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.

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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, assuming copper conductor at 20 °C — derate by ~10% for runs in enclosed hot spaces.

Two constraints apply at once, and the cable has to satisfy both: it must carry the current without overheating (ampacity), and it must not drop too much voltage. At short runs ampacity governs; at long runs voltage drop governs. These figures take whichever is larger, and land around 1% drop — comfortably inside the 3% limit rather than exactly on it. Sizing to exactly 3% would give noticeably thinner cable and leave nothing for terminal resistance, a hot bilge, or a motor you upgrade later.

At 96 V the same power draws half the current, so read the row for half your amps — a 200 A draw at 48 V becomes 100 A at 96 V and the cross-section falls accordingly. That is the single biggest practical argument for the 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.

For a lithium propulsion pack, use a Class T fuse. This matters more than the rating: what counts in a bolted fault is the fuse's interrupting capacity, and an LFP pack can deliver 5,000–20,000 A. Class T is rated to interrupt 20 kA at 125 V DC. MIDI and ANL fuses are rated far below that and can fail to clear the fault — arcing instead of breaking it — which is the one failure mode you are fitting the fuse to prevent. ANL is acceptable on light house circuits; it is not the right part on a propulsion pack.

Placement follows ABYC E-11: overcurrent protection within 7 in (178 mm) of the battery positive terminal, extendable to 40 in (1.02 m) only where the conductor runs inside a sheath or enclosure for its whole length. The fuse protects the cable from the battery, not the motor from itself — so the short stub between post and fuse is the one length on the boat that is unprotected, and it has to be mechanically faultless.

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.

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