Wire size calculator for 12 V and 24 V (AWG and mm²) • The Science 360

By Shounak Das · Updated November 2026 · 6 min read

What size wire for a 12 V run?

At 12 V, a 10 A load 5 m from the battery needs 10 AWG (5.26 mm²) to stay within a 3% drop, because the current makes a 10 m round trip. A 5 A fridge at the same distance needs 12 AWG. Ampacity only decides the size on short, heavy runs such as an inverter's battery cables.

For the background on voltage drop at 12 V, see what gauge wire for 12 V. For battery cables on an inverter, where the fuse and the heat set the size, see what size cable for a 2000 W inverter.

Key results from the calculator

5 A fridge, 5 m, 12 V: 12 AWG, 0.27 V drop, 11.73 V at the fridge.
10 A light bar, 3 m: 12 AWG; it reaches the 3% limit at 3.39 m.
145 A inverter, 1.5 m: 25 mm² at 12 V, 10 mm² for the same power at 24 V.
14 AWG at 10 A over 5 m drops 7.0%, more than twice the 3% target.

Six runs

Each example loads the voltage, current, one-way length and drop target.

A 5 A fridge 5 m from the battery

Compressor fridge

10 m of wire in the loop. A 3% drop allows 0.36 V, which needs at least 2.44 mm²: 12 AWG. The fridge sees 11.73 V.

Wire 12 AWG Drop 0.27 V At the fridge 11.73 V Longest run 6.78 m

A 10 A light bar 3 m away

LED lighting

Twice the current over a shorter run lands on the same size. 12 AWG drops 2.7%, close to the limit; 3.39 m is as far as it goes at 3%.

Wire 12 AWG Drop 2.7% Longest run 3.39 m Lost as heat 3.2 W

An 8 A pump 10 m away at 10%

Non-critical load

A 10% target is for loads that tolerate low voltage. It lets 12 AWG reach 10 m with 11.15 V left; at 3% the same run would need much thicker cable.

Wire 12 AWG Drop 7.1% At the pump 11.15 V Longest run 14.1 m

A 145 A inverter, 1.5 m of cable

Battery cables

About 1,600 W of AC at 92% efficiency draws 145 A from 12 V. The drop needs 21.2 mm², so 25 mm². Victron's manual asks for 70 mm² because the cable must also carry the 300 A fuse.

Wire 25 mm² Drop 0.31 V Lost as heat 44 W Ampacity 160 A

The same power at 24 V

Higher voltage

Half the current and twice the allowed drop: the drop alone needs 5.3 mm², a quarter of the 12 V figure. Heat then sets the size at 10 mm².

Wire 10 mm² Current 72 A Drop 1.6% Lost as heat 27 W

14 AWG carrying 10 A over 5 m

Common mistake

14 AWG is rated well above 10 A, so it will not overheat. It still drops 0.84 V (7.0%), leaving 11.16 V. Its 3% limit at 10 A is 2.13 m.

Drop 7.0% At the load 11.16 V Longest run 2.13 m Needed 10 AWG

Longest one-way run at 12 V and a 3% drop (copper at 25 °C)

Wire	mm²	5 A	10 A	20 A	ABYC rating
16 AWG	1.31	2.68 m	1.34 m	0.67 m	25 A
14 AWG	2.08	4.26 m	2.13 m	1.07 m	35 A
12 AWG	3.31	6.78 m	3.39 m	1.69 m	45 A
10 AWG	5.26	10.8 m	5.39 m	2.69 m	60 A
8 AWG	8.37	17.1 m	8.57 m	4.28 m	80 A
6 AWG	13.30	27.2 m	13.6 m	6.81 m	120 A
4 AWG	21.15	43.3 m	21.7 m	10.8 m	160 A
2 AWG	33.63	68.9 m	34.4 m	17.2 m	210 A
0 AWG	53.48	110 m	54.8 m	27.4 m	285 A

Lengths are one way; the wire in the loop is twice as long. Halve them for twice the current. At 24 V they double. Ratings are ABYC E-11 Table VII-A (two or fewer conductors bundled, 105 °C insulation, outside engine spaces).

How the calculator works

Resistance: copper resistivity 0.017241 Ω·mm²/m at 20 °C, rising 0.393% per °C (NBS Handbook 100), times the round-trip length, divided by the cross-section.

AWG areas: from the gauge definition, 0.46 in for 4/0 and 0.005 in for 36 AWG with 39 steps between them.

Size: the smallest size whose drop is within the target, or the smallest whose ABYC ampacity covers the current, whichever is larger.

mm² ratings: each metric size takes the ABYC rating of the largest AWG size no bigger than it, which errs on the safe side.

What the model leaves out

Connections: every lug, fuse and switch adds resistance; Victron lists about 0.06 mΩ per connection and 0.35 mΩ for a 150 A fuse.
Bundles and engine spaces: ABYC derates wires in bundles of three or more and inside engine rooms; this uses the figure for two or fewer, outside engine spaces.
Chassis returns: if the vehicle body is the return path, only the positive wire counts, but the body joint adds its own resistance.
Aluminium and CCA cable: copper-clad aluminium has higher resistance than copper of the same size.

Common misconceptions

"The wire is rated for 20 A, so it is fine at 20 A." That rating is about heat. At 12 V the drop usually needs a thicker wire first.

"Use the distance to the load." Use twice that: the current goes out and comes back.

"Thicker is always overkill." Each step up in size cuts the loss by about a fifth; on an inverter that is tens of watts.

Frequently asked questions

What size wire for 10 A at 12 V?

It depends on the run. For a 3% drop: 10 AWG to 5 m one way, and 14 AWG to 2 m. Ampacity alone would allow much thinner wire.

Do I double the length for voltage drop?

Yes, for a two-wire circuit. The current flows out on the positive and back on the negative, so a load 5 m away has 10 m of wire in its loop.

Is 3% or 10% voltage drop right?

ABYC E-11 sets 3% for panelboard feeders, bilge blowers, electronics, navigation lights and other circuits where drop must be kept low, and 10% for other lighting and circuits where it is not critical. Victron advises 2.5% for inverter cables.

Does 24 V need thinner wire than 12 V?

For the same power, yes: half the current and twice the allowed drop let the wire be a quarter of the area, until the heat limit takes over.

How hot can the wire be?

The ratings here are for 105 °C insulation in free air outside an engine space. Bundled wires and hot spaces carry less; the calculator also raises resistance with the temperature you enter.