1What your device needs Voltage drop
3 AWG
94 % of allowed drop
At most 10 % drop (1.20 V) at 110 A: calculated 49,271 circular mils (CM), rounded up to 3 AWG.
Marine Wire Size and Fuse Calculator, 12 V, 24 V and 48 V DC to ABYC E-11
Enter what you connect on board and how the wire runs. You instantly get wire size in AWG and the fuse to the guide values of ABYC E-11: not just by voltage drop, but also by heat, engine space and bundling.
Some inputs were invalid and have been replaced.
On a boat the hull is not a return conductor: the negative always runs back in its own wire. That is how we calculated.
2What the wire can take Heat
3 AWG ↑ thicker✓ fine
3 AWG: 56 % load
At 30 °C, 3 AWG carries up to 180 A continuously. With 100 A it runs at 56 %: fine.
3What the installation leaves Installation
3 AWG ↑ thicker✓ fine
3 AWG: 56 % load
Single, in free air: factor 1, each cable carries its full table value. 3 AWG runs at 56 %: fine.
4Which fuse fits Fuse (DC)
MEGA 175 A
none fits
In the fuse box at 65 °C it must hold 120 A continuously: MEGA 175 A (holds 125 A there). The cable carries 180 A, so the fuse protects it against short circuit and overload.
Place the fuse in the positive wire within 7 in (175 mm) of the battery, measured along the wire. Up to 72 in (1.83 m) is allowed only if the wire connects directly to the battery terminal and is in a sheath or enclosure all the way to the fuse; from other power sources up to 40 in (1.02 m). Cranking motor conductors are exempt.
Only for DC on board (12, 24, 48 V). Not for 120 V or 240 V AC: shore power, generator and inverter output belong in the hands of a qualified marine electrician.
How to go thinner
- calculate
- With 24 V instead of 12 V only half the current flows for the same power: 8 AWG. calculate
- calculate
- calculate
- It does not get thinner here without cutting into function or safety.
Lugs, crimps, fuse holders and switches must handle the same current. Most hot wires are really hot connections.
Working, tables and background
Working with your numbers
- 1 Power with margin: 1,200 W × 1.1 = 1,320 W
- 1 Design current at nominal voltage: 1,320 W ÷ 12 V = 110 A
- 1 Conductor length L: 25 ft × 2 = 50 ft (from the positive source connection to the device and back to the negative)
- 1 Allowed voltage drop E: 10 % of the nominal 12 V = 1.20 V
-
1
CM = 10.75 × I × L ÷ E = 10.75 × 110 A × 50 ft ÷ 1.20 V = 49,271 CM, rounded up 3 AWG - 2 Continuous current for heat and fuse: 1,200 W ÷ 12 V = 100 A, at nominal voltage and without margin, because a wire heats up over minutes.
- 2 33 CFR 183.425 Table 5, 105 °C wire column, 30 °C ambient: the smallest size carrying 100 A continuously is 6 AWG.
- 3 Factor for Single, in free air: 1. Needed: 100 A at 30 °C: 6 AWG.
- 4 Fuse: 100 A + 20 % safety factor = 120 A it must hold at 65 °C. Table: MEGA 175 A holds 125 A there. No larger than the wire's ampacity (strict reading of ABYC E-11): 3 AWG carries 180 A.
- = Result: the largest size of steps 1 to 4, at least 16 AWG. 3 AWG
What the result means
- Voltage drop with 3 AWG: 1.12 V (9.4 %). With 12 V at the battery, 10.88 V reach the device.
- Heat in the cable at continuous load: 102 W.
- Copper: 3.64 kg. The copper core is about 5.8 mm across; 3 AWG is about 26.66 mm² of cross-section.
- With 3 AWG the device may be up to 26.6 ft from the battery.
- Cross-check with the real strand resistance (0.666 mΩ/m at 20 °C) and the current without margin (100 A): 1.01 V (8.5 %). Fine, the margin covers the difference to the ABYC constant K = 10.75.
Fuses compared
| Type | Rating | holds at 65 °C | protects the cable |
|---|---|---|---|
| Blade fuse (ATO) | – A | – A | ✓ yesshort circuit only |
| MIDI | 200 A | 122 A | ✓ yesshort circuit only |
| MEGA | 175 A | 125 A | ✓ yesshort circuit only |
Why we calculate this way
Why is voltage drop not enough? Current heats every wire. Above a certain current it gets hotter than its insulation can take continuously, however short it is. That is why ABYC E-11 requires both, ampacity and voltage drop, and the larger size where they disagree.
Why nominal voltage? ABYC E-11 takes the allowed drop from nominal voltage: 3 % of 12 V is 0.36 V, 10 % is 1.2 V. So here we calculate current and drop with 12, 24 or 48 V. The camper profile instead uses an almost empty battery (10.7 V).
Why a 10 % margin? No device is lossless. An inverter with 1,000 W output draws about 1,050 to 1,100 W from the battery. ABYC works with the load current; the margin is our addition for efficiency and disturbances.
Why 3 or 10 %? ABYC E-11 (2008) calls for 3 % on panel main feeders, bilge blowers, electronics, navigation lights and other circuits where drop must stay minimal. Lighting other than navigation lights and other non-critical circuits get 10 %. We cautiously use 3 % for bilge pumps and fridges as well.
Why double length? ABYC measures the conductor from the positive source connection to the device and back to the negative. If the device is 10 ft (3 m) from the battery, that is 20 ft of wire. On board the negative is always its own wire; the hull is not a return conductor.
What is 10.75? The ABYC formula constant K for copper, in ohm circular mils per foot. Converted it equals a conductivity of about 56 m/(Ω·mm²), slightly more cautious than the 58 in the camper profile. The cross-check shows the margin covers fine-stranded wire.
Why 30 °C, and what about the engine space? ABYC E-11 gives what a single conductor carries continuously at 30 °C (86 °F), by insulation rating. In engine spaces it assumes 50 °C (122 °F): a 105 °C wire carries 85 % there, a 75 °C wire only 75 %. There the insulation must be oil resistant and rated at least 75 °C dry.
What does installation mean? A wire sheds heat through its surface. On its own in free air that works well. Bundled, sheathed or in conduit, conductors heat each other, so each may carry less current. ABYC counts conductors as bundled when they run together for more than 24 in (610 mm).
What counts? For DC, ABYC E-11 (2008) has a single step: a conductor bundled with at least one other current-carrying conductor carries 70 %. Positive and negative of one device are already enough. The further steps (60, 50, 40 %) are headed for AC circuits there. That is why the calculator asks for no count here.
What the fuse is for: it protects the wire, not the device. We keep it no larger than the wire's ampacity where it runs; older texts allowed up to 150 % in exceptions. If the wire is not enough, we size it up. Place it in the positive wire within 7 in (175 mm) of the battery. Fuses also weaken in heat: a MEGA 300 A holds only 205 A at 65 °C.
DC only: everything here applies to 12, 24 and 48 V circuits on board. 120 V and 240 V, shore power and generators follow the AC part of ABYC E-11, other protective devices (circuit breakers, GFCI and ELCI protection) and other tables.
Wire size and fuse for typical loads on board
Calculated for 12 V, negative as a wire, wire alone in free air, outside the engine space (30 °C), wire rated 105 °C. Columns: distance to battery in feet. In a bundle or the engine space it gets more, the calculator above shows how much.
| Load | Current | 3 ft | 6 ft | 10 ft | 15 ft | 25 ft | Fuse |
|---|---|---|---|---|---|---|---|
| LED navigation lights | 0.50 A | 16 AWG | 16 AWG | 16 AWG | 16 AWG | 16 AWG | Blade fuse (ATO) 1 A |
| Bilge pump (up to 5 A) | 5.00 A | 16 AWG | 16 AWG | 14 AWG | 12 AWG | 10 AWG | Blade fuse (ATO) 7.5 A |
| Bilge blower (up to 4 A) | 4.00 A | 16 AWG | 16 AWG | 14 AWG | 14 AWG | 10 AWG | Blade fuse (ATO) 7.5 A |
| tigerexped water pump small (2.9 GPM) | 8.00 A | 16 AWG | 16 AWG | 16 AWG | 16 AWG | 14 AWG | Blade fuse (ATO) 15 A |
| tigerexped water pump large (4.8 GPM) | 15.0 A | 16 AWG | 16 AWG | 14 AWG | 12 AWG | 10 AWG | Blade fuse (ATO) 25 A |
| Compressor fridge (approx. 5 A) | 5.00 A | 16 AWG | 16 AWG | 14 AWG | 12 AWG | 10 AWG | Blade fuse (ATO) 7.5 A |
| Chartplotter 12 inch (approx. 3 A) | 3.00 A | 16 AWG | 16 AWG | 16 AWG | 14 AWG | 12 AWG | Blade fuse (ATO) 5 A |
| VHF radio (25 W transmit, approx. 6 A) | 6.00 A | 16 AWG | 16 AWG | 14 AWG | 12 AWG | 10 AWG | Blade fuse (ATO) 10 A |
| Autopilot (drive up to 7 A) | 7.00 A | 16 AWG | 14 AWG | 12 AWG | 10 AWG | 8 AWG | Blade fuse (ATO) 15 A |
| Livewell pump (approx. 3 A) | 3.00 A | 16 AWG | 16 AWG | 16 AWG | 16 AWG | 16 AWG | Blade fuse (ATO) 5 A |
| Trolling motor 55 lb (up to 50 A) | 50.0 A | 8 AWG | 8 AWG | 8 AWG | 8 AWG | 6 AWG | MIDI 80 A |
| Anchor windlass (approx. 80 A) | 80.0 A | 4 AWG | 4 AWG | 4 AWG | 4 AWG | 4 AWG | MIDI 125 A |
| DC-DC charger 30 A | 41.7 A | 8 AWG | 8 AWG | 4 AWG | 4 AWG | 1 AWG | MIDI 70 A |
| Inverter 2000 W | 167 A | 2/0 AWG | 2/0 AWG | 2/0 AWG | 3/0 AWG | > 4/0 AWG | MEGA 300 A |
Frequently asked questions
What size wire do I need on my boat?
It depends on current, length and how the wire runs, not just the device. Following ABYC E-11 the calculator checks voltage drop (3 or 10 %), ampacity at 30 °C or in the engine space, bundling and the fuse, and takes the largest result. Every conductor is at least 16 AWG. For a CE boat in mm², switch to Boat EU (ISO 13297).
How do I calculate marine wire size with the ABYC formula?
CM = 10.75 × I × L ÷ E. CM is the circular mil area, I the current in amps, L the wire length in feet out and back, E the allowed drop in volts (3 % of 12 V is 0.36 V). Take the next larger gauge. Example: 10 A, 20 ft one way: 10.75 × 10 × 40 ÷ 0.36 = 11,944 CM, that is 8 AWG. Then check ampacity as well.
How much voltage drop does ABYC allow?
ABYC E-11 (2008) calls for 3 % on panel main feeders, bilge blowers, electronics, navigation lights and other circuits where drop must stay minimal, and 10 % for other lighting and non-critical circuits. Both are taken from nominal voltage: on 12 V that is 0.36 V or 1.2 V. We also use 3 % for bilge pumps and fridges.
What size wire for a 2000 W inverter on a boat?
On 12 V, 5 ft (1.5 m) from the battery, run singly with 105 °C boat cable outside the engine space: 2/0 AWG with MEGA 300 A. Bundled or in the engine space it gets thicker, on 24 V much thinner. If the inverter manual calls for more, follow it.
What size wire for a trolling motor?
A 12 V trolling motor with 55 lb thrust draws up to about 50 A. With the battery 20 ft (6 m) away one way, 10 % drop and run in free air: 6 AWG with MIDI 80 A. If the motor manual calls for heavier wire or a specific breaker, follow it.
What size wire for an anchor windlass?
A windlass with a 700 W motor draws around 80 A on 12 V, briefly much more when starting. At 20 ft (6 m) one way, 10 % drop and run in free air: 4 AWG with MIDI 125 A. If the windlass manual calls for heavier wire, follow it.
What size fuse do I need for my boat wire?
The fuse protects the wire, not the device. We take the largest rating that does not exceed the wire's ampacity where it runs, in a bundle or the engine space too: the strict reading of ABYC E-11. It must also hold the continuous current plus a 20 % safety factor when warm. Older texts allowed up to 150 % in exceptions; we do not use that.
Where does the fuse have to go?
In the positive wire, within 7 in (175 mm) of the battery, measured along the wire. If the wire connects directly to the battery terminal and runs in a sheath or enclosure all the way to the fuse, up to 72 in (1.83 m) is allowed; from other power sources 40 in (1.02 m). Cranking motor conductors are exempt.
Do I count positive and negative in the wire length?
Yes. ABYC measures from the positive source connection to the device and back to the negative. Enter the one-way distance in feet, the calculator doubles it. The hull or the bonding system is never the return path: on a boat the negative always runs back in its own wire.
What changes in the engine space?
ABYC assumes 50 °C (122 °F) there instead of 30 °C (86 °F). A 105 °C wire then carries 85 %, a 75 °C wire 75 % of its rating: 8 AWG at 105 °C carries 80 A outside and 68 A inside. In engine spaces the insulation must be oil resistant and rated at least 75 °C dry.
Does bundling reduce ampacity on DC circuits?
Yes. In ABYC E-11 (2008) a DC wire that is bundled, sheathed or in conduit with another current-carrying conductor for more than 24 in (610 mm) carries 70 % of its single-wire rating. Positive and negative of one device already count. The further steps (60, 50, 40 %) are headed for AC; some sources apply them to DC too.
How does AWG compare to mm²?
The higher the AWG number, the thinner the wire; above 1 AWG come 1/0 to 4/0. 16 AWG is about 1.31 mm², 10 AWG 5.26 mm², 8 AWG 8.37 mm², 4 AWG 21.2 mm², 1/0 53.5 mm² and 4/0 107 mm². Metric sizes do not match exactly: 10 mm² lies between 8 and 6 AWG. For mm², switch to Boat EU (ISO 13297).
DC only: everything here applies to 12, 24 and 48 V circuits on board. 120 V and 240 V, shore power and generators follow the AC part of ABYC E-11, other protective devices (circuit breakers, GFCI and ELCI protection) and other tables.
The calculator applies to DC, copper conductors and the assumptions stated in the calculation steps. Working on electrical systems requires expertise. Ampacities and engine room factors per 33 CFR 183.425, Table 5 (US federal regulation). Voltage drop, bundling (DC 0.7, cautious reading) and fuse rule derived from ABYC E-11. Fuses from manufacturer data (Littelfuse). ABYC, UL and Littelfuse are not affiliated with tigerexped and have neither reviewed nor endorsed this calculator.