12V wire size chart
Find the current down the left and the one-way length across the top. Each cell is the smallest copper gauge that keeps the drop within the limit at 12 V and is rated for the current. For example, 30 A over 15 ft at 12 V needs 6 AWG at 3%. Change any number in the calculator for your own run.
Use 6 AWG (13.3 mm²) copper or larger.
Voltage drop sets the size. 8 AWG would drop 4.71%, over your 3% limit. Ampacity alone would allow 10 AWG.
Check: 6 AWG (13.3 mm²)
| Load current | 30 A |
| Wire in the circuit (out and back) | 2 x 15 ft = 30 ft |
| Resistance of 6 AWG | 0.3951 ohm per 1000 ft; 0.01185 ohm for this circuit |
| Voltage drop | 0.356 V = 2.96% within 3% |
| Voltage at the load | 11.64 V |
| Power lost in the wire | 10.7 W |
| Ampacity of 6 AWG | 65 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 30 x 2 x 15 x 0.3951 / 1000 = 0.356 V, and 0.356 / 12 = 2.96%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 5.746 | 47.89 | 6.25 | 10* | drop and current |
| 16 | 1.31 | 3.614 | 30.12 | 8.39 | 13* | drop and current |
| 14 | 2.08 | 2.272 | 18.94 | 9.73 | 20 | drop and current |
| 12 | 3.31 | 1.429 | 11.91 | 10.57 | 25 | drop and current |
| 10 | 5.26 | 0.899 | 7.49 | 11.1 | 35 | too much drop |
| 8 | 8.37 | 0.565 | 4.71 | 11.43 | 50 | too much drop |
| 6 | 13.3 | 0.356 | 2.96 | 11.64 | 65 | smallest that passes |
| 4 | 21.2 | 0.224 | 1.86 | 11.78 | 85 | passes |
| 3 | 26.7 | 0.177 | 1.48 | 11.82 | 100 | passes |
| 2 | 33.6 | 0.141 | 1.17 | 11.86 | 115 | passes |
| 1 | 42.4 | 0.112 | 0.93 | 11.89 | 130 | passes |
| 1/0 | 53.5 | 0.088 | 0.74 | 11.91 | 150 | passes |
| 2/0 | 67.4 | 0.07 | 0.58 | 11.93 | 175 | passes |
| 3/0 | 85 | 0.056 | 0.46 | 11.94 | 200 | passes |
| 4/0 | 107 | 0.044 | 0.37 | 11.96 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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12V chart at 3% drop (critical loads)
| Amps | 5 ft 1.5 m | 10 ft 3 m | 15 ft 4.6 m | 20 ft 6.1 m | 25 ft 7.6 m | 30 ft 9.1 m | 40 ft 12.2 m | 50 ft 15.2 m |
|---|---|---|---|---|---|---|---|---|
| 5 A | 18 | 14 | 12 | 12 | 10 | 10 | 8 | 8 |
| 10 A | 14 | 12 | 10 | 8 | 8 | 6 | 6 | 4 |
| 15 A | 12 | 10 | 8 | 6 | 6 | 6 | 4 | 3 |
| 20 A | 12 | 8 | 6 | 6 | 4 | 4 | 3 | 2 |
| 30 A | 10 | 6 | 6 | 4 | 3 | 3 | 1 | 1/0 |
| 40 A | 8 | 6 | 4 | 3 | 2 | 1 | 1/0 | 2/0 |
| 50 A | 8 | 4 | 3 | 2 | 1 | 1/0 | 2/0 | 3/0 |
| 60 A | 6 | 4 | 3 | 1 | 1/0 | 1/0 | 3/0 | 4/0 |
| 80 A | 4* | 3 | 1 | 1/0 | 2/0 | 3/0 | 4/0 | over 4/0 |
| 100 A | 3* | 2 | 1/0 | 2/0 | 3/0 | 4/0 | over 4/0 | over 4/0 |
| 150 A | 1/0* | 1/0 | 2/0 | 4/0 | over 4/0 | over 4/0 | over 4/0 | over 4/0 |
| 200 A | 3/0* | 3/0* | 4/0 | over 4/0 | over 4/0 | over 4/0 | over 4/0 | over 4/0 |
12V chart at 10% drop (non-critical lighting)
| Amps | 5 ft 1.5 m | 10 ft 3 m | 15 ft 4.6 m | 20 ft 6.1 m | 25 ft 7.6 m | 30 ft 9.1 m | 40 ft 12.2 m | 50 ft 15.2 m |
|---|---|---|---|---|---|---|---|---|
| 5 A | 18 | 18 | 18 | 16 | 16 | 14 | 14 | 12 |
| 10 A | 18 | 16 | 14 | 14 | 12 | 12 | 10 | 10 |
| 15 A | 14* | 14 | 14 | 12 | 12 | 10 | 10 | 8 |
| 20 A | 14* | 14 | 12 | 10 | 10 | 10 | 8 | 6 |
| 30 A | 10* | 10* | 10 | 10 | 8 | 8 | 6 | 6 |
| 40 A | 8* | 8* | 8* | 8 | 6 | 6 | 4 | 4 |
| 50 A | 8* | 8* | 8 | 6 | 6 | 6 | 4 | 3 |
| 60 A | 6* | 6* | 6* | 6 | 6 | 4 | 4 | 3 |
| 80 A | 4* | 4* | 4* | 4 | 4 | 4 | 2 | 1 |
| 100 A | 3* | 3* | 3* | 3* | 3 | 3 | 1 | 1/0 |
| 150 A | 1/0* | 1/0* | 1/0* | 1/0* | 1/0* | 1/0* | 1/0 | 2/0 |
| 200 A | 3/0* | 3/0* | 3/0* | 3/0* | 3/0* | 3/0* | 3/0 | 4/0 |
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
30 A at 12 V, 15 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 15 = 30 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:
30 x 30 x 0.6282 / 1000 = 0.565 V, which is 4.71% of 12 V; over the 3% limit. - 6 AWG:
30 x 30 x 0.3951 / 1000 = 0.356 V, which is 2.96% of 12 V; passes both checks.
Answer: 6 AWG (13.3 mm²), rated 65 A, the smallest size that passes both.
Why 12V needs thinner wire than it would at a lower voltage
For the same power, current falls as voltage rises, and the allowed drop in volts rises with it. Both work in your favour: at 12 V a 1000 W load draws 83.3 A and may lose 0.36 V at 3%, against 83.3 A and 0.36 V at 12 V. Other charts: 24V, 36V, 48V.
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 size wire for 20 amps at 12V over 20 feet?
6 AWG copper at 3% drop, or 10 AWG at 10%, from the chart above.
What does the star in the chart mean?
The current rating, not the voltage drop, sets the size in that cell. A thinner wire would keep the drop low enough but is not rated for the current.