What Gauge Wire for 12 Volts?

The two tests every wire must pass

Wire size is given in AWG (American Wire Gauge). The smaller the number, the thicker the wire. 10 AWG is thicker than 14 AWG.

Test 1: can it carry the current? Push too many amps through a thin wire and it heats up, melts its insulation and can start a fire. The safe amount is called ampacity. On boats, the Coast Guard's table in 33 CFR 183.425 sets it.

Test 2: does enough voltage reach the device? Every foot of wire has a little resistance, so some voltage is lost on the way. That's voltage drop. At 12 volts there isn't much to spare. A 1-volt loss is over 8% of your power gone, and things like pumps and sonar can act up when their voltage sags.

Short runs are usually set by Test 1. Anything more than a few feet is usually set by Test 2. That's why a wire that's fine for 10 amps at 3 feet can be far too small for 10 amps at 20 feet.

How much drop is OK? Blue Sea Systems, following the ABYC standard (ABYC writes the boat wiring standards most builders follow), uses:

• 3% for circuits that affect safety, like running lights, bilge blowers, electronics and the feeds to fuse panels.
• 10% for everything else, like cabin lights and bait pumps.

Wire size chart: 3% drop

Use this for fish finders, GPS, radios, nav lights, fuse block feeds and trolling motors. Distance is one way, from the battery to the device. The math doubles it for the round trip.

Wire sizes in AWG for 12 volts, from our Wire Size Calculator: CM = 10.75 × amps × round-trip feet ÷ 0.36 volts allowed. Each size can also carry a fuse of 125% of the load (rounded up to a standard size) under 33 CFR 183.425 for 105 °C wire outside the engine space, single wire. 16 AWG is the smallest allowed.

Wire size chart: 10% drop

Use this only for things that aren't critical and don't mind a little sag, like interior lights, bait pumps and a cooler fan.

Same math and ampacity check as the 3% chart, with 1.2 volts allowed. At 24 volts you can usually go one or two sizes thinner, because the same percentage is twice as many volts.

These charts assume one wire on its own, outside the engine space. If your wire runs through the engine compartment or sits bundled with others, it can carry less current. Our Wire Size Calculator handles both, and any distance or amps.

Working it out yourself

The formula ABYC and wire makers like Ancor use for copper wire gives the thickness you need in circular mils (CM), a measure of the wire's cross-section:

CM = 10.75 × amps × round-trip feet ÷ allowed volts dropped

Example: a 20 amp load, 20 feet from the battery, at 12 volts and 3% drop.

• Round trip: 20 × 2 = 40 feet.
• Allowed drop: 3% of 12 volts = 0.36 volts.
• CM = 10.75 × 20 × 40 ÷ 0.36 = 23,889.
• Find the first wire that's at least that big:

Circular mils are standard wire sizes. Max amps from Table 5 of 33 CFR 183.425 for 105 °C insulation, single wire, with the rule's 0.85 engine-space factor.

6 AWG (26,240 CM) is the first size big enough, and it drops about 2.7%. If that same 20 amp load could live with 10% drop, the answer is 10 AWG, at about 6.9%. Same load, same distance, two very different wires. That's why the drop limit matters so much.

Then check Test 1. A 20 amp load gets a 25 amp fuse. 6 AWG can carry 120 amps, so it passes easily.

How many amps a wire can carry

The Coast Guard's ampacity table depends on three things:

• Insulation rating. Wire rated for higher heat can carry more. Most marine wire, including Ancor's, is rated 105 °C. The same 10 AWG wire is good for 60 amps at 105 °C but only 40 amps at 75 °C.
• Engine space. It's hotter there, so 105 °C wire in the engine space is multiplied by 0.85.
• Bundling. Wires bundled together can't shed heat as well. West Marine lists the cuts: 30% off for 3 wires, 40% for 4 to 6, and 50% for 7 to 24.

Blue Sea Systems gives a simple way to pick the fuse once the wire is chosen. The fuse should be at least 125% of the device's current, and no more than the wire can carry. Pick something in that range. If the device's manual names a fuse size, use that, and make sure the wire can carry it.

What kind of wire to buy

• Stranded copper only. 33 CFR 183.425 requires insulated, stranded copper wire on boats. Solid house wire can crack from engine and wave vibration.
• 16 AWG minimum. The same rule sets 16 AWG as the smallest wire run on its own. 18 AWG is allowed only inside a sheathed cable, like a factory power cord.
• Tinned copper. Ancor's marine wire uses tinned strands for "superior corrosion and electrolysis resistance."
• Fine stranding. Ancor calls its wire Type 3, a very flexible stranding that resists breaking from vibration.
• Size the negative the same as the positive. The formula counts both wires, so a thin ground wire undoes the good work of a thick positive.

Our marine wire picks list tinned wire in common sizes, and our terminals page has crimp and heat-shrink connectors to match.

Questions people ask

What gauge wire do I need for 20 amps at 12 volts?

It depends on the distance. For a short run of 5 feet, 12 AWG. At 10 feet, 8 AWG for 3% drop or 14 AWG for 10%. At 20 feet, 6 AWG for 3% or 10 AWG for 10%.

What size wire for 30 amps at 12 volts?

At 3% drop: 10 AWG at 5 feet, 6 AWG at 10 feet, 4 AWG at 15 to 20 feet. At 10% drop, 12 AWG works up to 10 feet.

Does the length include the ground wire?

Yes. Voltage drop happens in both wires, so you count the round trip. A device 15 feet from the battery is a 30-foot circuit. West Marine warns that guessing short leads to wire that's too small.

Can I use speaker wire or solid house wire on 12 volts?

Not on a boat. Federal rules require stranded copper, and 16 AWG or larger for a single wire. Thin speaker wire also drops a lot of voltage over any real distance.

Is 12 V wire different from 120 V house wire?

The copper is the same, but 12 volts loses a much bigger share of its power to voltage drop. That's why 12-volt runs often need thicker wire than the same amps would need in a house.

What gauge wire for a 12 V water pump 20 feet away?

Say it draws 7 amps. At 10% drop, 16 AWG just makes it at about 9.7%. At 3% you'd need 10 AWG. Members on a solar forum worked through a pump like this and also landed on 10 AWG for a 20-foot run.

Can wire be too big?

Not electrically. Thicker wire loses less voltage and runs cooler. The only limits are cost, weight and whether it fits your terminals and fuse block screws.