The essentials in 30 seconds
- The fuse protects the cable, not the device. It is placed as close as possible to the source, within 175 mm per ABYC E-11 (1.02 m or 1.83 m if the cable is sheathed), and its rating matches the manual.
- Fishfinder: the fuse for the factory cable (3 A on most 7 to 9 inch units, 8 A on an ECHOMAP Ultra 2, 5 A inline on a Raymarine Element), a dedicated ignition feed, and a cross-section calculated for a 3 % voltage drop.
- Electric motor: a Garmin Force requires a dedicated 24 or 36 V bank, a continuous 60 A circuit breaker rated for at least 42 V DC and 16 to 35 mm² depending on length; a Force Current requires 40 A and at least 32 V DC.
- One battery isolator per bank, rated above the current and voltage; the motor bank must remain separate from house and start banks.
- Crimped terminals sized to the stud, soldered splices then sheathed, tinned copper, torque-tightened: corrosion starts here.
Warning. A house battery can deliver several thousand amps in a short circuit; ABYC E-11 requires circuit breakers to have an interrupting rating of 1,500 to 5,000 A depending on the battery. This tutorial outlines best practices: it does not replace your device’s manual, which always takes precedence, nor a professional inspection. Disconnect the battery before working, install only ignition-protected components in a gasoline compartment (SAE J1171, ISO 8846), and have the system inspected before the first outing. The Carnac workshop and our partner installers perform these checks.
Contents
Three rules before plugging anything in
1. The fuse protects the cable. It is sized for the weakest conductor in the circuit, not for the device: ABYC E-11 requires a rating that does not exceed the current-carrying capacity of the cable, with the next standard rating tolerated up to 150 %. The device itself retains its inline fuse.
2. As close as possible to the source. ABYC E-11.10.1.1.1: each positive conductor is protected within 175 mm of its connection to the source, measured along the cable. If the cable is sheathed or run in conduit for its entire length: 1.83 m from a battery terminal, 1.02 m from a switchboard or busbar. Cable tails shorter than 175 mm are exempt.
3. The rating comes from the manual. Garmin, Simrad, Lowrance and Raymarine publish it and require keeping the original fuse holder; Garmin voids the warranty if the fuse is missing.
In Europe, ISO 13297 (electrical installations for small craft, CE marking) addresses the same points in clauses 12 and 18 and Annex E, with a voltage drop limited to 10 %, and to 3 % for safety equipment. We cite ABYC E-11 because its tables are published.
Powering a fishfinder or a combo unit
Fishing combos are 12 V devices, to be connected to the ignition or a switch, with the fuse in their cable. Values from the manuals, recorded on September 30, 2026:
| Model | Acceptable voltage | Max consumption | Fuse (manual) | Cable extension (manual) |
|---|---|---|---|---|
| Garmin ECHOMAP UHD2 5x/7x (keypad) | 9 to 18 V | 8.4 W (5") | 3 A fast-blow, 125 V | 0.82 mm² (18 AWG) minimum |
| Garmin ECHOMAP UHD2 6/7/9 sv | 9 to 18 V | 18.3 W (7"), 20.7 W (9") | 3 A fast-blow, supplied | not specified |
| Garmin ECHOMAP Ultra 2 10/12 | 9 to 18 V | 34 / 36 W | 8 A fast-blow, supplied | 1.5 mm² up to 3 m, 2.5 mm² up to 6.1 m |
| Lowrance Eagle 7 / 9 | 10.8 to 18 V | 6.6 / 7.7 W | 3 A recommended | not specified |
| Lowrance Elite FS | 10 to 17 V | 28 W | 3 A | not specified |
| Lowrance HDS Pro 9/10 (12/16) | 10.8 to 17 V | 40 W (48 / 57 W) | 3 A (5 A) | not specified |
| Simrad GO XSE 7/9 (12) | 10 to 17 V, 24 V excluded | 12 W (20 W) | 3 A ATC, in the cable | not specified |
| Simrad NSX 3007 / 3009 / 3012 | 10 to 17 V | 11.5 / 18.8 / 26.2 W | 2 A / 3 A / 3 A | not specified |
| Raymarine Element 7/9/12 HV | 8 to 16 V | 16.8 / 18 / 24 W | 5 A inline, or 3 A thermal | "sufficient cross-section for total load" |
| Furuno FCV-600 | 12 to 24 V | 1.0 to 0.6 A | see installation manual | not specified |
Installation instructions from Garmin, Lowrance, Simrad and Raymarine, Furuno page (list at the end of the article). Simrad accepts a circuit breaker of the closest rating instead of a fuse.
The power feed is properly set up in three ways: an inline ATO/ATC waterproof fuse holder with the manual’s fuse if the device is standalone; a 6-circuit ST Blade negative bus fuse block; or an ATO/ATC fuse per circuit, as soon as there are fishfinders, VHF and NMEA 2000; or a switchboard circuit breaker of the closest rating. On a 24 V boat, these displays are not connected directly: a 24/12 V DC-DC converter sized for the load, or a device that accepts 24 V like the Furuno FCV.
Cross-section and length: voltage drop
For electronics, ABYC E-11 limits voltage drop to 3 %, calculated over the round-trip length from the battery to the device and back. Two checks, in this order: voltage drop, then the cable’s current-carrying capacity; keep the larger of the two cross-sections. The ABYC formula, in its units: cross-section in circular mils = 10.75 × I (amperes) × L (feet, round-trip) ÷ E (allowed drop in volts, 0.36 V for 3 % on 12 V).
| Direct current | 3 m round-trip | 6 m | 9 m | 12 m |
|---|---|---|---|---|
| 5 A (fishfinder 7 to 9") | 1 mm² | 2.5 mm² | 4 mm² | 6 mm² |
| 10 A | 2.5 mm² | 6 mm² | 6 mm² | 10 mm² |
| 15 A | 4 mm² | 6 mm² | 10 mm² | 16 mm² |
| 20 A | 6 mm² | 10 mm² | 16 mm² | 16 mm² |
| 40 A (Force Current) | 10 mm² | 16 mm² | 25 mm² | 35 mm² |
| 60 A (Force, 12 V) | 16 mm² | 25 mm² | 35 mm² | 50 mm² |
Cross-sections for 3 % drop on 12 V: Skysat calculation from ABYC E-11 TABLE IX (AWG sections), converted to the next higher commercial metric section (18 AWG → 1 mm², 14 → 2.5, 12 → 4, 10 → 6, 8 → 10, 6 → 16, 4 → 25, 2 → 35, 1 → 50); ABYC allows metric sections of equivalent area. On 24 V, at equal current, the table drops by one to two steps.
| Section (AWG equivalent) | Current-carrying capacity outside engine compartment | Inside engine compartment |
|---|---|---|
| 1.5 mm² (16) | 25 A | 21.3 A |
| 2.5 mm² (14) | 35 A | 29.8 A |
| 4 mm² (12) | 45 A | 38.3 A |
| 6 mm² (10) | 60 A | 51 A |
| 10 mm² (8) | 80 A | 68 A |
| 16 mm² (6) | 120 A | 102 A |
| 25 mm² (4) | 160 A | 136 A |
| 35 mm² (2) | 210 A | 178.5 A |
ABYC E-11 TABLE VI-A, 105 °C insulation, single conductor; when grouped over more than 61 cm, conductors lose current-carrying capacity (TABLE VI-B). Since metric sections are larger than the reference AWG, their actual ampacity is at least that shown in the table.
Cables are sold by the meter: MPRX 2 × 2.5 mm², 2 × 1.5 mm² and tinned 1 mm² for a fishfinder; HI FLEX 10, 16, 25 and 35 mm² for an engine, in red and black.
Powering an electric trolling motor
A bow thruster draws high current for hours: it is the most demanding circuit on the boat. Garmin’s requirements:
| Motor | Battery bank | Continuous current | Required protection | Cable |
|---|---|---|---|---|
| Force Kraken, Force Pro | 24 or 36 V, capable of 60 A continuous | 60 A (2 160 W at full power under 36 V) | 60 A continuous circuit breaker, at least 42 V DC, not supplied; up to 90 A only under high heat or shared circuit, after wiring verification | 1.2 m supplied; extension 16 mm² from 0 to 3 m, 25 mm² from 3 to 4.6 m, 35 mm² from 4.6 to 9.1 m, soldered and sleeved joint; 60 A plug and socket at least optional |
| Force Current | 12 or 24 V | 40 A | 40 A continuous circuit breaker or fuse, at least 32 V DC | 1.62 m with eyelet terminals; Skysat raw cables 16 mm², 4 m |
At the battery end, Rebelcell requires « always a fuse between the battery and the motor », sized according to the motor’s manual, and offers Quick Connect cables with 60 A or 100 A fuse holders; its 24V75 EDGE and 12V75 EDGE accept 100 A continuous discharge. A lithium battery’s BMS protects the battery itself; it does not replace cable protection.
Which device? A thermal circuit breaker Blue Sea series 187 or series 285 in 60 A, rated 48 V DC by Blue Sea, covers a Force on 24 V or 36 V and serves as a switch at the helm; a 187 in 40 A is suitable for a Force Current. A MRBF 60 A terminal fuse (or 40 A) on its terminal block protects the cable from the battery, up to 58 V DC. A single device is enough if it is within 175 mm of the terminal, or 1.83 m with a sleeved cable; otherwise, MRBF at the terminal and a circuit breaker near the helm.
Breaking capacity. ABYC E-11 requires the main circuit breaker to have 1 500 A for a battery up to 650 CCA, 3 000 A up to 1 100 CCA, 5 000 A beyond; an upstream fuse may satisfy this. Blue Sea rates the MRBF at 10 000 A at 14 V (5 000 A at 32 V) and 20 000 A at 125 V DC for Class T; on a lithium bank, the recommended practice is a Class T fuse on its block at the bank’s head, with the motor’s circuit breaker downstream.
Battery switch and bank isolation
Each bank has its own battery switch on the positive, rated above the continuous current and voltage. According to Blue Sea datasheets, an m-Series mini handles 300 A continuous at 48 V DC, IP66, flame-resistant; an e-Series 350 A at 48 V DC; the HD series reaches 600 A but stops at 32 V DC and is not suitable for a 36 V bank. Negatives are joined on a busbar, never by stacking terminals on a single post.
Separating battery banks is not a luxury: Garmin requires for the Force a 24 V or 36 V bank capable of 60 A continuous, Raymarine demands for the Element a battery separate from the starter battery, Simrad forbids taking power from the NMEA 2000 network on the terminals of the starter battery, the autopilot computer or the bow thruster. One bank for the engine, one for the house loads and electronics, the starter battery separate: three battery isolators.
Protect the NMEA 2000 network
The bus has its own 12 V power supply, protected by a 3 A fuse (Garmin’s power cable integrates it, Simrad requires it), injected at a single point and switched by the ignition or a switch; otherwise bus devices drain the battery when stopped. Garmin forbids 24 V; bus devices operate between 9 and 16 V, a LEN draws 50 mA. Architecture, lengths and diagnostics are in our article NMEA 2000; power cables are in the network collection.
Connections and corrosion
ABYC E-11.16 sets the standard: ring or captive fork terminals sized to the stud; no terminal blocks; soldering is never the sole mechanical fastening method. Garmin extends the Force cable with a soldered junction under heat-shrink tubing and, at Force Pro service, requires terminal nuts tightened to 4 N·m with corrosion brushed off. Rebelcell mandates M8 terminals tightened with a wrench and forbids crocodile clips. Blue Sea publishes its torque values: 13.56 N·m on an m-Series, 15.82 N·m on an e-Series.
In our catalogue: 16/8 tinned copper tubular terminals, 25/8 and 35/8 for engine cables, insulated ring terminals or Ancor heat-shrink ring terminals for transducer leads, heat-shrink adhesive-lined sleeving per meter or in 4:1 ratio. The Rebelcell ANEN SA50 connector is rated 50 A and 16 mm²: enough for a Force Current (40 A), not for a Force, for which Garmin requires at least a 60 A socket.
Limits and pitfalls
- No installation is documented by the workshop: this guide is based on ABYC E-11, ISO 13297 and the cited manuals, not on a Skysat job site.
- Manuals take precedence: a wire size or section published here does not override those in your device manual; Garmin ties its warranty to the original fuse.
- Wire gauge table is a calculation derived from ABYC tables at 12 V; the 36 V table is absent, so we apply the Garmin table and, where missing, the 24 V table.
- Blue Sea interrupt ratings and voltages come from the manufacturer’s datasheets; ISO 13297 is cited from its public summary, the full text is paywalled.
- Gasoline compartment: only ignition-protected components are allowed inside; a circuit breaker or uncertified block must be installed outside.
Workshop verdict
- A standalone 7 to 9 inch depth sounder for a 12 V boat: start via ignition, inline fuse block with the fuse specified in the manual, 2.5 mm² up to 6 m round trip, 4 mm² up to 9 m, 6 mm² up to 12 m; crimped terminals, adhesive-lined heat shrink tubing.
- Depth sounder, VHF and NMEA 2000: ST Blade block with negative bus, one fuse per circuit, 3 A dedicated to the bus, all circuits cut by the ignition.
- Force bow motor in 24 or 36 V: dedicated battery bank, 60 A MRBF on the terminal or 187/285 60 A circuit breaker within 175 mm, minimum 16 mm² and 25 to 35 mm² from 3 m extension onwards, m-Series or e-Series battery switch, Class T at the battery bank head if lithium. Have it checked before the first outing.
- Force Current in 12 or 24 V: 40 A protection rated at least 32 V DC, 16 mm², M8 terminals on the battery, 50 A connector for quick disconnect.
Sources
- ABYC E-11: §11.10.1, §11.14.2.6-7, TABLE IV-A, VI-A and IX, excerpts published by Paneltronics; tables IX and X reproduced by BoatHowTo; §11.16 cited from Practical Sailor and Marine How To.
- ISO 13297:2020, Small craft — Electrical systems — Alternating and direct current installations (public summary: clauses 12, 18, annex E).
- Garmin: Force Kraken Installation Instructions, Force Kraken, Force Pro and Force Current manuals, ECHOMAP UHD2 and Ultra 2 instructions, Technical Reference NMEA 2000.
- Lowrance: Eagle 7 & 9, Elite FS, HDS PRO installation guides; Simrad: GO XSE and NSX manuals; Raymarine: Element 87360 rev 6, §4.3 and §8.1; Furuno: FCV-600 page.
- Rebelcell: Use, maintenance, charging and connection, 36V70 BT manual. Blue Sea Systems: MRBF specifications, Class T, 187 and 285 series, m-Series, e-Series and HD published on manufacturer datasheets.
Disclosure: Skysat distributes Blue Sea Systems, BEP, Garmin, Lowrance, Simrad, Raymarine, Furuno and Rebelcell, and sells its own cut-to-length cables and terminals; links point to our catalogue. Article neither sponsored nor reviewed by the brands listed.
