GearPort
Marine wire size chart: pick the gauge in one look
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 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 limits the standard sets: 3 percent for critical circuits and 10 percent for everything else. The full reasoning lives in the marine wiring guide; this page is the chart.
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. Measuring one way is the most common mistake and it yields wire one or two sizes too thin.
Decide whether the circuit is critical. Navigation lights, bilge pumps and electronics are critical, held to a 3 percent drop so they get full voltage. Cabin lights and accessories are non-critical and allowed 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, so the 3 percent chart says 10 AWG.
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 |
Computed from the ABYC formula CM = 10.75 x amps x round-trip feet / 0.36 V, rounded up to the next standard gauge, with 16 AWG as the practical minimum for a circuit run. 12 V system, conductors not bundled.
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 | 16 AWG | 12 AWG | 10 AWG | 10 AWG |
Same formula at a 1.2 V allowable drop. Even on non-critical circuits the wire must still carry the current safely and be protected by a fuse or breaker sized to the conductor.
The caveats that keep the chart honest
The chart sizes for voltage drop, not for heat. At these currents and gauges the drop requirement is the stricter one, which is why it drives the table, but every conductor still needs overcurrent protection sized to the wire so the wire can never become the fuse.
Bundling derates. Many conductors bundled tightly or run in conduit shed heat poorly. If the run is heavily bundled, step up a size.
24 volt systems can run thinner wire for the same power because the current halves and the allowable drop doubles in volts. These tables are for 12 volt systems; do not apply them to 24 volt sizing 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 guide covers why automotive wire that looks identical fails early on a boat.
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.
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.
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 a voltage drop chart. 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.
- What size wire for a bilge pump?
- Bilge pumps are critical circuits, held to a 3 percent drop. 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 produces wire one or two sizes too small.
- Which boat circuits count as critical?
- ABYC holds circuits like navigation lights, bilge pumps and electronics to the tighter 3 percent voltage drop, because reduced voltage there means reduced safety or function. General lighting and accessories may use the 10 percent allowance.
- 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.