Electric boat cable sizing: the complete guide

Cable sizing is the most under-discussed part of an electric boat installation — and the part most likely to cause a fire, a failed motor controller, or premature battery degradation.

The core problem: DC current in a propulsion system is very high. A 10 kW motor on a 48 V system draws over 200 A at full throttle. At those currents, even small errors in cable size or connection quality generate significant heat.

→ Get your DC current, cable length, and drop % calculated automatically: /electric-boat-spec

Why voltage drop matters more than heat

Most wiring guides focus on thermal limits — the maximum current a cable can carry before the insulation degrades. That matters, but for boat propulsion there is a second and often more important constraint: voltage drop.

At 48 V, a 3% voltage drop across the main cable represents 1.44 V. That 1.44 V appears as reduced voltage at the motor controller input. Motor controllers typically have a low-voltage cutoff; if voltage sags during a hard manoeuvre, the controller may trip and cut power exactly when you need it most — entering a berth, for instance.

The formula:

Voltage drop (V) = Current (A) × Resistance (Ω)
Resistance (Ω) = (Resistivity × Length × 2) ÷ Cross-section

The factor of 2 accounts for both the positive and return conductor. The run length is the one-way distance from battery to motor controller.

Choosing cable cross-section

The table below gives minimum recommended cross-sections for common run lengths at 48 V and a 3% maximum drop target. These assume copper conductor at 20 °C — derate by ~10% for runs in enclosed hot spaces.

| Continuous current | 2 m run | 4 m run | 6 m run | 8 m run | |---|---|---|---|---| | 100 A | 16 mm² | 25 mm² | 35 mm² | 50 mm² | | 150 A | 25 mm² | 35 mm² | 50 mm² | 70 mm² | | 200 A | 35 mm² | 50 mm² | 70 mm² | 95 mm² | | 250 A | 50 mm² | 70 mm² | 95 mm² | 120 mm² | | 300 A | 70 mm² | 95 mm² | 120 mm² | 150 mm² |

At 96 V (or 80 V), the same power requires roughly half the current, so you can drop two columns to the left — one of the key practical benefits of higher bus voltage.

Marine cable vs automotive cable

Use tinned copper, marine-grade, flexible cable rated for at least 105 °C insulation. This matters for three reasons:

1. Tinned copper resists the corrosion that untinned cables develop in the bilge environment within a season.
2. Flexible (stranded fine-wire) construction is essential wherever the cable moves or bends, including anywhere near the motor or battery terminals.
3. 105 °C rated insulation gives a meaningful safety margin in a bilge that can reach 50–60 °C in summer.

Do not use automotive or household cable in marine propulsion applications. The consequences of a wiring failure at 200 A are severe.

Fusing

Every battery positive must be protected by a fuse or circuit breaker rated for the battery's short-circuit current, not just the motor's operating current. For LiFePO₄ packs, short-circuit current can exceed 2,000 A for even a brief fault.

Use ANL fuses (bolt-down blade) or MIDI fuses for runs up to approximately 250 A. Above that, use a marine-rated DC circuit breaker or a properly rated DC contactor with a separate fuse.

The fuse should be mounted within 30 cm of the battery positive terminal — not at the motor controller end. Its purpose is to protect the cable from the battery, not to protect the motor.

Terminal connections

At high DC current, a poor terminal connection is effectively a resistor in series with your propulsion circuit. A 5 mΩ contact resistance at 200 A dissipates 20 W of heat — enough to soften solder joints and eventually fail.

Use hydraulically crimped lugs rather than soldered or mechanically compressed connections for cables above 35 mm². Solder joints under high current vibration fatigue and crack. Hydraulic crimps make permanent cold-welds that do not loosen.

Torque all terminal fasteners to specification and apply a light coat of non-conductive dielectric grease at each connection to prevent oxidation.

Common wiring mistakes to avoid

• Undersized cable on the return conductor. The negative cable carries the same current as the positive. They must be the same cross-section.
• Coiled cable. Coiling cable in a tight bundle acts as an inductor and can cause voltage spikes at the motor controller. Run cables flat and parallel.
• Missing pre-charge. Connecting a motor controller directly to a fully charged battery bank causes an inrush spike that can destroy contactor contacts and the controller's capacitor bank. Always use a pre-charge resistor circuit.
• Cable runs through bilge water. Route cables high in the boat where possible. Where bilge routing is unavoidable, use IP68-rated connectors and secure the cable clear of standing water.

Calculating your own cable spec

The inputs you need:

1. Continuous motor current (A) — from your motor datasheet or the spec calculator
2. One-way cable run length (m) — measured from battery terminals to motor controller
3. Bus voltage (V)

With those three numbers, the spec calculator outputs cross-section, estimated voltage drop, and the correct fuse rating.

→ Calculate your cable spec now: /electric-boat-spec