Marine electrical
How much voltage drop does your marine wire run actually have?
Quick answer
The published marine voltage drop formula is drop volts equals 2 times 10.75 times amps times length, divided by circular mils, and ABYC E-11 caps critical circuits like a trolling motor feed at 3 percent drop. At 50 amps over a 10 foot one-way run, 6 AWG already exceeds that 3 percent limit on a 12 volt system.
Voltage drop is the voltage lost to resistance along a conductor's length before it reaches whatever it is feeding, and on a boat it is the reason a trolling motor can seem underpowered even though the battery reads full charge and the motor itself is fine. The formula behind it is published rather than approximated, and the single most common mistake in applying it is forgetting that current makes a round trip: it flows out along the positive conductor and returns along the negative, so the effective length in the formula is twice the one-way run.
ABYC E-11 sets the acceptable limit rather than leaving it to preference: 3 percent drop for critical circuits, where a trolling motor feed, navigation lights and electronics all live by this site's deliberate choice, and 10 percent for circuits where a small voltage sag does not matter. The tables below apply the published formula to real run lengths and currents so the abstract percentage becomes an actual number you can check a specific run against.
On this page
- The voltage drop formula, stated explicitly
- ABYC E-11 drop limits by circuit type
- Drop at a fixed gauge and current, across run length and system voltage
- Smallest gauge holding the 3 percent limit, by run length
- The same calculation at a lighter 30 amp load
- Why the run length is measured one-way and doubled inside the formula
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 The 2 accounts for the round trip: current flows out on the positive conductor and returns on the negative, doubling the effective conductor length used in the calculation Forgetting the 2 halves every answer and is the most common error in marine wire sizing
This is the standard marine voltage drop calculation, identical to the one used on the marine wire gauge chart.
ABYC E-11 drop limits by circuit type
ABYC E-11 sets a 3 percent drop limit for critical circuits and 10 percent for non-critical circuits, and a trolling motor feed is treated as a 3 percent critical circuit on this site 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% | Trolling motor feeds, panel feeds, electronics, navigation lights, bilge blowers |
| Non-critical | 10% | Circuits where a small voltage sag does not affect safety or function |
A motor still runs at 10 percent drop, it simply delivers less of the power the battery paid for. Treating the motor feed as a 3 percent circuit is the more conservative and cheaper-in-the-long-run choice.
Drop at a fixed gauge and current, across run length and system voltage
Holding the gauge and current fixed and varying only the run length and the system voltage shows why the same cable and the same motor can pass on a 24 volt boat and fail on a 12 volt one.
A 6 AWG run at 50 amps stays under the 3 percent critical limit to about 10 feet one-way on a 12 volt system, but the same cable and current does not exceed 3 percent until roughly 30 feet one-way on a 24 volt system.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Run length (one-way) | Voltage drop | Percent of 12V system | Percent of 24V system |
|---|---|---|---|
| 5 ft | 0.20V | 1.7% | 0.9% |
| 10 ft | 0.41V | 3.4% | 1.7% |
| 15 ft | 0.61V | 5.1% | 2.6% |
| 20 ft | 0.82V | 6.8% | 3.4% |
| 25 ft | 1.02V | 8.5% | 4.3% |
| 30 ft | 1.23V | 10.2% | 5.1% |
Calculated from the published formula at 6 AWG and 50 amps. The same drop in volts is a larger percentage of a 12 volt system than of a 24 volt system, which is why higher voltage banks tolerate longer runs at the same gauge and current.
Smallest gauge holding the 3 percent limit, by run length
Working the formula backward from the 3 percent ABYC limit at a fixed 50 amp load shows exactly which gauge is the smallest one that still qualifies as the one-way run gets longer.
At 50 amps on a 12 volt system, a 10 foot one-way run needs at least 4 AWG to hold the 3 percent critical drop limit, and a 30 foot run needs 1/0.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Run length (one-way) | Smallest AWG holding 3% drop |
|---|---|
| 5 ft | 8 AWG |
| 10 ft | 4 AWG |
| 15 ft | 2 AWG |
| 20 ft | 2 AWG |
| 25 ft | 1 AWG |
| 30 ft | 1/0 AWG |
Calculated from the published formula and the 3 percent ABYC critical circuit limit at a fixed 50 amp load and 12 volt system. This is a voltage drop result only; it does not check ampacity, which this site does not publish by gauge.
The same calculation at a lighter 30 amp load
Current matters as much as run length. Dropping the load from 50 amps to 30 amps, a realistic figure for a smaller bow mount motor, changes which gauge qualifies at the same run lengths.
The same 20 foot one-way run that needed 2 AWG at 50 amps needs only 4 AWG at a lighter 30 amp load.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Run length (one-way) | Smallest AWG holding 3% drop |
|---|---|
| 10 ft | 6 AWG |
| 20 ft | 4 AWG |
| 30 ft | 2 AWG |
| 40 ft | 1 AWG |
Calculated from the published formula and the 3 percent ABYC critical circuit limit at a fixed 30 amp load and 12 volt system. Always use your own motor's published maximum draw rather than an assumed figure.
Why the run length is measured one-way and doubled inside the formula
Measure the actual distance from the battery to the motor along the path the cable will follow, not a straight line through the hull, and enter that single one-way distance as the length in the formula. The formula itself applies the round trip factor of 2 internally, accounting for the return path along the negative conductor. Doubling the length yourself before entering it into the formula, on top of the formula's own factor of 2, is a common mistake that produces a gauge recommendation twice as heavy as actually needed. Enter the one-way distance once and let the formula's own factor of 2 do the rest.
Frequently asked questions
What is the marine voltage drop formula?
Drop in volts equals 2 times the copper constant 10.75, times amps, times the one-way length in feet, divided by the conductor's circular mil area. The 2 accounts for the round trip current makes, out along the positive conductor and back along the negative, so the effective conductor length used in the calculation is double the one-way distance you measure.
Why do I multiply by 2 in the voltage drop formula?
Because current does not travel one way. It flows from the battery to the load along the positive conductor and returns from the load to the battery along the negative conductor, so the total length of conductor the current actually passes through is twice the one-way distance between the battery and the device. Leaving out the factor of 2 halves the calculated drop and can lead to undersizing the cable.
What is the ABYC voltage drop limit for a trolling motor feed?
This site treats a trolling motor feed as a critical circuit under ABYC E-11, which caps drop at 3 percent. The motor will still run at a higher drop percentage, it simply delivers less of the power the battery is actually producing, so the 3 percent target is a deliberate choice to protect performance rather than a requirement the motor cannot function without.
Does a higher system voltage really allow a longer wire run at the same gauge?
Yes, because voltage drop is measured as a percentage of the system voltage, and the same drop in volts is a smaller percentage on a 24 volt system than on a 12 volt system. A 6 AWG cable carrying 50 amps holds the 3 percent limit to roughly three times the run length on a 24 volt system compared to a 12 volt system at the identical gauge and current.
Do I measure the wire run length one way or round trip?
Measure and enter the one-way distance from the battery to the load along the actual cable path. The published formula already applies the round trip factor of 2 internally to account for the return path, so doubling the length yourself before using the formula counts the round trip twice and results in an oversized gauge recommendation.
Does passing the voltage drop check mean the wire gauge is fully sized correctly?
No. Voltage drop is one of two separate checks a conductor has to pass, the other being ampacity, and this site does not publish an ampacity figure by gauge alone because it depends on insulation temperature rating, engine space location and conductor bundling. A gauge that passes the voltage drop calculation above still needs checking against the ABYC E-11 ampacity table for your specific installation.
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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.