How to Choose Marine Wire Gauge (AWG): A Boat Owner's Guide
Undersized wire is the most common electrical mistake we see on boats. It doesn't fail on the dock. It fails when the bilge pump has been running for 40 minutes in a seaway. Three numbers settle it: current (amps), round-trip length and acceptable voltage drop.
1. Find the current
Use the nameplate amps, or watts ÷ volts. A 60 W light on 12 V draws 5 A. Size for the maximum continuous draw, and add the startup surge for motors like windlasses and bilge pumps.
2. Measure the round trip
Measure from the source (battery, panel or bus bar) to the device and back. A pump 12 ft from the panel is a 24 ft circuit.
3. Pick your voltage-drop target
- 3%: critical and electronic loads such as navigation lights, bilge pumps, electronics and panel feeders.
- 10%: non-critical loads such as cabin lights and accessories.
| Amps | 10 ft | 20 ft | 30 ft | 40 ft |
|---|---|---|---|---|
| 5 A | 18 | 14 | 12 | 12 |
| 10 A | 14 | 12 | 10 | 8 |
| 20 A | 12 | 8 | 6 | 6 |
| 40 A | 8 | 6 | 4 | 2 |
| 100 A | 4 | 1 | 1/0 | 2/0 |
AWG at 3% drop, 12 V, round-trip length, 105 °C insulation. This is a quick reference. Always check ampacity for engine-room bundles too.
4. Buy tinned marine wire and do the terminations right
Tinned copper resists the green corrosion that creeps up untinned strands. Pair it with adhesive-lined heat-shrink terminals and a ratcheting crimper. Pliers-style crimps are the second most common failure we see.
- Ancor Red 2/0 AWG Battery Cable Sold by the Foot $13.99
- Ancor Red 50' 6 AWG Wire $114.27
- Ancor Red 50' 4 AWG Wire $130.35
- Ancor 12-10 #10 Heat Shrink Spade Terminals 100pk $58.98
- Ancor Single Crimp Ratchet Tool $72.44
- Ancor Cut/Strip/Crimp Multi Tool 22-10 AWG $9.76
5. Protect every conductor
Put a fuse or breaker within 7 in of the source (up to 72 in if the wire is sheathed), rated at or below the wire's ampacity. For big battery feeds, a Class T fuse is the standard.
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FAQ
What gauge wire do I need for a 12V bilge pump?
Most 1,100–2,000 GPH pumps draw 4–8 A. On a 25 ft round trip that's typically 14 AWG for a 10% drop, or 12 AWG if the pump is a critical circuit you want at 3%.
Is automotive wire OK on a boat?
No. Marine wire is tinned, fine-stranded and has heavier insulation (ABYC/UL 1426). Untinned automotive wire wicks salt water and corrodes from the inside out.
Do I measure one-way or round-trip length?
Round trip: from the source to the load and back along the negative. Voltage drop happens on both conductors.