An introduction to marine electrical for new techs, taught the way ESI actually wires a boat. Fourteen chapters cover the general foundation — volts and amps, battery types, why LiFePO4 is safe, the meter, wire, terminations, overcurrent protection and fuse types, the bank, charge sources, commissioning, and the ESI standard for working efficiently and leaving the boat cleaner — with ABYC cited as the reason behind each step. The complete DC & AC system design and a full new-40′ install come next, in Electrical Tech 2.
Before you start
The general foundation of marine electrical for new ESI techs — taught the way we actually wire a boat, not from a textbook. It covers the electricity itself, the battery types, why LiFePO4 is safe and effective, wire and terminations, and overcurrent protection and fuse types. ABYC is cited as the reason a step is done that way, never as the starting point. The complete DC & AC system design and a full new-40′ install are covered next in Electrical Tech 2. A short check ends each chapter and an in-depth final exam unlocks your certificate.
Fundamentals taught through a real install, step by step.
The meter is the truth source — prove every connection.
The fuse protects the wire. That rule never bends.
Chapter 1
Before you touch a wire, know what's moving through it.
Electricity on a boat is simple once you see it as water in a hose. Get these three words right and everything else — wire size, fuses, the whole install — follows.
If you know the watts, you know the amps. A 120-watt load on a 12-volt system draws 120 ÷ 12 = 10 amps. That 10 amps is what decides the wire and the fuse — not the 12 volts.
Heat in a wire follows the current. Push too many amps through a wire that's too small and it gets hot — hot enough to melt insulation and start a fire. A short circuit is the extreme: near-zero resistance, enormous current, fire in seconds. Everything we do downstream exists to keep current under control.
Chapter 2
Before the chemistry of the install, know the chemistries themselves.
Four battery types turn up on boats. Three are lead-acid in different packaging; one is lithium. Knowing the difference is the foundation for everything else in this course.
Lithium iron phosphate (LiFePO4) is a different animal: about 100% of its capacity is usable, it's roughly a third to a half the weight of the lead bank it replaces, it delivers thousands of cycles, holds a flat voltage as it discharges, and charges much faster. Every Epoch we fit has a built-in BMS (battery management system) watching the cells — covered next chapter.
| Type | Usable capacity | Weight | Cycle life | Charge speed |
|---|---|---|---|---|
| Flooded | ~50% | Heavy | Low | Slow |
| AGM | ~50% | Heavy | Low–med | Medium |
| Gel | ~50% | Heavy | Low–med | Slow |
| LiFePO4 | ~100% | Light | Very high | Fast |
Chapter 3
The fire fear comes from a different chemistry. Know the facts.
The single most common objection you'll hear is “aren't lithium batteries a fire risk?” The honest, sourced answer is: the batteries in those headlines aren't ours.
The phone, laptop and EV fires people picture use cobalt-based lithium (NMC/Li-ion). Lithium iron phosphate (LiFePO4) uses a phosphate cathode that is far more thermally stable — its structure resists releasing oxygen under stress, which is exactly what feeds a thermal-runaway fire. LiFePO4 has a dramatically higher thermal-runaway threshold than NMC. Same word “lithium,” very different chemistry.
At the request of the US Coast Guard, ABYC's technical team ran fire tests on LiFePO4 batteries — deliberately abusing them and even adding cells to an already-burning fire — to see if they would ignite.
The testers could not produce spontaneous combustion. That's the answer to the misconception: a properly built LiFePO4 battery is extraordinarily hard to set alight, and its safety cutoffs act long before things get dangerous.
Every Epoch has an integrated BMS that monitors cell voltage, temperature, current and state of charge in real time, and protects against overcharge, over-discharge, short circuit and thermal extremes. It balances the cells (keeping each near the LiFePO4 limits of ~3.65 V charging / ~2.5 V discharge), adds low-temperature charge protection and self-heating, and reports it all over Bluetooth — down to individual cell voltages. Epoch's Essential series is built to UL 1973, CE and RoHS.
Safe and better: ~100% usable capacity, a fraction of the weight, thousands of cycles, a flat voltage that runs equipment happily, and fast charging that actually uses a high-output alternator or solar. Safety isn't a trade-off against performance here — you get both.
Sources: John Adey / ABYC LiFePO4 fire testing (Ocean Navigator; ABYC). Epoch BMS, balancing, low-temp protection & UL 1973 per Epoch Batteries technical pages. Confirm current product certifications on the Epoch spec sheet for the model installed.
Chapter 4
Never guess what a wire is doing. Measure it.
The meter is the one tool that tells you the truth. Three measurements cover almost everything you'll do on this install.
Set the meter to DC volts. Put the red probe on positive, black on negative — across the two points you want to compare. A healthy 12 V bank reads ~13.2–13.6 V at rest. Voltage is always measured in parallel (across), never in line.
With the circuit de-energized, the continuity setting beeps when there's a complete path. Use it to confirm a wire isn't broken, a fuse is good, or two points are actually connected. Never check continuity on a live circuit.
This is how you find a hidden bad connection. With the circuit under load, measure the voltage across a single connection or run. A good joint reads nearly 0 V of drop. A corroded lug or undersized wire will drop voltage and get warm — the meter finds it before it becomes a fire.
Chapter 5
Every boat is wired a little differently. Learn this one first.
The fastest way to cause a problem is to start cutting before you understand the boat. Spend the first part of any job tracing what's already there.
Before anything comes apart, photograph it and label the wires you'll disturb. Future-you (and the next tech) needs to know what was where. A traced, labeled system is a system you can work on safely.
Chapter 6
Picking the right wire is two questions, not one.
The wire is the part that carries — and the part that burns if it's wrong. Two things decide the gauge.
We use tinned, finely-stranded marine wire — the tin resists corrosion in a salt environment, and the fine strands survive constant vibration that would fatigue solid or coarse wire. Household wire has no place on a boat.
Always size for whichever answer is bigger.
In the engine space, ambient heat is high — we use wire rated for 105 °C there so the rating holds at temperature.
Chapter 7
Good wire with a bad end is a bad circuit.
Most electrical failures aren't the wire — they're the ends. A loose or corroded connection adds resistance, and resistance under load makes heat. Master the termination.
Use the correct connector and the matching crimp die — a full, even crimp with no nicked or cut-off strands. Then seal it with adhesive-lined heat shrink so water can't wick in and corrode the joint.
Battery and bank cables land on lugs — LiFePO4 terminals are generally M8. Land them clean, stack in the right order, and torque to spec. Too loose makes heat; too tight strips or cracks. Use a torque tool, not a guess.
A solder-only joint goes stiff and brittle right where the wire flexes — boat vibration eventually cracks it. We crimp for the mechanical and electrical connection (solder, if used, is only in addition).
Chapter 8
The single most important safety concept on the boat.
If you remember one thing from this whole course, make it this: the fuse protects the wire. Not the radio, not the battery — the wire. If a wire faults, the fuse must blow before that wire becomes a heating element.
The overcurrent device (fuse or breaker) is sized to the wire's safe ampacity, so it opens before the wire overheats. An oversized fuse on a small wire is how boats burn — the wire cooks while the fuse sits there happy.
Protection goes as close to the power source as practical — within about 7 inches of the battery positive (a little more if it's in protective sheathing). The whole point is to protect the run that leaves the battery, so the fuse has to be near the start of that run.
A LiFePO4 bank can deliver a massive short-circuit current — far more than lead-acid. The main fuse must be able to safely interrupt that, so we use a high-interrupt-capacity fuse (Class T) on the lithium main.
Chapter 9
Same job — protect the wire — but not every fuse can do it on lithium.
Last chapter: the fuse protects the wire, sized to the wire, within ~7 inches of the source. This chapter is about which fuse — because on lithium the wrong type can fail catastrophically even at the right amp rating.
Every fuse has an amp rating — the current at which it opens. It also has an AIC (Ampere Interrupt Capacity) — the maximum fault current it can safely break without arcing across, melting, or blowing apart. They are not the same number, and the AIC is the one people forget.
A LiFePO4 bank can dump a colossal short-circuit current — many thousands of amps, far more than a lead-acid bank of the same size. If a dead short happens, the main fuse has to interrupt all of it instantly. A fuse whose AIC is below that fault current can arc and keep conducting — the worst-case failure. So on lithium we choose by AIC first.
| Fuse type | Typical AIC @12V | Use |
|---|---|---|
| Class T | ~20,000 A | Lithium main (and paralleled banks) — our default |
| MRBF | ~10,000 A | Terminal-mount; smaller lead/lithium where AIC allows |
| ANL | ~6,000 A | Lead-acid mains; not enough AIC for a lithium main |
| Mega / Maxi | ~2,000 A | Not for battery main protection — too low |
| Blade (ATO/ATC) | ~1,000 A | Branch / accessory circuits only |
On a LiFePO4 bank we put a Class T fuse on the main, sized to the cable, within ~7 inches of the positive terminal. Branch circuits get appropriately-rated fuses or breakers for their wire. AIC is checked, not assumed.
Chapter 10
A battery that can move is a hazard.
With the fundamentals in hand, the bank goes in. Two priorities: it can't move, and it's connected cleanly.
The bank is mounted and restrained so it can't shift in a seaway. A loose battery can short, chafe a cable, or break a terminal — all of them dangerous. It also needs to sit where it won't take spray or standing water.
We run two cables per battery (one positive, one negative) landed on the M8 terminals, rather than stacking four. It keeps each connection clean and avoids unnecessary draw and imbalance between cells.
The whole bank is LiFePO4. We never leave a lead-acid battery on the boat — mixing chemistries is a top cause of destroyed batteries and fires. The Epoch's built-in cranking (and a KBI capacitor on big engines) means we never need a lead start.
Chapter 11
Three ways power comes in — and why the BMS changes things.
A lithium house bank takes charge from three places. Each one connects through the right device.
A standard internally-regulated alternator isn't safe on lithium. If the BMS disconnects mid-charge (it protects the cells), an internally-regulated alternator can spike and destroy itself and nearby electronics. An external regulator (default Wakespeed WS500, alternate Balmar MC-618) controls charge current and watches alternator temperature so it charges hard but safely.
The BMS is the battery's brain — it will open the circuit to protect the cells from over-voltage, over-current, or temperature. The install has to expect that disconnect and manage the voltage transient it causes, especially on the charging side.
Chapter 12
Where the power is controlled — and how it should look.
Between the bank and the loads sits the switching and distribution. Done right, it's clean, labeled, and controllable.
The bank feeds a main switch and bus bars that distribute positive and negative to the circuits. We increasingly use the Egis XD series — remote battery switches controllable over NMEA 2000, so the owner can switch and monitor banks from the chartplotter, and the install carries fewer heavy control wires.
Chapter 13
It's not done until you've measured it.
The last step separates a real install from a science project: you prove it. Power up deliberately and verify.
Bring the system up deliberately — main protection in, bank on, then charge sources and loads — watching for anything unexpected (a spark, a warm spot, a wrong reading) at each step rather than throwing everything on at once.
Photograph the finished system and record what went in. A system built to standard is one you can prove — with readings and pictures — to the owner, the next tech, and a surveyor.
Chapter 14
How we work is part of the work. This applies to every job.
Skill gets the install right; discipline gets it done profitably and hands the customer a boat that's better than we found it. These habits run through every ESI job, electrical or not.
Before you walk to the boat, plan the job and gather the tools, parts and equipment you'll need. Think the job through end to end and stage it. A planned job is a fast job; a forgotten tool is a trip you didn't have to make.
Use a cart for each job. Stage your tools, parts and consumables on it so everything travels with you to the boat and stays in one place — not scattered, and not back in the shop.
Every trip back to the shop is unbillable time the customer doesn't pay for and the job eats. Walking costs money. Keep your tools organized and within reach so you're working, not searching or fetching.
Count the extra trips you make to the shop and record those numbers. It's a real efficiency metric: trips per job, tracked over time, show where pre-planning is slipping and where we can tighten up. Honest numbers, every job.
ESI runs on Kaizen — continuous improvement. Small better-ways found on the boat, captured every day, compound into a faster, sharper shop. That only works if you write it down.
Final certification
A short check ran at the end of each chapter. Pass this final at 80% to unlock your certificate.