Marine electrical
What gauge marine wire do you actually need?
Quick answer
Marine wire is sized for voltage drop using the published formula drop volts equals 2 times 10.75 times amps times length, divided by circular mils, where the 2 accounts for the round trip out and back. This chart deliberately never publishes an ampacity figure by gauge alone, because that number depends on insulation, location and bundling.
Wire gauge on a boat gets sized by two entirely separate checks, and this chart deliberately covers only one of them. The first check is voltage drop: does the conductor stay thin enough in resistance, over its actual length, that the device at the far end still receives close to full voltage. The second check is ampacity: how much current the conductor can carry continuously without overheating its insulation. This site publishes the first and refuses to publish the second, and the reason is not caution for its own sake.
ABYC E-11's ampacity tables depend on the conductor's insulation temperature rating, on whether the run passes through an engine space, and on how many current-carrying conductors are bundled together in the same run. A single number indexed by gauge alone would be wrong for most real installations, and wrong in the direction that lets a cable run hotter than it should. So the sizing below runs entirely on the published voltage drop formula and the published AWG circular mil standard, and the gap where ampacity would sit is called out explicitly rather than filled with an estimate.
On this page
The voltage drop formula, stated explicitly
Marine voltage drop
Published figuredrop volts = ( 2 x K x amps x length ) / circular mils K = 10.75, the published copper resistivity constant used in the standard marine calculation The 2 is the round trip: current flows out on the positive conductor and returns on the negative, so the effective conductor length is twice the one-way run Forgetting the 2 is the single most common wire sizing error, and it halves every answer
This is the standard marine voltage drop calculation, used consistently across every reference and calculator page on this site.
AWG circular mil area, the published basis for the calculation
The circular mil figure is the actual cross sectional area of the conductor, and it is what the voltage drop formula divides by. A heavier gauge simply has more copper to carry current through with less resistance loss over a run.
Circular mil area runs from 1,620 circular mils at 18 AWG up to 211,600 circular mils at 4/0, the published American Wire Gauge standard used in every voltage drop calculation on this site.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| AWG gauge | Circular mils |
|---|---|
| 18 | 1,620 |
| 16 | 2,580 |
| 14 | 4,110 |
| 12 | 6,530 |
| 10 | 10,380 |
| 8 | 16,510 |
| 6 | 26,240 |
| 4 | 41,740 |
| 2 | 66,360 |
| 1 | 83,690 |
| 1/0 | 105,600 |
| 2/0 | 133,100 |
| 3/0 | 167,800 |
| 4/0 | 211,600 |
This is the published American Wire Gauge circular mil standard, not a marine-specific figure. It does not change by cable brand or by manufacturer.
Voltage drop by gauge at a fixed load and run length
Running the formula above at a fixed 50 amp load over a 10 foot one-way run shows how much gauge actually matters at a real trolling motor current.
At 50 amps over a 10 foot one-way run, 6 AWG produces a 3.4 percent drop on a 12 volt system, just over the ABYC critical circuit limit, while 4 AWG stays at 2.2 percent.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| AWG gauge | Voltage drop | Percent of a 12V system |
|---|---|---|
| 8 | 0.65V | 5.4% |
| 6 | 0.41V | 3.4% |
| 4 | 0.26V | 2.2% |
| 2 | 0.16V | 1.4% |
Calculated from the formula above at 50 amps, 10 feet one-way, using the round trip factor of 2. This is not a measurement, it is the published formula applied to a stated example.
What decides which gauge is correctly sized here
ABYC E-11 sets a 3 percent drop limit for critical circuits and 10 percent for non-critical circuits, and this site treats a trolling motor feed as a 3 percent circuit by deliberate choice.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Circuit type | Maximum drop | Example circuits |
|---|---|---|
| Critical | 3% | Panel feeds, electronics, navigation lights, bilge blowers, trolling motor feeds |
| Non-critical | 10% | Circuits where a small voltage sag does not affect safety or function |
A motor will still run at 10 percent drop, it simply delivers less of the power the battery paid for. The cable is the cheapest part of the system to size correctly once.
A gauge by typical circuit, without a current figure attached
Signal and accessory wiring typically runs 18 to 16 AWG, while a trolling motor feed typically runs 6 to 4 AWG or heavier depending on the run length and current.
Rule of thumb Boating and fishing convention rather than a published standard. Taught everywhere, useful, and not a specification. Nothing enforces it and no body publishes it.
| Gauge range | Typical circuit |
|---|---|
| 18 to 16 AWG | Signal wiring and low current accessories |
| 14 to 12 AWG | Navigation lights, bilge blower, small pumps |
| 10 to 8 AWG | Distribution panel feeds, mid size pumps |
| 6 to 4 AWG | Trolling motor feeds on a typical run length |
| 2 AWG and heavier | Main battery cable and other high current runs |
These are typical circuit associations from marine wiring convention, not an ampacity rating. The correct gauge for any specific circuit still comes from the voltage drop calculation above, checked against your own run length and current.
Frequently asked questions
What gauge marine wire do I need for my trolling motor?
Run the published voltage drop formula using your motor's actual maximum current draw and your own one-way run length, then check the result against the ABYC E-11 ampacity table for your insulation and installation. There is no single correct gauge for a trolling motor in general, since the correct answer depends entirely on your specific run length and current.
Why does this site not publish an ampacity chart by wire gauge?
Because ABYC E-11's ampacity figures depend on the conductor's insulation temperature rating, whether the run passes through an engine space, and how many current-carrying conductors are bundled together, none of which a single number by gauge alone can account for. Printing one figure would be wrong for most real installations, in the direction that lets a cable run hotter than it should, so this site sizes for voltage drop and points to the E-11 table instead.
What is a circular mil, and why does the voltage drop formula use it?
A circular mil is the published unit of cross sectional area used for the American Wire Gauge standard, and it is what the voltage drop formula divides by. A larger circular mil figure means more copper cross section, which means less resistance and less voltage lost over the same run length and current, which is why a heavier gauge shows a smaller drop in the formula.
Do I need to know my wire's insulation rating to size it for voltage drop?
No, voltage drop sizing only needs the conductor's circular mil area, the run length and the current, none of which depend on insulation rating. Insulation temperature rating matters for the separate ampacity check, which this site does not publish by gauge, so you need that figure only when you move on to confirming ampacity against the ABYC E-11 table.
Is tinned copper wire sized differently for voltage drop than bare copper?
No, the voltage drop formula and the AWG circular mil figures are the same for tinned and bare copper conductors of the same gauge, since the tin coating adds negligible resistance compared to the copper itself. Tinned construction is chosen for corrosion resistance in a marine environment, not because it changes the voltage drop or ampacity math for a given gauge.
What happens if I undersize the wire gauge on my boat?
The circuit sees more voltage drop than intended, which on a trolling motor shows up as reduced thrust and a battery gauge that reads fine while the motor underperforms. Beyond voltage drop, an undersized conductor relative to its actual ampacity limit can also run hotter than its insulation is rated for, which is a separate and more serious problem than lost performance alone.
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Researched, not professional advice. This page is compiled from published engineering and regulatory figures, manufacturer specifications and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are boating convention rather than published standards, and they are labelled that way wherever they appear. Marine electrical work is not house wiring. Use tinned, finely stranded marine cable, size it for voltage drop and for the ABYC E-11 ampacity table rather than one or the other, and put overcurrent protection within seven inches of the battery positive terminal, because everything between the post and the fuse is unprotected. A lithium battery needs a charger with a lithium profile. Never load a boat past its capacity plate, and remember that canoes and kayaks carry no federal capacity plate at all, so their stated capacity is the manufacturer own figure. Wear the life jacket, and treat early season water as the hazard it is: cold water immersion takes your breath and then your hands long before it takes your core.