DC wire size calculator
Enter the system voltage, the load in amps or watts and the one-way length from the battery or panel to the load. The calculator doubles the length for the return path, finds the smallest copper gauge that keeps the voltage drop within your limit and is rated for the current, and shows the drop, the voltage at the load and the power lost for any gauge. Use 3% for critical loads and 10% for non-critical lighting.
Use 8 AWG (8.37 mm²) copper or larger.
Voltage drop sets the size. 10 AWG would drop 3.33%, over your 3% limit. Ampacity alone would allow 14 AWG.
Check: 8 AWG (8.37 mm²)
| Load current | 20 A |
| Wire in the circuit (out and back) | 2 x 10 ft = 20 ft |
| Resistance of 8 AWG | 0.6282 ohm per 1000 ft; 0.01256 ohm for this circuit |
| Voltage drop | 0.251 V = 2.09% within 3% |
| Voltage at the load | 11.75 V |
| Power lost in the wire | 5.03 W |
| Ampacity of 8 AWG | 50 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 20 x 2 x 10 x 0.6282 / 1000 = 0.251 V, and 0.251 / 12 = 2.09%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 2.554 | 21.28 | 9.45 | 10* | drop and current |
| 16 | 1.31 | 1.606 | 13.39 | 10.39 | 13* | drop and current |
| 14 | 2.08 | 1.01 | 8.42 | 10.99 | 20 | too much drop |
| 12 | 3.31 | 0.635 | 5.29 | 11.36 | 25 | too much drop |
| 10 | 5.26 | 0.4 | 3.33 | 11.6 | 35 | too much drop |
| 8 | 8.37 | 0.251 | 2.09 | 11.75 | 50 | smallest that passes |
| 6 | 13.3 | 0.158 | 1.32 | 11.84 | 65 | passes |
| 4 | 21.2 | 0.099 | 0.83 | 11.9 | 85 | passes |
| 3 | 26.7 | 0.079 | 0.66 | 11.92 | 100 | passes |
| 2 | 33.6 | 0.063 | 0.52 | 11.94 | 115 | passes |
| 1 | 42.4 | 0.05 | 0.41 | 11.95 | 130 | passes |
| 1/0 | 53.5 | 0.039 | 0.33 | 11.96 | 150 | passes |
| 2/0 | 67.4 | 0.031 | 0.26 | 11.97 | 175 | passes |
| 3/0 | 85 | 0.025 | 0.21 | 11.98 | 200 | passes |
| 4/0 | 107 | 0.02 | 0.16 | 11.98 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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Worked example
20 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:
- 10 AWG:
20 x 20 x 0.9989 / 1000 = 0.4 V, which is 3.33% of 12 V; over the 3% limit. - 8 AWG:
20 x 20 x 0.6282 / 1000 = 0.251 V, which is 2.09% of 12 V; passes both checks.
Answer: 8 AWG (8.37 mm²), rated 50 A, the smallest size that passes both.
12V wire size chart at 3% drop
| 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 |
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.
Charts for every voltage: 12V, 24V, 36V, 48V.
Wire gauge, resistance and rated amps
| AWG | mm² | ohm per 1000 ft | ohm per km | Rated amps | Basis |
|---|---|---|---|---|---|
| 18 | 0.82 | 6.385 | 20.9482 | 10 | chassis-wiring guidance |
| 16 | 1.31 | 4.016 | 13.1759 | 13 | chassis-wiring guidance |
| 14 | 2.08 | 2.525 | 8.2841 | 20 | NEC 310.16, 75 C |
| 12 | 3.31 | 1.588 | 5.21 | 25 | NEC 310.16, 75 C |
| 10 | 5.26 | 0.9989 | 3.2772 | 35 | NEC 310.16, 75 C |
| 8 | 8.37 | 0.6282 | 2.061 | 50 | NEC 310.16, 75 C |
| 6 | 13.3 | 0.3951 | 1.2963 | 65 | NEC 310.16, 75 C |
| 4 | 21.2 | 0.2485 | 0.8153 | 85 | NEC 310.16, 75 C |
| 3 | 26.7 | 0.197 | 0.6463 | 100 | NEC 310.16, 75 C |
| 2 | 33.6 | 0.1563 | 0.5128 | 115 | NEC 310.16, 75 C |
| 1 | 42.4 | 0.1239 | 0.4065 | 130 | NEC 310.16, 75 C |
| 1/0 | 53.5 | 0.09827 | 0.3224 | 150 | NEC 310.16, 75 C |
| 2/0 | 67.4 | 0.07793 | 0.2557 | 175 | NEC 310.16, 75 C |
| 3/0 | 85 | 0.0618 | 0.2028 | 200 | NEC 310.16, 75 C |
| 4/0 | 107 | 0.04901 | 0.1608 | 230 | NEC 310.16, 75 C |
Resistance: copper at 20 C. Rated amps for 14 AWG and larger: NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors together; hotter spaces and bundles lower it. 18 and 16 AWG are not in that table; their figures are common chassis-wiring guidance. NEC 240.4(D) also limits overcurrent protection on 14, 12 and 10 AWG to 15, 20 and 30 A.
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
Should I enter the one-way length or the total wire length?
The one-way length, from the source to the load. The calculator doubles it, because the current flows out on the positive and back on the negative. If your negative returns through a chassis or hull bond, the real circuit is different and you should measure it.
Why does the calculator sometimes pick a thicker wire than the voltage drop needs?
Every conductor has a current rating. On short runs a thin wire can keep the drop small and still be rated for less than the load draws, so the ampacity check decides the size.
What drop should I use?
A common rule is 3% for loads that suffer from low voltage, such as electronics, pumps, fridges, inverters and charging, and 10% for lighting and other non-critical loads. Marine tables built on ABYC E-11 use the same two figures.
Does it handle stranded wire?
The resistance figures are the standard values for copper at 20 C. Stranded cable of the same gauge has slightly higher resistance, and warm wire more again, so leave some margin when a result is close to the limit.