Marine Electrical
AWG circular mil chart: the numbers behind marine voltage drop
Quick answer
American Wire Gauge sizes cable by circular mil area, published from 1,620 circular mils at 18 AWG to 211,600 circular mils at 4/0. The standard voltage drop formula uses that area directly: drop volts equals 2 times 10.75 times amps times length, divided by circular mils, and the 2 accounts for the round trip.
Every marine wire gauge has a published circular mil area, and that area, not the gauge number itself, is what the standard voltage drop formula actually uses. This chart lists the American Wire Gauge circular mil table exactly as published, then works through the copper constant and the round trip factor that most wire sizing mistakes on a boat come down to.
This page covers voltage drop only. Ampacity, the separate question of how much current an insulation can carry, depends on the conductor's temperature rating, whether the run passes through an engine space, and how many current carrying conductors are bundled together. ABYC E-11 covers that in a table this site does not reproduce, because a single number by gauge would be wrong for most installations.
On this page
What is a circular mil?
A circular mil is the cross sectional area of a wire exactly one thousandth of an inch in diameter, squared. It is the unit the American Wire Gauge standard actually publishes for each gauge, and it is what the voltage drop formula multiplies against, not the gauge number. A larger circular mil figure means a physically thicker conductor and a lower resistance per foot, and it is the reason two cables with very different AWG numbers can carry the same current the same distance with very different results.
AWG circular mil chart
The table below is the published American Wire Gauge standard, unchanged, for the sizes this site sizes marine circuits around.
The published American Wire Gauge standard runs from 1,620 circular mils at 18 AWG to 211,600 circular mils at 4/0, and this area, not the gauge label, is what the voltage drop formula actually uses.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| AWG gauge | Circular mils (published) |
|---|---|
| 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 |
Why does the voltage drop formula have a 2 in it?
Current runs out to the load along the positive conductor and back to the battery along the negative conductor, so the electrical length of the circuit is twice the one way distance measured with a tape. Leaving that factor of 2 out of the formula halves the computed drop, understates it, and is the single most common error in marine wire sizing.
The formula is drop volts equals 2 times K times amps times length, divided by circular mils, and K, the copper constant, is 10.75, a physical property of copper rather than a convention.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Term | Value | What it means |
|---|---|---|
| K, the copper constant | 10.75 | Circular mil ohms per foot for copper, published and fixed |
| The factor of 2 | 2 | Round trip: current travels out on the positive conductor and back on the negative, doubling the effective conductor length |
A worked voltage drop example
Take a 55 lb thrust trolling motor with a published maximum draw of 50 amps, on a 20 foot one way run from the battery. The table below runs that draw through several gauges using the same formula and compares the result against the ABYC critical circuit limit.
In this worked example, only 2 AWG and larger hold the drop under the ABYC 3 percent critical circuit limit, and 10 AWG, still a common size in older wiring, loses more than 17 percent.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| AWG gauge | Drop volts | Drop percent at 12V |
|---|---|---|
| 10 | 2.07 | 17.3% |
| 8 | 1.30 | 10.8% |
| 6 | 0.82 | 6.8% |
| 4 | 0.52 | 4.3% |
| 2 | 0.32 | 2.7% |
| 1 | 0.26 | 2.1% |
This is a worked example for one draw and one length. Change either input and the answer changes; the formula, not this specific row, is the fixed part. This table sizes for voltage drop only and says nothing about ampacity, which depends on insulation rating, engine space location and conductor bundling and is covered by the separate ABYC E-11 table.
ABYC voltage drop limits
ABYC E-11 allows 3 percent drop on critical circuits such as electronics, navigation lights and bilge blowers, and 10 percent on circuits where voltage matters less.
Published figure A published engineering, regulatory or manufacturer figure. It does not change because somebody disagrees with it.
| Circuit type | Maximum allowed drop |
|---|---|
| Panel feeds, electronics, navigation lights, bilge blowers (critical) | 3% |
| Circuits where voltage matters less (noncritical) | 10% |
Tools for measuring your own run
A ratchet crimper that will not release until the crimp is fully formed removes the most common failure in a homemade cable, and something like iCrimp AP-50BI battery cable lug crimper, 8 to 2 AWG is built specifically for heavy battery cable lugs rather than small terminal crimps. A cable such as End Game 6 gauge marine battery cable, USCG and ABYC compliant arrives with tinned lugs already fitted and states USCG and ABYC compliance in its own listing, which is rarer than the marketing phrase marine grade suggests. Measuring the actual drop on a finished run, rather than trusting the math alone, is worth a real meter such as Fluke 15B+ digital multimeter.
Frequently asked questions
What is a circular mil?
A circular mil is the cross sectional area of a wire exactly one thousandth of an inch in diameter, squared. It is the unit the American Wire Gauge standard publishes for every gauge, and the voltage drop formula multiplies amps and length against that area rather than against the gauge number itself. A larger circular mil figure means a physically thicker conductor and a lower resistance per foot of run.
Why does the marine voltage drop formula include a 2?
Current flows out to the load along the positive conductor and returns to the battery along the negative conductor, so the electrical path is twice the one way distance measured with a tape. Leaving the 2 out of the formula, drop volts equals 2 times the copper constant times amps times length divided by circular mils, cuts the computed answer in half and is the single most common wire sizing mistake made on a boat.
What copper constant does this chart use?
This chart uses 10.75, the published circular mil ohm per foot constant for copper used in the standard marine voltage drop calculation. It is a physical property of copper rather than a convention, so it does not change with the boat, the brand of wire, or the length of the run. It is one of the two published constants, along with the AWG circular mil table, behind every drop figure on this page.
Does a larger circular mil size make a cable safe for more amps?
Not by itself. Circular mil area decides voltage drop over a run, but the separate question of ampacity, how much current the insulation can carry, depends on the conductor's temperature rating, whether the run passes through an engine space, and how many conductors are bundled together. This site does not publish an ampacity figure by gauge, so check the ABYC E-11 table for that number instead.
What gauge wire does a 55 lb thrust trolling motor need?
It depends on the run length, not a single fixed answer. A 55 lb thrust motor draws up to a published 50 amps at maximum, and running that through the voltage drop formula over a 20 foot one way run points to at least 2 AWG to hold the drop under the ABYC 3 percent critical circuit limit. A shorter or longer run changes the answer, so run the numbers for the actual installation rather than copying a table.
Is voltage drop the same thing as ampacity?
No. Voltage drop is how much voltage a conductor loses over its length at a given current, governed by the circular mil area and the run length. Ampacity is how much current the insulation can carry without overheating, governed by the temperature rating, the location and how many conductors are bundled together. A cable has to satisfy both limits at once, and they come from two different tables.
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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.