The DC side of an electric boat — the wiring, fuses, contactors and isolation between your LFP pack and the rest of the boat — is the part where mistakes burn boats down. The BMS protects the cells. Everything between the pack terminals and the loads is protected by the hardware described here, and each piece answers one specific fault.
→ The spec calculator sizes cables and main fuses based on your motor and pack → — pair it with this guide for the protective hardware.
Five components, five faults
| Component | The fault it answers |
|---|---|
| Main DC fuse | A short circuit in the cable or a load |
| Main contactor, controlled by the BMS | A cell outside its safe voltage or temperature range |
| Pre-charge | The current surge when the contactor closes onto empty controller capacitors |
| Manual DC isolator | Working on the system, or isolating it when the BMS itself has failed |
| Galvanic isolator or isolation transformer | Stray current through the shore-power earth, which corrodes underwater metal |
Main DC fuse: Class T
A lithium bank's very low internal resistance lets it push a very large current into a short circuit. A fuse has to interrupt that current at the pack's voltage, not just carry the running current. Blue Sea Systems' fuse guide gives:
| Fuse type | Interrupting capacity | Maximum voltage |
|---|---|---|
| Class T | 20,000 A | 160 V DC |
| ANL | 6,000 A | 32 V DC |
| MIDI / AMI | 5,000 A | 32 V DC |
ANL and MIDI fuses aren't rated for a 48 V pack's voltage at all. Use Class T.
Sizing
The calculator sizes the main fuse at 1.25 × the full-power DC current, taken at the pack's minimum voltage:
Fuse_A = (motor_W ÷ (V_min × efficiency)) × 1.25
For a 10 kW motor on a 48 V pack (40 V minimum, 90% efficiency):
- Full-power current = 10,000 ÷ (40 × 0.90) = 278 A
- Fuse = 278 × 1.25 = 348 A → a 350 A Class T
Size from minimum voltage, not nominal. Dividing by 48 V gives 208 A and a fuse that blows on a long run into a headwind with a low pack. The cable must be rated to carry more than the fuse, so check the two together; the cable sizing guide has the table.
Where to place it
Mount the fuse as close to the battery positive terminal as the installation allows. ABYC E-11 sets the maximum distance, with a longer allowance for conductors that are sheathed along their whole run; check the current edition for the figures. Whatever length you end up with between post and fuse is unprotected cable, and it is the section most likely to chafe and short.
Main contactor: BMS-controlled, not user-switched
The main contactor is what the BMS uses to physically disconnect the pack on a fault. It must:
- Carry the continuous current, at the cable size you're using, with margin
- Interrupt current if it ever has to open under load
- Be failsafe-open: no coil power means contacts open
- Be driven by the BMS, not by a switch the crew can override
Contactor ratings depend on conditions, so read them carefully. TE's Kilovac LEV200, for example, is rated 500 A continuous at 65 °C with 400 MCM conductors, and to interrupt 2,000 A at 320 V DC. Use thinner cable and the carry rating falls; the datasheet says to consult TE above 500 A.
Separate control of charging and discharging
The contactor works best as the backstop, not the first line of defence. Victron's Lynx Smart BMS NG shows the pattern. Its allowed-to-charge and allowed-to-discharge contacts switch chargers and loads off individually, so a charging fault doesn't cut propulsion. The contactor opens only if those controls fail. The Lynx Smart BMS NG guide covers the wiring.
Pre-charge: the most-forgotten component
A motor controller has a bank of capacitors across its DC input. Discharged, they look like a short circuit to the pack, so closing the main contactor onto them draws a surge that arcs across the contactor tips. Curtis's manual for its 1232E–1238E controllers describes pre-charge as protecting "controller internal components and main contactor tips", and says it "decreases the arcing that would otherwise occur when the contactor is closed with the capacitor bank discharged".
Pre-charge puts a resistor in the path first, so the capacitors fill slowly; then the main contactor closes with almost no voltage across it. Two ways to get it:
- In the controller. Curtis's 1232E–1238E controllers pre-charge their capacitor bank through the key-switch input before the main contactor closes, and flag a fault if pre-charge fails.
- In the BMS or a separate circuit. The Lynx Smart BMS NG has pre-charge built in for capacitive loads such as inverters. Otherwise a resistor and a small relay, sequenced before the contactor, do the same job.
Size a separate pre-charge resistor from the controller maker's instructions: the capacitance, and how long you can wait, decide the resistor value and its energy rating. Don't skip it on the grounds that the system "worked without it" the first time; contactor tips wear with every unprotected close.
DC isolator: the manual disconnect
Besides the BMS-controlled contactor, every installation needs a manually operated DC isolator for:
- Service work, because you can't rely on the BMS being functional during a fault
- Emergency isolation, with the lever or handle reachable from the helm or companionway
- Long-term storage, to stop parasitic drain
The isolator must be rated for the full pack voltage, not just the current. Much marine DC gear is built for 12–24 V boats: in Blue Sea's own fuse guide, several fuse types and fuse blocks top out at 32 V DC. Check the voltage rating before fitting any switch to a 48 V or 96 V bank.
Place it close to the pack. Most installations put the main fuse first and the isolator straight after it, because anything between the battery post and the fuse lengthens the unprotected cable. Opening the isolator then makes everything downstream dead; to work on the fuse itself, disconnect at the battery. ABYC E-11 covers where the switch may go.
Galvanic isolation on the AC side
This sits on the shore-power side rather than the propulsion side, which is why owners forget it. If the boat's DC negative is bonded to the AC safety earth, as it usually is, a shore connection links your underwater metal to every other boat on the pontoon. Small DC currents then flow through the water and corrode it.
Two options:
- Galvanic isolator: back-to-back diodes in the shore earth line. They block the small DC voltages that drive corrosion and still pass AC fault current. ABYC A-28 covers them.
- Isolation transformer: a 1:1 transformer at the shore inlet that separates the boat's AC system from the marina's entirely. It costs more and weighs more, and it solves the problem completely.
High-power DC charging straight into the pack is still an immature area for leisure craft. There's no settled leisure-marine equivalent of the automotive CCS standard, and inlet, isolation and communication requirements vary by vendor. If you're specifying a DC charge inlet, agree the isolation design with your charger supplier and surveyor.
The standards
The documents that govern this area are ABYC E-11 (AC and DC electrical systems on boats), ABYC E-30 (electric propulsion systems) and ABYC A-28 (galvanic isolators) in the US. Internationally, ISO 16315 covers electric propulsion on small craft up to 24 m, for systems below 1,500 V DC; its second edition, "Small craft — Electrical systems used for electrical propulsion", was published in 2026. IEC 60092-507 covers electrical installations in small vessels more generally. All of them are sold, not published free, and all are revised from time to time. We haven't summarised their clauses here, because a summary of an old edition is worse than none. Buy the current edition, or have the installation checked by a surveyor who has it.
The fault-tree summary
| Fault scenario | What protects you |
|---|---|
| Cable short between pack and contactor | Main Class T fuse |
| Cell over- or under-voltage, or too cold to charge | BMS: allowed-to-charge / allowed-to-discharge first, contactor as backstop |
| Contactor closing onto empty capacitors | Pre-charge |
| Service work on the pack | Manual DC isolator |
| Stray current from shore power | Galvanic isolator or isolation transformer |
| Cell internal fault | BMS sensing, contactor, and a battery compartment designed to contain and vent it |
Sources
Checked on 14 November 2026:
- Blue Sea Systems — Guide to fuse types (Blue Sea document, copy hosted by The Boat Warehouse): interrupting capacity and maximum voltage of Class T, ANL and MIDI/AMI fuses
- TE Connectivity — KILOVAC LEV200 series datasheet (copy hosted by RS): carry rating of 500 A at 65 °C with 400 MCM conductors, 2,000 A interrupt at 320 V DC
- Curtis Instruments — 1232E–1238E/SE manual, OS 31: Precharge Enable parameter, ENABLE_PRECHARGE() function, and pre-charge via KSI before main contactor closure
- Victron Energy — Lynx Smart BMS NG: see the sources listed in our Lynx Smart BMS NG guide for the ATC/ATD contacts, contactor and pre-charge
- ISO — ISO 16315 catalogue entry and the ISO 16315:2016 preview: scope (small craft up to 24 m, below 1,500 V DC) and the 2026 second edition's title
- The calculator's sizing model: the fuse sizing rule and the 10 kW example
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