Electrical
How to size marine wire by voltage drop, not an ampacity chart
Quick answer
Size marine wire by voltage drop, not by an ampacity chart. Use drop volts equals two times 10.75 times amps times length, divided by circular mils, hold three percent drop on circuits where voltage matters, and check the ABYC E-11 ampacity table separately for the current limit your insulation, engine space and bundling actually allow.
Sizing a conductor on a boat is really two separate checks wearing one name. The first is voltage drop: whether the wire is thick enough that the device on the far end still gets a usable voltage after the run loses some of it to resistance. The second is ampacity: whether the wire can carry that current continuously without overheating its insulation. A cable has to pass both. Most articles on this subject collapse the two into a single gauge chart and hand you a number, which looks helpful and is exactly the kind of shortcut that gets a boat wired wrong.
This guide only does the first half, on purpose. Voltage drop has one published formula, one published copper constant, and an answer that does not depend on where the cable happens to run. Ampacity depends on the insulation's temperature rating, whether the run passes through an engine space, and how many other current carrying conductors are bundled next to it, which is why the published ABYC E-11 ampacity tables are tables, not a single number, and why nothing on this page tries to compress them into one. Work the drop calculation here, then take the result to the E-11 table for the ampacity check before you buy cable.
On this page
Two checks, not one, and this page only does the first
A marine conductor has to satisfy voltage drop and ampacity at the same time, and they are not the same question. Voltage drop asks whether enough of the source voltage survives the trip down the cable to actually run the device, which matters most on electronics, navigation lights, bilge blowers and anything with a motor winding sensitive to low voltage. Ampacity asks whether the copper and its insulation can carry that current indefinitely without the insulation breaking down from heat. A cable sized only for drop can still be a fire risk if it is undersized for ampacity, and a cable sized only for ampacity can leave a trolling motor starved for voltage over a long run. Both checks are required. This page walks the drop calculation in full and treats the ampacity check as a separate, deliberate step handled by the published E-11 tables rather than by anything printed here.
The voltage drop formula, and the 2 that gets forgotten
Marine voltage drop, published formula
Published figuredrop volts = ( 2 x K x amps x length ) / circular mils K = 10.75 for copper length is the one-way run in feet, from source to device
The 2 is the round trip. Current flows out along the positive conductor and returns along the negative, so the conductor length the resistance actually acts over is twice the one-way distance you measure with a tape. This is the single most common error in marine wire sizing, and leaving the 2 out understates the drop by exactly half, which is enough to pass a circuit that should have failed.
How much drop ABYC actually allows
ABYC E-11 sets two drop limits rather than one. Circuits where voltage actually matters, meaning panel feeds, electronics, navigation lights and bilge blowers, are held to three percent of system voltage. Circuits where a little sag does not change the outcome are allowed ten percent. A trolling motor feed is treated as a three percent circuit on this site, and that is a deliberate choice rather than something E-11 spells out by name for that exact device: a motor running at ten percent drop still runs, it simply delivers less thrust for the amp hours you are paying to draw from the battery, and the cable is the cheapest part of the whole system to size correctly the first time. On a twelve volt system, three percent is about 0.36 volts of allowable drop, and ten percent is about 1.2 volts.
Work the sizing steps in order
- Measure the one-way run, not the round trip. Run a tape or a piece of string along the actual path the cable will take, including every bend around a bulkhead or under a deck, from the battery to the device. This is the length that goes into the formula. The formula itself doubles it for you.
- Get the real current draw, not a guess. Use the device manufacturer's own published maximum current draw where one exists, such as a trolling motor's stated maximum amps at full thrust. Where no figure is published, a clamp meter reading on the actual device beats an estimate every time.
- Pick the drop limit for the circuit type. Three percent for panel feeds, electronics, navigation lights, bilge blowers and trolling motor feeds. Ten percent for circuits where a little sag genuinely does not matter, such as some accessory lighting.
- Solve for the circular mils you need. Rearrange the formula: circular mils needed equals two times 10.75 times amps times length, divided by the allowed drop in volts. Compare that number against the AWG circular mil table and round up to the next size.
- Check the ampacity separately, using the E-11 table for your insulation and location. The gauge that passes voltage drop is not automatically large enough for continuous current. Look up your conductor's ampacity in the ABYC E-11 tables using its actual insulation temperature rating and whether the run passes through an engine space, then use whichever of the two gauges, drop or ampacity, comes out larger.
- Buy tinned, finely stranded marine cable rather than automotive wire. The AWG circular mil figures the formula relies on describe the conductor, and a marine grade cable like End Game 6 gauge marine battery cable, USCG and ABYC compliant gives you that copper cross section in a tinned, finely stranded build that survives a damp, vibrating boat instead of corroding from the inside.
Required gauge to hold a 3 percent drop, sample circuits
Every row below is the smallest AWG gauge that holds ABYC's three percent limit on a 12 volt system, calculated from the formula above rather than looked up on a chart. Run your own numbers before buying cable: a different length or a different published current draw changes the answer.
Doubling the one-way run from 10 feet to 20 feet on a 50 amp trolling motor feed pushes the required gauge from 4 AWG to 2 AWG, which is the length of the run mattering as much as the current it carries.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Circuit | One-way length | Current | Gauge needed for 3% drop |
|---|---|---|---|
| Bilge pump feed | 15 ft | 5 A | 12 AWG |
| Panel branch circuit | 10 ft | 10 A | 12 AWG |
| Accessory circuit | 15 ft | 20 A | 6 AWG |
| Trolling motor feed | 10 ft | 50 A | 4 AWG |
| Trolling motor feed | 20 ft | 50 A | 2 AWG |
Each figure is calculated from the formula above with a 3 percent limit at 12 volts. It is a voltage drop answer only. Confirm the ampacity for your insulation rating and installation location against the ABYC E-11 table before final sizing.
Buy the cable and the crimper as one purchase
A correctly sized run of tinned marine cable is only as good as the connection at each end of it. End Game 6 gauge marine battery cable, USCG and ABYC compliant arrives with tinned lugs already fitted for exactly this reason, and if you are making up your own ends, a proper crimper matters as much as the cable gauge does: Ancor Marine Grade single crimp ratchet tool covers terminal sizes and iCrimp AP-50BI battery cable lug crimper, 8 to 2 AWG handles heavier battery cable lugs that a terminal crimper cannot compress correctly. A ratchet tool that will not release until the crimp is fully formed removes the single most common cause of a joint that looks fine and fails within a season.
Sources
- ABYC E-11, AC and DC Electrical Systems on Boats, and the American Wire Gauge circular mil standard
Frequently asked questions
What is the single biggest mistake people make sizing marine wire
Forgetting that the formula's 2 accounts for the round trip. Current flows out on the positive conductor and returns on the negative, so the electrical length the resistance acts over is twice the one-way distance measured with a tape. Leaving the 2 out of the calculation understates the drop by exactly half, which can make an undersized cable appear to pass a three percent check that it actually fails by a wide margin once the real math is run correctly.
Does going up one wire size always fix a voltage drop problem
Usually, but check the number rather than assuming it. Circular mil area does not increase linearly with AWG number, so the improvement from one step up in gauge is larger at some points on the scale than others. Recalculate the drop with the new gauge's circular mil figure rather than assuming one size up automatically clears the ABYC limit, especially on a long run where the answer can still fall short.
Why will this site not tell me the ampacity of a given wire gauge
Because ABYC E-11 ampacity is not a single number per gauge. It depends on the conductor's insulation temperature rating, whether the run passes through an engine space, and how many other current carrying conductors are bundled with it. A single figure by gauge alone would be wrong for most installations and wrong toward overheating a cable, so this site sizes by voltage drop and sends you to the published E-11 table for the ampacity check.
Should a trolling motor feed use the 3 percent or 10 percent drop limit
Three percent, treated on this site as a deliberate choice rather than a rule ABYC spells out by device name. A motor running at ten percent drop still runs, but it delivers less usable thrust for the same battery draw, effectively wasting capacity you paid for. Cable is the cheapest part of the whole system to size correctly once, so holding the tighter three percent limit on the motor feed is worth the slightly heavier gauge it calls for.
Can I use automotive wire instead of tinned marine cable
The copper cross section for a given AWG is the same either way, so the voltage drop formula works identically. What differs is corrosion resistance: automotive wire uses bare copper strands that corrode from the inside where you cannot see it happening until a joint heats up, while tinned marine cable resists that corrosion in a damp, vibrating environment. The full comparison, including where automotive wire is genuinely fine, is covered on the tinned wire versus automotive wire page.
How do I measure the length to plug into the formula
Follow the actual physical path the cable will take from the battery to the device, including every turn around a bulkhead, through a conduit or under a deck, rather than the straight line distance between the two points. Add a foot or two of slack for a real installation, since the formula is sensitive to length and a run measured short will understate the drop and undersize the cable.
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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.