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Free tool · Boat electrical

What gauge wire does your 12V circuit need?

The answer a static chart can't give you, because it doesn't know your run length. Enter your load, distance, and voltage, and get the AWG that satisfies both voltage drop and ampacity, the marine way, with the fuse to match. Built on the ABYC E-11 method for 12, 24, and 48 volt DC.

Read first: undersized wire overheats and is a real fire risk, so this tool sizes conservatively and rounds up. It is a planning guide that follows the ABYC E-11 method, not a substitute for the standard or a qualified marine electrician, and it sizes the wire only, not your fuse or breaker. It assumes 105°C marine-grade stranded wire. Confirm any wire you install against ABYC E-11, and when in doubt, go heavier.

Your circuit

Follows the ABYC E-11 sizing method for 105°C marine wire. Sizes the conductor only, not the fuse. Confirm against the standard before installing.

How to size marine DC wire

The calculator above is just this six-step ABYC method. You can run it by hand.

Step 1: Size for voltage drop and ampacity, then take the larger

ABYC E-11 is explicit: size the wire two ways and use the bigger result. Voltage drop governs long runs at modest current; ampacity (the heat the wire can shed without melting its insulation) governs short, high-current runs like a windlass or inverter feed. A calculator that checks only voltage drop will happily undersize a short 200-amp run, and that is how wires catch fire. This tool always runs both and reports which one won.

Step 2: Measure the round trip, not the one-way distance

Current flows out to the device and back, so the resistance that drops your voltage is the full there-and-back length. The single most common wiring mistake is sizing off the one-way distance, which halves the apparent resistance and leaves you a gauge or two too thin. Enter the one-way run from the source to the device and the tool doubles it for you.

Step 3: Hold critical circuits to 3 percent, the rest to 10

ABYC allows up to 3 percent voltage drop on circuits where it matters: navigation lights, bilge pumps, electronics, and the main panel feeders. Everything else, like cabin lighting and accessory outlets, can run to 10 percent. A bilge pump on a 10 percent circuit pumps slowly when the battery is low, which is exactly when you need it, so when in doubt design to 3 percent. Many installers wire the whole boat to 3 percent for margin. One caveat: the formula uses the standard copper constant for cool wire, so a long run that sits in a hot engine space near full load drops a little more than the math shows, and it is worth leaving margin there.

Step 4: Derate for heat: the engine space and bundling

Wire carries less current when it is hot or crowded. A conductor inside the engine space is rated lower than the same wire in open air, so pick the right location. And when current-carrying conductors are bundled together for more than two feet, they cannot all shed heat, so the bundle is derated. ABYC applies this once two or more conductors are bundled, and for DC circuits the derate stops at 0.70, no lower, even in a large bundle, so when in doubt treat a run lashed in with others as bundled. The tool applies the location and the bundle, never a separate temperature factor on top.

Step 5: Round up, never down, and never below 16 AWG

Wire is sized in steps, so once the math lands between two gauges you take the larger one. Heavier wire runs cooler, drops less voltage, and costs a few dollars more. There is no upside to shaving it. ABYC also sets a floor of 16 AWG for general boat wiring regardless of how little current a circuit carries, because thinner wire is too fragile for the vibration and flexing of a boat.

Step 6: Fuse the wire, not the load

A fuse or breaker protects the conductor, so its rating sits in a window: at or below the wire's ampacity, and at or above the load. The tool suggests the smallest standard size in that window, and ABYC lets you step to the next size up to 150 percent of the wire's ampacity. Panel feeders, DC motors like a windlass, and engine cranking each follow their own rules, so treat the number as a starting point. ABYC also wants that device within about 7 inches of the power source, or 72 inches if the wire runs in a sheath or conduit. Circuit protection is its own requirement, separate from wire sizing.

Marine wire ampacity chart (105°C)

ABYC E-11 allowable current for a single 105°C conductor, outside and inside the engine space. This is the ampacity half of the sizing. The voltage-drop half depends on your run length and voltage, which is what the calculator works out. Bundling two or more conductors derates these further (for DC, down to 0.70).

AWG Outside engine space (A) Inside engine space (A)
16 25 21.3
14 35 29.8
12 45 38.3
10 60 51
8 80 68
6 120 102
4 160 136
2 210 178.5
1 245 208.3
1/0 285 242.3
2/0 330 280.5
3/0 385 327.3
4/0 445 378.3

Wiring the rest of the system

Once the gauge is sized, here is the gear it connects.

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Common questions

What size wire do I need for a given load and distance?

Size it two ways and take the larger. First for voltage drop, using the round-trip length and your system voltage: thinner wire over a long run drops too much voltage. Second for ampacity, the current the wire can carry without overheating. Long runs are usually limited by voltage drop, while short high-current runs like a windlass or inverter are limited by ampacity. Enter your load in amps, the one-way run, the voltage, and the circuit type above, and the calculator runs both checks and tells you which one governs.

Why does the calculator double my run length?

Because current has to flow out to the device and back to the battery, so the resistance that causes voltage drop is the full round-trip length of the wire, both the positive and the negative conductor. If you size off the one-way distance you halve the resistance in the math and end up a gauge or two too thin. Enter the one-way distance from the source to the device and the tool doubles it for the calculation.

What is voltage drop and why 3 percent versus 10 percent?

Voltage drop is the voltage lost to the resistance of the wire between the battery and the device. ABYC E-11 allows up to 3 percent on critical circuits (navigation lights, bilge pumps, electronics, and main feeders) and up to 10 percent on non-critical ones like cabin lighting. The reason to favor 3 percent is that a low-voltage device underperforms: dim nav lights, a slow bilge pump, electronics that brown out. When in doubt, design to 3 percent.

Does the wire size change inside the engine compartment?

Yes. A conductor in the hot engine space is rated for less current than the same wire in open air, because it has less room to shed heat. The calculator has an inside-engine-space option that uses the lower ampacity column. If current-carrying conductors are also bundled together for more than two feet, the bundle is derated further, and ABYC applies this once two or more are bundled, though for DC circuits that derate stops at 0.70. Worth knowing too: real engine bays, especially around a turbo or a tightly enclosed diesel, can run hotter than the table assumes, so lean conservative there.

What gauge wire for a windlass, inverter, or bilge pump?

Enter the device's current draw and run length and let the tool size it, because the answer depends entirely on the amps and the distance. High-draw DC motors like a windlass or a large inverter are usually ampacity-limited and need surprisingly heavy cable over even a short run, and they also have their own ABYC fuse rules tied to the motor rather than the generic wire rule. A bilge pump is a critical circuit, so size it to 3 percent voltage drop.

What size fuse do I need for my wire?

The fuse or breaker protects the wire, so its rating has to sit at or below the wire's ampacity, and at or above your load so it does not nuisance-trip. The calculator suggests the smallest standard size in that window, and ABYC lets you step to the next size up to 150 percent of the wire's ampacity. Main panel feeders, DC motor circuits, and engine cranking each follow their own rules. ABYC also wants the device within about 7 inches of the power source, or 72 inches if the wire runs in a sheath. Confirm overcurrent protection against the standard; it is a separate requirement from wire sizing.

Is marine wire different from regular wire?

Yes. Boat wiring uses stranded copper, not solid, because solid wire fatigues and breaks under vibration. It is almost always rated to 105 degrees C, and tinned copper is widely recommended for corrosion resistance in the salt air, though that is a best practice rather than an ABYC requirement. Marine ampacity is set by the ABYC E-11 tables, which differ from the household wiring tables, so do not size boat wire off a house-wiring chart. This calculator assumes 105 degrees C marine-grade wire. One sourcing check: buy AWG-rated marine wire, not SAE-rated, which carries about 10 percent less copper for the same gauge number and is not interchangeable with these tables.

Does this replace ABYC or a marine electrician?

No. It is a planning guide that implements the ABYC E-11 sizing method to get you to the right gauge and help you shop and understand a quote, not a substitute for the published standard or for a qualified installer. It sizes the conductor only, not the full circuit protection, and it cannot see your terminations, corrosion, or unusual heat sources. Confirm any wire you intend to install against ABYC E-11 or have an ABYC-certified technician review it.

Sources & further reading

The numbers here follow ABYC E-11. Verify against the standard for any installation.

Disclaimer

This calculator provides wire-sizing estimates based on the ABYC E-11 method for AC and DC electrical systems on boats, for 105°C marine-grade copper conductors. It is not a substitute for the published standard or for a qualified marine electrician. It sizes the conductor only and does not size your fuse or circuit breaker, which is a separate ABYC requirement. Results must not be the sole basis for an installation: any wire size selected here should be reviewed for adequacy before installation by a professional applying the applicable industry standards. This tool does not account for heated terminations, corrosion, wiring errors, data-entry errors, unusual environmental heat sources, defective insulation, or overcurrent-protection sizing. Sorted Gear expressly disclaims responsibility for any use of this calculator that results in inadequate wire size or circuit protection. Consult an ABYC-certified technician for final installations, and when in doubt, size up. Last reviewed June 2026.

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