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These charts show the longest one-way run of copper cable, from the battery or source to the load, that keeps the voltage drop at 3% or less. A dash means the cable cannot safely carry a fuse sized at 125% of that load, using conservative standard-insulation (60 °C) current ratings. For your own numbers, use the wire gauge and fuse calculator.
12 V chart, AWG sizes
| Load | 14 | 12 | 10 | 8 | 6 | 4 | 2 | 1/0 | 2/0 | 4/0 |
|---|---|---|---|---|---|---|---|---|---|---|
| 5 A | 14 ft · 4.3 m | 22 ft · 6.8 m | 36 ft · 10.8 m | 56 ft · 17.2 m | 90 ft · 27.4 m | 143 ft · 43.6 m | 227 ft · 69.1 m | 361 ft · 110.1 m | 455 ft · 138.7 m | 722 ft · 220.1 m |
| 10 A | 7 ft · 2.1 m | 11 ft · 3.4 m | 18 ft · 5.4 m | 28 ft · 8.6 m | 45 ft · 13.7 m | 72 ft · 21.8 m | 113 ft · 34.6 m | 181 ft · 55.0 m | 227 ft · 69.3 m | 361 ft · 110.1 m |
| 15 A | – | 7 ft · 2.3 m | 12 ft · 3.6 m | 19 ft · 5.7 m | 30 ft · 9.1 m | 48 ft · 14.5 m | 76 ft · 23.0 m | 120 ft · 36.7 m | 152 ft · 46.2 m | 241 ft · 73.4 m |
| 20 A | – | – | 9 ft · 2.7 m | 14 ft · 4.3 m | 22 ft · 6.8 m | 36 ft · 10.9 m | 57 ft · 17.3 m | 90 ft · 27.5 m | 114 ft · 34.7 m | 181 ft · 55.0 m |
| 30 A | – | – | – | 9 ft · 2.9 m | 15 ft · 4.6 m | 24 ft · 7.3 m | 38 ft · 11.5 m | 60 ft · 18.3 m | 76 ft · 23.1 m | 120 ft · 36.7 m |
| 40 A | – | – | – | – | 11 ft · 3.4 m | 18 ft · 5.5 m | 28 ft · 8.6 m | 45 ft · 13.8 m | 57 ft · 17.3 m | 90 ft · 27.5 m |
| 50 A | – | – | – | – | – | 14 ft · 4.4 m | 23 ft · 6.9 m | 36 ft · 11.0 m | 45 ft · 13.9 m | 72 ft · 22.0 m |
| 75 A | – | – | – | – | – | – | – | 24 ft · 7.3 m | 30 ft · 9.2 m | 48 ft · 14.7 m |
| 100 A | – | – | – | – | – | – | – | 18 ft · 5.5 m | 23 ft · 6.9 m | 36 ft · 11.0 m |
| 150 A | – | – | – | – | – | – | – | – | – | – |
| 200 A | – | – | – | – | – | – | – | – | – | – |
12 V chart, metric sizes (mm²)
| Load | 1.5 | 2.5 | 4 | 6 | 10 | 16 | 25 | 35 | 50 | 70 | 95 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 5 A | 10 ft · 3.1 m | 17 ft · 5.1 m | 27 ft · 8.2 m | 40 ft · 12.3 m | 67 ft · 20.6 m | 108 ft · 32.9 m | 169 ft · 51.4 m | 236 ft · 72.0 m | 337 ft · 102.9 m | 472 ft · 144.0 m | 641 ft · 195.4 m |
| 10 A | – | 8 ft · 2.6 m | 13 ft · 4.1 m | 20 ft · 6.2 m | 34 ft · 10.3 m | 54 ft · 16.5 m | 84 ft · 25.7 m | 118 ft · 36.0 m | 169 ft · 51.4 m | 236 ft · 72.0 m | 321 ft · 97.7 m |
| 15 A | – | – | 9 ft · 2.7 m | 13 ft · 4.1 m | 22 ft · 6.9 m | 36 ft · 11.0 m | 56 ft · 17.1 m | 79 ft · 24.0 m | 112 ft · 34.3 m | 157 ft · 48.0 m | 214 ft · 65.1 m |
| 20 A | – | – | – | 10 ft · 3.1 m | 17 ft · 5.1 m | 27 ft · 8.2 m | 42 ft · 12.9 m | 59 ft · 18.0 m | 84 ft · 25.7 m | 118 ft · 36.0 m | 160 ft · 48.9 m |
| 30 A | – | – | – | – | 11 ft · 3.4 m | 18 ft · 5.5 m | 28 ft · 8.6 m | 39 ft · 12.0 m | 56 ft · 17.1 m | 79 ft · 24.0 m | 107 ft · 32.6 m |
| 40 A | – | – | – | – | – | 13 ft · 4.1 m | 21 ft · 6.4 m | 30 ft · 9.0 m | 42 ft · 12.9 m | 59 ft · 18.0 m | 80 ft · 24.4 m |
| 50 A | – | – | – | – | – | – | 17 ft · 5.1 m | 24 ft · 7.2 m | 34 ft · 10.3 m | 47 ft · 14.4 m | 64 ft · 19.5 m |
| 75 A | – | – | – | – | – | – | – | – | 22 ft · 6.9 m | 31 ft · 9.6 m | 43 ft · 13.0 m |
| 100 A | – | – | – | – | – | – | – | – | – | 24 ft · 7.2 m | 32 ft · 9.8 m |
| 150 A | – | – | – | – | – | – | – | – | – | – | – |
| 200 A | – | – | – | – | – | – | – | – | – | – | – |
24 V chart, AWG sizes
At 24 V the same cable carries the same current with twice the voltage to spare, so the allowed length doubles for the same load in amps. In practice the same power draws half the amps at 24 V, so the run can be longer still.
| Load | 14 | 12 | 10 | 8 | 6 | 4 | 2 | 1/0 | 2/0 | 4/0 |
|---|---|---|---|---|---|---|---|---|---|---|
| 5 A | 28 ft · 8.6 m | 45 ft · 13.6 m | 71 ft · 21.6 m | 113 ft · 34.4 m | 180 ft · 54.7 m | 286 ft · 87.2 m | 454 ft · 138.2 m | 722 ft · 220.1 m | 910 ft · 277.3 m | 1444 ft · 440.2 m |
| 10 A | 14 ft · 4.3 m | 22 ft · 6.8 m | 36 ft · 10.8 m | 56 ft · 17.2 m | 90 ft · 27.4 m | 143 ft · 43.6 m | 227 ft · 69.1 m | 361 ft · 110.1 m | 455 ft · 138.7 m | 722 ft · 220.1 m |
| 15 A | – | 15 ft · 4.5 m | 24 ft · 7.2 m | 38 ft · 11.5 m | 60 ft · 18.2 m | 95 ft · 29.1 m | 151 ft · 46.1 m | 241 ft · 73.4 m | 303 ft · 92.4 m | 481 ft · 146.7 m |
| 20 A | – | – | 18 ft · 5.4 m | 28 ft · 8.6 m | 45 ft · 13.7 m | 72 ft · 21.8 m | 113 ft · 34.6 m | 181 ft · 55.0 m | 227 ft · 69.3 m | 361 ft · 110.1 m |
| 30 A | – | – | – | 19 ft · 5.7 m | 30 ft · 9.1 m | 48 ft · 14.5 m | 76 ft · 23.0 m | 120 ft · 36.7 m | 152 ft · 46.2 m | 241 ft · 73.4 m |
| 40 A | – | – | – | – | 22 ft · 6.8 m | 36 ft · 10.9 m | 57 ft · 17.3 m | 90 ft · 27.5 m | 114 ft · 34.7 m | 181 ft · 55.0 m |
| 50 A | – | – | – | – | – | 29 ft · 8.7 m | 45 ft · 13.8 m | 72 ft · 22.0 m | 91 ft · 27.7 m | 144 ft · 44.0 m |
| 75 A | – | – | – | – | – | – | – | 48 ft · 14.7 m | 61 ft · 18.5 m | 96 ft · 29.3 m |
| 100 A | – | – | – | – | – | – | – | 36 ft · 11.0 m | 45 ft · 13.9 m | 72 ft · 22.0 m |
| 150 A | – | – | – | – | – | – | – | – | – | – |
| 200 A | – | – | – | – | – | – | – | – | – | – |
Fuse size chart
A common rule is to fuse at about 125% of the continuous load current, rounded up to the next standard rating, and never above the cable’s current rating.
| Load | 2 A | 5 A | 10 A | 15 A | 20 A | 30 A | 40 A | 50 A | 80 A | 100 A | 150 A | 200 A |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Fuse | 3 A | 7.5 A | 15 A | 20 A | 25 A | 40 A | 50 A | 70 A | 100 A | 125 A | 200 A | 250 A |
Cable sizes and current ratings
| AWG | Copper area | Rating, standard insulation (60 °C) | Rating, high temperature (90 °C) |
|---|---|---|---|
| 18 | 0.823 mm² | 7 A | 7 A |
| 16 | 1.31 mm² | 10 A | 10 A |
| 14 | 2.08 mm² | 15 A | 15 A |
| 12 | 3.31 mm² | 20 A | 20 A |
| 10 | 5.26 mm² | 30 A | 30 A |
| 8 | 8.37 mm² | 40 A | 55 A |
| 6 | 13.3 mm² | 55 A | 75 A |
| 4 | 21.2 mm² | 70 A | 95 A |
| 2 | 33.6 mm² | 95 A | 130 A |
| 1 | 42.4 mm² | 110 A | 145 A |
| 1/0 | 53.5 mm² | 125 A | 170 A |
| 2/0 | 67.4 mm² | 145 A | 195 A |
| 3/0 | 85 mm² | 165 A | 225 A |
| 4/0 | 107 mm² | 195 A | 260 A |
The ratings come from copper values in the US National Electrical Code table for conductors, with the small-size limits for 18 to 10 AWG applied. Metric sizes are matched to the next smaller AWG row, which is conservative. Bundled or hot cable should be derated. Marine, vehicle and building codes differ, so check the rules where you live.
How the length is calculated
Maximum one-way length = allowed voltage drop × cable area ÷ (2 × 0.0175 × current). The 0.0175 is the resistivity of copper in ohm-mm² per meter, and the 2 is for the return path. Allowed voltage drop is 3% of the system voltage.
Common questions
What gauge wire do I need for 12 V and 30 amps?
It depends on the distance. For a 30 A load, a fuse of 40 A and 3% voltage drop, 8 AWG covers about 9 ft one way and 6 AWG about 15 ft. The calculator finds the size for your exact length.
How far can I run 12 V wire at 20 amps?
With 10 AWG cable, a 20 A load stays within a 3% voltage drop for about 9 ft (2.7 m) one way. 8 AWG about doubles that. Longer runs need a thicker cable or a higher system voltage.
What is the difference between AWG and mm²?
AWG is the American wire gauge, where a smaller number means a thicker wire, and mm² is the copper cross-section area used in most other countries. They do not match exactly, so choose the nearest size that is large enough in the system your shop uses.
Why does the allowed length shrink as the current goes up?
A cable wastes voltage in proportion to the current through it. Doubling the current doubles the voltage drop on the same cable, so the same drop allows only half the length.
Do I always need a fuse?
Yes, on the positive cable close to the battery. The fuse protects the cable. Choose a rating at about 125% of the load and below the cable’s current rating, and use a fuse rated for DC at your system voltage.
Is 3% voltage drop the right target?
A common guideline for sensitive circuits such as electronics and lighting is 3%, and up to 10% for less critical loads. The wire gauge and fuse calculator lets you choose 2%, 3%, 5% or 10%.
Related
Calculate your exact cable and fuse with the wire gauge and fuse calculator. Charging from the alternator? See the alternator charge time calculator. For the terms used here, see the off-grid power glossary.