Sizing battery cable on a boat is less forgiving than on land. DC systems run at low voltage where every tenth of a volt matters, and the marine environment attacks every strand of copper it can reach. Here's how to size it the way ABYC expects.
The Two Rules That Drive Everything
ABYC E-11 practice sizes conductors by two constraints, and the cable must satisfy both:
- Ampacity. The conductor must safely carry the load current.
- Voltage drop. 3% maximum for critical circuits (starting, navigation, electronics, bilge pumps); 10% for non-critical loads (cabin lights, accessories).
On 12V systems, voltage drop almost always governs. A 3% drop at 12V is just 0.36V, and long cable runs eat that fast.
12V Sizing Table (3% Drop, Critical Circuits)
Sizes shown for round-trip circuit length, battery to load and back:
| Load | 10 ft | 15 ft | 20 ft | 30 ft | 40 ft |
|---|---|---|---|---|---|
| 50 A | #6 | #4 | #2 | #1 | 1/0 |
| 100 A | #2 | #1 | 1/0 | 3/0 | 4/0 |
| 150 A | #1 | 2/0 | 3/0 | 4/0 | |
| 200 A | 1/0 | 3/0 | 4/0 |
Starter circuits are a special case: they draw hundreds of amps for seconds. Follow the engine manufacturer's cable size, and keep runs short.
Why Tinned Copper Is Non-Negotiable
Salt air wicks along bare copper strands and corrodes them from the inside out. You'll see green powder at the terminations while the middle of the cable quietly loses cross-section. Tin plating on every strand blocks that corrosion path. Marine-grade battery cable also uses Class K fine stranding for vibration resistance and flexibility through tight chases.
Installation Checklist
- Use marine-rated ring terminals, crimped with the correct die. Never solder-only on battery cable.
- Seal terminations with adhesive-lined heat shrink.
- Fuse every positive conductor within 7 inches of the battery (ABYC), except the starter circuit.
- Support cable every 18 inches; protect from chafe at every bulkhead penetration.
- Size the negative return the same as the positive.
Bigger Boats, Bigger Cable
For inverters, windlasses, and thrusters pulling 200+ amps, you're into 2/0 through 4/0 territory where DLO cable (tinned, fine-stranded, 2kV) is a popular choice on workboats and larger vessels. For shipboard power and control wiring, see our shipboard and marine cable line built to IEEE 1580/UL 1309.
Run your own numbers with the wire size calculator. It handles DC circuits and copper conductors directly.
