12V wire size calculator

Find the right wire for a DC circuit on a boat, RV, van or solar setup. Enter the load and the distance; you get the size that keeps the voltage drop in limits and carries the current safely. Works for 12, 24 and 48 V, in AWG or mm².

System voltage

Load

Distance, one way

From the battery or panel to the device. The calculator counts the wire there and back.

Allowed voltage drop

3%: electronics, navigation lights, pumps, fans, main feeds10%: lighting and other loads that tolerate itOther:%

Wire rating and routing

Insulation rating (printed on the wire)Runs through an engine room (hotter, carries less)Bundled with other current-carrying wires

Use

10 AWG

Voltage drop 0.31 V (2.6%), so the device gets 11.69 V. This wire can carry up to 60 A here.

The distance decides the size: a thinner wire would carry 15 A safely but lose too much voltage.

Size	Drop	Max amps
18 AWG	16.6%	20 A
16 AWG	10.4%	25 A
14 AWG	6.5%	35 A
12 AWG	4.1%	45 A
10 AWG	2.6%	60 A
8 AWG	1.6%	80 A
6 AWG	1.0%	120 A
4 AWG	0.6%	160 A
2 AWG	0.4%	210 A
1 AWG	0.3%	245 A
1/0 AWG	0.3%	285 A
2/0 AWG	0.2%	330 A
3/0 AWG	0.2%	385 A
4/0 AWG	0.1%	445 A

Red: too much voltage drop, or more current than the wire is rated for. Bigger than the pick is always fine.

What it checks

Voltage drop. Every wire has some resistance, so part of the voltage is lost on the way. Too much, and pumps run slow, lights dim and electronics reset. The longer the run and the higher the current, the thicker the wire has to be.

Current rating. A wire carrying too much current heats up. The rating depends on the insulation, on whether the wire runs through a hot engine room, and on how many current-carrying wires share a bundle.

The answer is the smallest wire that passes both checks.

3% or 10% drop?

The boat standard (ABYC E-11) asks for 3% on main feeds from the battery to the panel, on electronics, navigation lights and bilge blowers, and on anything where a low voltage causes trouble.

10% is allowed for cabin lighting and other loads that still work with a little less voltage. When in doubt, use 3%: the wire costs a little more, the system works better.

12V wire size charts

Quick answers for a 12 V system with 105 °C wire, not bundled and not in an engine room. Distances are one way, from the battery to the device. For other cases, use the calculator above.

AWG at 3% voltage drop
Load	5 ft	10 ft	15 ft	20 ft	25 ft
5 A	18	14	12	12	10
10 A	14	12	10	8	8
15 A	12	10	8	6	6
20 A	12	8	6	6	4
30 A	10	6	4	4	2
40 A	8	6	4	2	2
50 A	8	4	2	2	1
60 A	6	4	2	1	1/0
80 A	6	2	1	1/0	2/0
100 A	4	2	1/0	2/0	3/0

AWG at 10% voltage drop
Load	5 ft	10 ft	15 ft	20 ft	25 ft
5 A	18	18	18	16	16
10 A	18	16	14	14	12
15 A	18	14	14	12	10
20 A	16	14	12	10	10
30 A	14	12	10	8	8
40 A	12	10	8	8	6
50 A	10	10	8	6	6
60 A	10	8	8	6	4
80 A	8	8	6	4	4
100 A	6	6	4	4	2

mm² at 3% voltage drop
Load	1 m	2 m	3 m	5 m	8 m
5 A	0.75	1	1.5	2.5	4
10 A	1	2.5	4	6	10
15 A	1.5	4	6	10	16
20 A	2.5	4	6	10	16
30 A	4	6	10	16	25
40 A	4	10	16	25	35
50 A	6	10	16	25	50
60 A	6	16	25	35	50
80 A	10	16	25	50	70
100 A	16	25	35	50	95

mm² at 10% voltage drop
Load	1 m	2 m	3 m	5 m	8 m
5 A	0.75	0.75	0.75	0.75	1.5
10 A	0.75	0.75	1	1.5	2.5
15 A	0.75	1	1.5	2.5	4
20 A	1	1.5	2.5	4	6
30 A	2.5	2.5	4	6	10
40 A	4	4	4	6	10
50 A	6	6	6	10	16
60 A	6	6	6	10	16
80 A	10	10	10	16	25
100 A	16	16	16	16	25

— means no single wire up to 4/0 AWG or 120 mm² is enough.

How it is calculated

Voltage drop uses the ABYC formula: circular mils = 10.75 × amps × length there and back (ft) ÷ allowed drop (V). For mm² the same copper constant is used (0.0179 Ω·mm²/m).

Current ratings come from the US Coast Guard table for boats (33 CFR 183.425, the same values as ABYC E-11) for AWG, and from ISO 13297 for metric cable, with their engine-room and bundling factors.

This is a sizing aid. For a boat, follow ABYC or ISO; for a vehicle or building, your local rules and the wire maker's rating come first.

Drawing the wiring for your boat or van?

Sketch it on paper, take a photo, and Sketch2scheme turns it into a clean wiring diagram with proper symbols, ready to edit in draw.io or print.

See examples

Questions

Why does the distance count twice?

Current flows out to the device and back to the battery, so it passes through both wires. A device 10 feet away has 20 feet of wire in its circuit. Enter the one-way distance; the calculator doubles it.

Can I use a thicker wire than the calculator says?

Yes, always. A thicker wire loses less voltage and runs cooler. The only downsides are cost, weight and fitting it into terminals.

What size fuse do I need?

The fuse protects the wire, so its rating must not be higher than the wire can carry (the "max amps" in the table). Pick one above the normal load and at or below the wire rating, and fit it as close to the battery as you can.

Does it work for 24 V and 48 V?

Yes. For the same power, a higher voltage needs less current, so the wire can be much thinner. That is why larger solar and van systems often move to 24 or 48 V.

Which wire should I buy for a boat?

Tinned, stranded copper rated 105 °C, sold as marine or boat cable. Solid household wire cracks with vibration and is not allowed on boats.

Reading a wiring diagram? See the electrical symbols chart.