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Small boat · 12 V accessories

Single battery boat wiring diagram

Start a small boat’s 12 V accessory plan with one battery, a main switch and two fused circuits for lights and electronics. Follow both the positive supply and the negative return, compare AWG and metric cable sizes, then open this exact diagram to adapt it to your boat.

For lights and electronics. Engine starting, charging and always-on bilge circuits are outside this example.

Open this diagram in Crimp →See every cable and fuse choice ↓

One battery, two accessory circuits

5 components · 7 conductors · 2 A lights + 5 A electronics

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Live Crimp simulation. Pan and zoom to inspect the conductors. Choose Simulate to try the main switch and load controls, or open the diagram to edit equipment and lengths.

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Red = positive · dark = return. Crossings are not junctions. Read the assumptions before adapting a circuit.

A starting plan, with protection still to verify. The fuse values are preliminary cable-protection choices. Select actual devices and holders, check battery-end placement and review the 3 current engine findings before using the plan for an installation.

Follow the circuit

Positive supply and negative return

The switch controls the positive feed to both accessories. Each branch has its own positive supply and negative return.

  1. 01

    Battery to switch

    The positive feed runs through the preliminary main-fuse choice to the on/off switch, then continues to the fuse block. The switch disconnects the loads; it does not protect a wire against overcurrent.

  2. 02

    Fuse block to accessories

    The lights and electronics each have a separate positive branch and fuse choice. The Blue Sea 5025 is a fuse block with a negative bus; two of its six circuits are used here.

  3. 03

    Every load returns to the battery

    Each load returns to the block’s negative bus, which connects to battery negative. This drawing uses dedicated return wires; it does not use the hull as a return.

A bilge pump that must run with the main switch off needs a separate protected supply and its own controls. Navigation lights also need the correct switching arrangement for the boat. Neither is represented by the generic lights in this example.

Cable and fuse table for this diagram

Lengths are one-way conductor runs. The positive battery-to-panel route is 3.3 ft in total; the negative route is also 3.3 ft. Sizes come from Crimp’s engine with a 3% planning target and 105°C copper outside engine spaces, without added heat or bundling derating. AWG labels are approximate counterparts to the metric conductor areas.

Single battery boat connections, cable sizes, preliminary fuses and voltage drop in AWG and metric
C1 · 12 V AGM example battery +
↔ Battery on/off switch A		One-way length0.8 ft25 cmWire size≈ AWG 132.5 mm²Design current7 AFuse choice10 APreliminaryDrop / target0.10% / 3%
C2 · Battery on/off switch B
↔ Blue Sea 5025 fuse block IN +		One-way length2.5 ft75 cmWire size≈ AWG 132.5 mm²Design current7 AFuse choiceNo separate fuseReview upstream C1 protectionDrop / target0.31% / 3%
C3 · Blue Sea 5025 fuse block IN −
↔ 12 V AGM example battery −		One-way length3.3 ft1 mWire size≈ AWG 132.5 mm²Design current7 AFuse choiceNone on returnDrop / target0.41% / 3%
C4 · Blue Sea 5025 fuse block 1 +
↔ Deck / cabin lights +		One-way length9.8 ft3 mWire size≈ AWG 161.5 mm²Design current2 AFuse choice3 APreliminaryDrop / target0.58% / 3%
C5 · Deck / cabin lights −
↔ Blue Sea 5025 fuse block 1 −		One-way length9.8 ft3 mWire size≈ AWG 161.5 mm²Design current2 AFuse choiceNone on returnDrop / target0.58% / 3%
C6 · Blue Sea 5025 fuse block 2 +
↔ Fish finder / electronics +		One-way length6.6 ft2 mWire size≈ AWG 161.5 mm²Design current5 AFuse choice7.5 APreliminaryDrop / target0.97% / 3%
C7 · Fish finder / electronics −
↔ Blue Sea 5025 fuse block 2 −		One-way length6.6 ft2 mWire size≈ AWG 161.5 mm²Design current5 AFuse choiceNone on returnDrop / target0.97% / 3%

Keep at least the stated metric conductor area; an approximate AWG label is not an interchangeable cable size. Design currents are allowances, not measurements. Each drop value is the calculated loss in that individual drawn conductor. The complete circuit includes the positive feed, its branch and the negative return; a single row is not the total circuit loss. Fuse ratings do not establish device compatibility or placement, and the switch-to-panel wire still needs its upstream protection checked.

Adapt the example

Adapt the battery, loads and cable lengths

Open the same diagram, make one useful change, then keep the resulting project as your own plan.

  1. 01

    Replace the placeholders

    Choose the actual battery and accessories. Enter the device current and any startup requirements; 2 A and 5 A are only this example’s allowances. Battery capacity needs a separate usage and charging budget.

  2. 02

    Measure the real route

    Change each conductor length to its physical route, including the return. Review cable size and voltage drop again when you add loads, move the fuse block or run through hotter or bundled spaces.

  3. 03

    Resolve protection, then save

    Select the actual fuses and holders, verify their battery fault-current suitability and location, and review the diagnostics. Save the edited project so you can return to the same equipment and cable plan.

Open this diagram in Crimp →Check one cable first →

Assumptions and current engine findings

This example covers a switched DC accessory group. Engine starting, alternator or solar charging, always-on bilge pumps, navigation-light switching, AC shore power and bonding need separate designs. It does not establish whole-boat compliance or battery runtime. The editor retains these findings; opening the diagram does not turn the example into a verified installation.

Review 3 current engine findings

Exact fuse selection and battery-end placement are unfinished. Any interrupt check uses the engine’s chemistry-based fault-current assumption, not measured battery evidence.

  • warning: Check fuse distance: 12 V AGM example battery(C1)
  • warning: Check fuse suitability: 12 V AGM example battery(C1, C4, C6)
  • warning: No charging source(system)
Components and example choices
  • 12 V AGM example battery

    12 V · 100 Ah

    Generic 100 Ah AGM catalog placeholder, not a selected battery or runtime recommendation. Confirm battery specifications, available fault current, charging and battery-end protection for the actual installation. The diagram covers accessory loads only.

  • Battery on/off switch

    Blue Sea 6005 single-circuit ON/OFF switch. This disconnects both illustrated accessory branches together. It is not an overcurrent device. Engine starting, charging and circuits that must remain powered are outside this drawing. Source: Blue Sea 6005 product page, checked 21 October 2026.

  • Blue Sea 5025 fuse block

    Blue Sea 5025 with negative bus: 6 ATO/ATC branches, 100 A maximum block total, 30 A maximum per branch, 32 V DC. Two branches are used here. Branch fuses are separate from the battery-feed protection; fuses are sold separately. Source: Blue Sea product page and linked schematic, checked 21 October 2026.

  • Deck / cabin lights

    12 V · 2 A load allowance

    2 A continuous planning allowance, not a specific product or measured current. Replace with the actual device, startup demand and manufacturer fuse instructions. Terminal limits are placeholders.

  • Fish finder / electronics

    12 V · 5 A load allowance

    5 A continuous planning allowance, not a specific product or measured current. Replace with the actual device, startup demand and manufacturer fuse instructions. Terminal limits are placeholders.

Read how Crimp sizes conductors and what the checks cover. Actual marine installation conditions, protection, mounting and compliance still need review.

Common questions

Planning a small boat’s wiring

What does a single battery boat wiring diagram need?

For the DC accessory group shown here: a 12 V battery, battery-feed protection, an on/off switch, a fused positive distribution block, and a negative return from each load to the battery. This example includes lights and electronics. Engine starting, charging, bilge-pump controls and navigation-light switching need their own circuits.

What size wire should I use on a small boat?

Choose each conductor from its current, actual route length, voltage-drop target and installation conditions. This page’s table is calculated for these example lengths and loads; it is not a universal wire-size list. Open the diagram, change the lengths and equipment, and review the recalculated cable sizes and diagnostics.

Does the fuse block replace a fuse near the battery?

No. The branch fuses protect the outgoing branch conductors; the feed from the battery needs its own protection. This drawing includes a preliminary feed-fuse rating, but its exact device, interrupt rating, holder and distance from the battery remain unverified. The editor keeps those findings visible.

Can I connect the engine or automatic bilge pump to this diagram?

This diagram does not size an engine-starting circuit. It also disconnects both shown accessory loads when the main switch is off, so an automatic bilge pump that must remain powered needs a separate protected supply and the pump manufacturer’s controls. Use the fuller boat example to explore more circuits, then review the actual installation requirements.

Source notes

Equipment and circuit references

Manufacturer references for the distribution hardware and circuit layout. Checked 21 October 2026; the battery and loads remain planning placeholders.

Blue Sea Systems5025 fuse block specifications and wiring schematicThe exact block shown: negative bus, ATO/ATC branch fuses, 100 A block / 30 A branch limits and 32 V DC maximum. Checked 21 October 2026.Blue Sea Systems6005 single-circuit on/off battery switchThe switch used to disconnect this accessory group. Product specifications and instructions checked 15 October 2026.Blue Sea SystemsMain and branch circuit protectionWhy positive accessory wiring needs overcurrent protection and why ignition protection depends on the installation environment.Blue Sea SystemsSwitched accessories and circuits that remain poweredApplication diagrams distinguish switched electronics from separately protected 24-hour circuits such as a bilge pump. This is an architecture reference, not a current installation standard.
Keep goingFuller boat wiring diagramsExplore house and starter-bank examples when your system needs more circuits.Keep goingCheck voltage dropCheck a known cable size against current and run length.Keep goingChoose a cable sizeWork through ampacity, voltage drop and installation assumptions for one run.Keep goingWhat Crimp checksSee the limits of the diagram's checks before adapting it.