit (out and back) | 2 x 10 ft = 20 ft |
| Resistance of 6 AWG | 0.3951 ohm per 1000 ft; 0.0079 ohm for this circuit |
| Voltage drop | 0.316 V = 2.63% within 3% |
| Voltage at the load | 11.68 V |
| Power lost in the wire | 12.6 W |
| Ampacity of 6 AWG | 65 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 40 x 2 x 10 x 0.3951 / 1000 = 0.316 V, and 0.316 / 12 = 2.63%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 5.108 | 42.57 | 6.89 | 10* | drop and current |
| 16 | 1.31 | 3.213 | 26.77 | 8.79 | 13* | drop and current |
| 14 | 2.08 | 2.02 | 16.83 | 9.98 | 20 | drop and current |
| 12 | 3.31 | 1.27 | 10.59 | 10.73 | 25 | drop and current |
| 10 | 5.26 | 0.799 | 6.66 | 11.2 | 35 | drop and current |
| 8 | 8.37 | 0.503 | 4.19 | 11.5 | 50 | too much drop |
| 6 | 13.3 | 0.316 | 2.63 | 11.68 | 65 | smallest that passes |
| 4 | 21.2 | 0.199 | 1.66 | 11.8 | 85 | passes |
| 3 | 26.7 | 0.158 | 1.31 | 11.84 | 100 | passes |
| 2 | 33.6 | 0.125 | 1.04 | 11.87 | 115 | passes |
| 1 | 42.4 | 0.099 | 0.83 | 11.9 | 130 | passes |
| 1/0 | 53.5 | 0.079 | 0.66 | 11.92 | 150 | passes |
| 2/0 | 67.4 | 0.062 | 0.52 | 11.94 | 175 | passes |
| 3/0 | 85 | 0.049 | 0.41 | 11.95 | 200 | passes |
| 4/0 | 107 | 0.039 | 0.33 | 11.96 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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40 A wire size by length
| One-way length | 12V, 3% | 12V, 10% | 24V, 3% |
|---|---|---|---|
| 5 ft (1.5 m) | 8 | 8* | 8* |
| 10 ft (3 m) | 6 | 8* | 8 |
| 15 ft (4.6 m) | 4 | 8* | 6 |
| 20 ft (6.1 m) | 3 | 8 | 6 |
| 25 ft (7.6 m) | 2 | 6 | 4 |
| 30 ft (9.1 m) | 1 | 6 | 4 |
| 40 ft (12.2 m) | 1/0 | 4 | 3 |
| 50 ft (15.2 m) | 2/0 | 4 | 2 |
Copper, minimum AWG. * ampacity, not voltage drop, sets this size. "over 4/0" means no single copper conductor up to 4/0 passes: shorten the run, use a higher system voltage or run conductors in parallel. Length is one way; the math uses twice that, out and back.
Worked example
40 A at 12 V, 10 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 10 = 20 ft of wire, and the allowed drop is 12 x 3 / 100 =
0.36 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 8 AWG:
40 x 20 x 0.6282 / 1000 = 0.503 V, which is 4.19% of 12 V; over the 3% limit. - 6 AWG:
40 x 20 x 0.3951 / 1000 = 0.316 V, which is 2.63% of 12 V; passes both checks.
Answer: 6 AWG (13.3 mm²), rated 65 A, the smallest size that passes both.
Same current at 24V
At 24 V the same 40 A may drop twice as many volts for the same 3%, so a 10 ft run needs 8 AWG instead of 6 AWG. The current rating does not change with voltage, so the ampacity floor stays the same. See the 24V chart.
Before you buy wire
- Ampacity figures for 14 AWG and larger are from NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors bundled together. Hot spaces, bundles and conduit lower them. 18 and 16 AWG figures are chassis-wiring guidance, not NEC ratings.
- On boats, follow ABYC E-11 for DC wiring. Engine rooms are hot, and conductors there are allowed less current than the same wire elsewhere.
- Use wire rated for the location: heat, oil, water, sunlight and abrasion all matter. Stranded copper is the norm in vehicles and boats, where wire vibrates.
- Fuse every positive run close to the source, sized to protect the wire. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG to 15, 20 and 30 A.
- This calculator does arithmetic. It does not certify an installation; check your work against the standards and the equipment makers' instructions, or have it checked.
Questions
What gauge wire for 40 amps at 12V over 20 feet?
3 at 3% drop and 8 at 10%, copper AWG.
How much voltage is lost carrying 40 amps through 10 feet of 6 AWG?
0.316 V, which is 2.63% of 12 V, and 12.6 W turned into heat in the wire. That uses 20 ft of wire, out and back.