What gauge wire do I need for a 12 volt boat circuit?

Take the current the device draws and the round-trip length of the run, then read the gauge from the charts above. As an example, 10 amps over a 30 foot round trip needs 10 AWG on a critical circuit at 3 percent drop, or 14 AWG on a non-critical circuit at 10 percent. Both figures come from the ABYC E-11 voltage drop formula the tables are built on.

What size wire for a bilge pump?

Bilge pumps are normally treated as critical circuits and sized to a 3 percent drop. On the chart above, a pump drawing 10 amps needs 14 AWG at a 10 foot round trip, 12 AWG at 20 feet and 10 AWG at 30 feet. Check the amp draw on your pump's label and use the round-trip length of its actual wiring path.

Why does the chart use round-trip length?

Because voltage is lost over the whole circuit: out along the positive conductor and back along the negative. A device 15 feet from the panel is a 30 foot circuit, and sizing from the one-way distance halves the conductor area the formula calls for.

Which boat circuits count as critical?

Circuits where reduced voltage means reduced safety or function. Panel main feeds, navigation lights, bilge pumps, blowers and electronics are the ones usually held to the tighter 3 percent voltage drop target in ABYC E-11. General lighting and accessories are sized to the 10 percent target.

Is a bigger wire ever a problem?

Electrically no; a thicker conductor only reduces the drop further. The costs are money, weight and termination: a larger gauge needs matching terminals and may not fit a small device's connections. When the chart puts you between sizes, go thicker.

Buying guide

Marine wire size chart: pick the gauge in one look

By Austin · Updated September 10, 2026

Sizing boat wire comes down to two numbers: the current the device draws, and the length of the run there and back. From those, the charts below read off the gauge directly.

They are computed from the ABYC E-11 voltage drop formula for copper conductors rather than copied from a catalog, using the round-trip length, a 12 volt system, and the two drop targets the standard recommends for conductor sizing: 3 percent for critical circuits and 10 percent for everything else. Each result is then checked against the conductor ampacity table in the same standard, because a gauge that satisfies the drop calculation still has to carry the current without overheating.

This page owns the numbers. For the wire itself, its circuit protection and its connections, see the marine wiring basics guide.

How to read the charts

Use the round trip. Measure from the power source out to the device and back to the negative bus, following the actual path of the wire. A pump 15 feet away is a 30 foot circuit. Sizing from the one-way distance is an easy mistake to make, and it halves the conductor area the calculation actually asks for.

Decide whether the circuit is critical. Panel main feeds, navigation lights, bilge pumps, blowers and electronics are typically treated as critical and sized to the tighter 3 percent target, so they get close to full voltage. Cabin lights and general accessories are non-critical and sized to 10 percent, where the only symptom is slight dimming.

Between sizes, go bigger. A lower AWG number is a thicker wire. If your amps or length fall between rows, take the next thicker gauge; copper is cheaper than pulling a run twice.

Worked example: a bilge pump drawing 10 amps, mounted 15 feet from the panel. Round trip 30 feet, critical circuit. The formula asks for 8,958 circular mils, the next standard size up is 10 AWG at 10,380 circular mils, and 10 AWG is rated far above 10 amps, so 10 AWG it is.

Critical circuits: 3 percent drop at 12 volts

For navigation lights, bilge pumps, electronics and anything else whose full performance matters. Gauge by current and round-trip length:

Current	10 ft round trip	20 ft	30 ft	40 ft
5 A	16 AWG	14 AWG	12 AWG	12 AWG
10 A	14 AWG	12 AWG	10 AWG	8 AWG
15 A	12 AWG	10 AWG	8 AWG	6 AWG
20 A	12 AWG	8 AWG	6 AWG	6 AWG
25 A	10 AWG	8 AWG	6 AWG	4 AWG

Source: ABYC E-11. Circular mils = 10.75 x amps x round-trip feet / allowable volts drop, here 0.36 V (3 percent of 12 V), rounded up to the next standard AWG size, with 16 AWG as this chart's floor for a circuit run. Each result is then checked against the ABYC E-11 allowable amperage for 105 C insulated conductors outside engine spaces (16 AWG 25 A, 14 AWG 35 A, 12 AWG 45 A, 10 AWG 60 A, 8 AWG 80 A, 6 AWG 120 A, 4 AWG 160 A). Where that limit is the stricter of the two it governs and the chart prints the heavier gauge; no cell in this table needed that override. 12 V system, conductors not bundled and outside engine spaces.

Non-critical circuits: 10 percent drop at 12 volts

For general lighting and accessories where a slightly lower voltage at the far end is acceptable:

Current	10 ft round trip	20 ft	30 ft	40 ft
5 A	16 AWG	16 AWG	16 AWG	16 AWG
10 A	16 AWG	16 AWG	14 AWG	14 AWG
15 A	16 AWG	14 AWG	14 AWG	12 AWG
20 A	16 AWG	14 AWG	12 AWG	10 AWG
25 A	14 AWG	12 AWG	10 AWG	10 AWG

Source: ABYC E-11. Same formula and the same 16 AWG floor at a 1.2 V allowable drop (10 percent of 12 V), then the same ampacity check. Here the ampacity does govern one cell: 25 A over a 10 ft round trip needs only 2,240 circular mils on voltage drop alone, but 16 AWG is rated at exactly 25 A, and this chart never runs a conductor at its full rating, so it prints 14 AWG instead. The conductor still needs a fuse or breaker sized to it. 12 V system, conductors not bundled and outside engine spaces; inside an engine space the allowable amperage falls by roughly 15 percent, so re-check any row where the current is close to the gauge's rating.

The caveats that keep the chart honest

Two constraints, not one. On longer runs the voltage drop calculation is the stricter of the two and it sets the gauge. On short runs at high current the drop calculation alone would allow a thin conductor, and the ampacity limit governs instead, which is why 25 A over a 10 foot round trip is printed as 14 AWG on the non-critical chart rather than 16 AWG. Either way, every conductor needs overcurrent protection sized to the wire so the wire is not left as the weakest link in the circuit.

Bundling derates. Many conductors bundled tightly or run in conduit shed heat poorly, and ABYC E-11 publishes correction factors for that based on how many current-carrying conductors are in the bundle. Look the factor up and apply it rather than assuming a single step up in size. Conductors inside an engine space derate as well.

24 volt systems need less copper for the same power. The same percentage allowance at 24 volts gives twice as many volts of headroom, and the current a given wattage draws halves. That does not work out to a fixed number of gauge sizes, so run the formula again at 24 volts rather than reading these 12 volt tables in reverse.

Marine wire only. The gauge is only half the specification; the other half is tinned, finely stranded, marine-rated conductor. The marine wiring basics guide covers why automotive wire that looks identical fails early on a boat, and how to protect and terminate the run once it is sized.

Tinned marine wire and cable

Ancor and Pacer tinned copper primary wire, duplex and triplex cable, and battery cable in the gauges the charts call for.

Ancor · 60264Ancor Green 18 AWG Tinned Copper Primary Wire 100310 100 ft$38.99Only 4 left

Sea-Dog · 112412Sea-Dog 10 AWG White Primary Wire 8010050 25 ft$41.99Only 2 left

Sea-Dog · 112417Sea-Dog Primary Wire 8010060 10 AWG Blue 25 ft$41.99Only 4 left

Ancor · 31870Ancor Grey 16 AWG Primary Wire 102410 100 ft Tinned Copper Marine Grade$41.99Only 5 left

Ancor · 60353Ancor Marine Grade Tinned Copper Wire 102110 16 AWG Dark Blue 100 ft Spool$41.99In stock

Ancor · 60358Ancor Marine Grade Primary Wire 102210 16 AWG Brown 100 ft$41.99Only 7 left

Ancor · 60410Ancor Marine Grade Primary Wire 102510 16 AWG Orange Tinned Copper 100 ft$41.99In stock

Sea-Dog · 112563Sea-Dog 14 AWG Green Primary Wire 8014042 100 ft$43.99Only 3 left

Terminals, lugs and circuit protection

Adhesive-lined heat-shrink terminals, tinned lugs for heavy gauges, and the fuse blocks and breakers that protect the run.

Ancor · 55445Ancor 16-14 Gauge #8 Heat Shrink Ring Terminal 311203 3 Pack$22.99In stock

Ancor · 48373Ancor Heavy Duty Tinned Copper Lug 252244 6 AWG 1/4in Stud 2-Pack$22.99In stock

Ancor · 55447Ancor 16-14 Gauge 1/4 in Heat Shrink Ring Terminal 311403 3 Pack$22.99Only 8 left

Ancor · 55451Ancor Heat Shrink Ring Terminal 311503 16-14 Gauge 5/16in 3-Pack$22.99In stock

Ancor · 68579Ancor 16-14 AWG Nylon Butt Connector 210110 25-Pack$24.99Only 1 left

Blue Sea Systems · 40584Blue Sea Systems In-Line ATO ATC Fuse Holder 5064 30A Max 12 AWG Pigtails$25.99In stock

Ancor · 68456Ancor 22-18 AWG #6 Nylon Ring Terminal 210201 25-Pack$25.99Only 5 left

Egis Mobile Electric · 113740Egis Mobile Electric ATM/MINI 5 Amp Circuit Breaker 4720-05 Type 3$26.99Only 9 left

Common questions

What gauge wire do I need for a 12 volt boat circuit?: Take the current the device draws and the round-trip length of the run, then read the gauge from the charts above. As an example, 10 amps over a 30 foot round trip needs 10 AWG on a critical circuit at 3 percent drop, or 14 AWG on a non-critical circuit at 10 percent. Both figures come from the ABYC E-11 voltage drop formula the tables are built on.
What size wire for a bilge pump?: Bilge pumps are normally treated as critical circuits and sized to a 3 percent drop. On the chart above, a pump drawing 10 amps needs 14 AWG at a 10 foot round trip, 12 AWG at 20 feet and 10 AWG at 30 feet. Check the amp draw on your pump's label and use the round-trip length of its actual wiring path.
Why does the chart use round-trip length?: Because voltage is lost over the whole circuit: out along the positive conductor and back along the negative. A device 15 feet from the panel is a 30 foot circuit, and sizing from the one-way distance halves the conductor area the formula calls for.
Which boat circuits count as critical?: Circuits where reduced voltage means reduced safety or function. Panel main feeds, navigation lights, bilge pumps, blowers and electronics are the ones usually held to the tighter 3 percent voltage drop target in ABYC E-11. General lighting and accessories are sized to the 10 percent target.
Is a bigger wire ever a problem?: Electrically no; a thicker conductor only reduces the drop further. The costs are money, weight and termination: a larger gauge needs matching terminals and may not fit a small device's connections. When the chart puts you between sizes, go thicker.

Sources

ABYC E-11, AC and DC Electrical Systems on Boats: the 3 percent and 10 percent voltage drop design targets, the circular mil conductor sizing formula, the allowable amperage table for insulated conductors (105 C column, with the 0.85 correction for engine spaces that gives the roughly 15 percent reduction quoted here), and the correction factors for bundled conductors
ASTM B258, standard nominal diameters and cross-sectional areas of AWG sizes of solid round wires used as electrical conductors: the circular mil area of each gauge used in the calculation

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