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 right, ABYC-style.
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 — 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 2/0–4/0 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.
