A boat's electrical system is one of the few things aboard that can fail quietly, catch fire slowly, or sink a hull months after a bad connection was made. Marine electrical work sits in its own category for a reason. Salt air, constant vibration, moisture, and the electrochemical soup a vessel floats in all conspire to break connections that would last decades in a house. This guide walks through how a boat electrical system actually works, from the 12v boat wiring that feeds your lights and pumps to the 120/240V AC shore power that runs your air conditioning. You will learn boat wiring fundamentals, how to size wire by amps and length, how fusing and breakers protect a circuit, why marine-grade tinned wire matters, and how grounding, bonding, and galvanic isolation keep your boat and your body safe. Whether you plan to do the work yourself or hire it out, understanding the marine electrical system on your vessel makes you a better owner.
Most recreational and working vessels carry two separate electrical systems that live on the same boat but almost never touch. The first is a low-voltage DC system, usually 12 volts on smaller boats and 24 volts on larger yachts and commercial craft. The second is an AC system, typically 120 volts in North America or 230 to 240 volts in Europe and much of the rest of the world, fed by shore power at the dock or by an onboard generator or inverter. Understanding a marine electrical system starts with keeping these two worlds straight in your head, because they follow different rules, use different wire, and fail in different ways.
The DC side is the heart of the boat. It runs from your batteries and powers navigation lights, bilge pumps, electronics, the engine starter, cabin lighting, refrigeration on many boats, and the instrument panel. DC electricity flows in one direction, from the positive terminal of the battery, through the load, and back to the negative terminal. Because DC voltage is low, the current has to be high to move the same amount of power, and high current means fat wire and careful attention to voltage drop. That single fact shapes almost every decision in boat electrical wiring.
The AC side behaves more like the wiring in your house. It alternates direction sixty times per second in North America, fifty times per second in most other places, and it arrives through a shore power cord or is generated aboard. AC at 120 or 240 volts can kill you in a way that 12 volts cannot, so the safety rules around grounding, polarity, and isolation are stricter and less forgiving. Many boats run for years on DC alone and only add AC when the owner wants air conditioning, a water heater, or household appliances.
The two systems are joined at exactly one point on a properly built boat: the safety ground. The AC grounding conductor connects to the boat's DC negative and bonding system so that a fault has a defined path and so the two systems share a common reference. Beyond that single tie, they stay separate. Mixing them anywhere else is one of the most dangerous mistakes an amateur can make, and it is a leading cause of the corrosion and shock hazards that plague poorly wired boats.
Trace the path of electricity through a well-built 12v boat wiring system and you see a logical chain. It begins at the battery bank, passes through a main battery switch, feeds a positive distribution bus or the main panel, splits into individual protected circuits, runs out to each load, and returns through a negative bus back to the battery. Every part of that chain has a job, and skipping any of them creates a weak point.
The battery bank stores the energy your boat runs on. Most cruising boats separate a starting battery, sized to crank the engine, from a house bank, sized to run everything else while the engine is off. Keeping them separate means a night of running the fridge and the anchor light will not leave you unable to start the engine in the morning. Batteries are joined with heavy cable, usually 2/0 or 4/0 gauge on larger banks, and the interconnects carry enormous current, so the connections must be clean, tight, and protected from corrosion.
Battery type matters for the wiring around it. Flooded lead-acid, AGM, gel, and lithium iron phosphate banks each have different charging profiles and different tolerance for heat and vibration. Lithium banks in particular demand a battery management system and careful attention to charge sources, because they can accept current far faster than older chemistries and will happily overwhelm an undersized cable or a charger that was never meant for them.
Right after the battery comes the main battery switch, the big red rotary knob that disconnects the bank from the boat. ABYC standards require a way to disconnect each battery bank, and the switch has to be rated for the current it will carry, including the brief but massive surge of an engine start. Between the battery positive terminal and the switch, or very close to it, sits the main fuse or circuit breaker that protects the entire feed. That protection has to be within seven inches of the battery, or within seventy-two inches if the cable is inside a sheath or conduit, because an unprotected cable that chafes through to a grounded surface can dump the full short-circuit current of the bank and start a fire in seconds.
From the switch, the positive feed lands on a positive bus bar, a solid metal bar that distributes power to the branch circuits. A matching negative bus bar collects all the return wires. Good bus bars are tinned copper or brass, rated for the total current, and mounted where they stay dry. The negative bus is the single most important tidy point on the boat, because every return current in the DC system passes through it, and a loose or corroded negative connection causes flickering lights, weird electronics behavior, and voltage readings that make no sense.
The distribution panel is where individual circuits are switched and protected. Each branch circuit gets its own breaker or fuse sized to the wire it protects, and a switch to turn it on and off. A well-labeled panel is a gift to your future self, because when a circuit fails at anchor in the dark, you want to know instantly which breaker feeds the bilge pump versus the stereo. The panel is fed by a single heavy positive wire from the distribution bus and returns through the negative bus.
Sizing wire correctly is the single skill that separates safe boat electrical wiring from the mess found on so many older vessels. Two independent factors decide how big a wire needs to be. The first is ampacity, the amount of current a wire can carry without overheating. The second is voltage drop, the amount of voltage lost along the length of the run. On a boat, voltage drop almost always wins the argument, meaning the wire you need to keep the voltage healthy is usually bigger than the wire you need to keep it cool.
Every conductor has resistance, and current flowing through resistance makes heat. Push too much current through too small a wire and the insulation cooks, softens, and eventually fails, which is how electrical fires start. Ampacity ratings tell you the maximum continuous current a given wire gauge can carry at a given temperature. Marine ampacity is derated for engine spaces and for bundles of wires running together, because heat has nowhere to go when wires are packed into a loom inside a hot compartment.
Voltage drop is where 12v boat wiring gets demanding. At 12 volts, losing even half a volt in a wire run is a big percentage of your total voltage. A bilge pump that only sees 11 volts pumps less water. An anchor windlass starved of voltage strains, draws even more current, and can stall. ABYC recommends keeping voltage drop under three percent for critical circuits like navigation lights, electronics, and safety equipment, and allows up to ten percent for non-critical loads like cabin lighting and general accessories.
Voltage drop depends on three things: the current the load draws, the round-trip length of the wire (out to the load and back, not just one way), and the wire's cross-sectional area. Double the length and you double the drop. Double the current and you double the drop. Go up two wire gauges and you roughly halve the resistance. Because boats have long runs from a battery amidships to a masthead light forty feet away, the round-trip distance adds up fast, and the wire ends up far larger than a beginner expects.
The table below gives approximate values for stranded copper marine wire. It shows the ampacity for a conductor and the maximum length of a round-trip run at 12 volts for both a three percent drop, used for critical circuits, and a ten percent drop, used for general circuits, at a representative current. Always confirm against the current ABYC tables for your exact load and conditions.
| Wire Gauge (AWG) | Approx. Ampacity (outside engine space) | Typical Use | Max Round-Trip Length at 10A, 12V, 3% drop | Max Round-Trip Length at 10A, 12V, 10% drop |
|---|---|---|---|---|
| 18 AWG | 10 A | Small indicator lights, sensors | 7 ft | 24 ft |
| 16 AWG | 15 A | Cabin lights, small electronics | 12 ft | 39 ft |
| 14 AWG | 20 A | Nav lights, pumps, accessories | 19 ft | 62 ft |
| 12 AWG | 25 A | Larger pumps, blowers, outlets | 29 ft | 98 ft |
| 10 AWG | 40 A | Windlass feeds, larger loads | 48 ft | 160 ft |
| 8 AWG | 55 A | Sub-panels, inverter feeds | 76 ft | 253 ft |
| 6 AWG | 75 A | High-draw loads, chargers | 120 ft | 400 ft |
| 4 AWG | 100 A | Battery feeds, thrusters | 190 ft | 636 ft |
| 2 AWG | 140 A | Main feeds, large inverters | 301 ft | 1005 ft |
| 1/0 AWG | 195 A | Battery interconnects, starters | 480 ft | 1600 ft |
| 2/0 AWG | 265 A | Large starter cable, big banks | 605 ft | 2020 ft |
| 4/0 AWG | 360 A | Heavy house banks, thrusters | 960 ft | 3200 ft |
Notice how at 24 volts the same wire carries the same power at half the current, which halves the voltage drop and lets you use smaller cable for the same load. That is one of the main reasons larger yachts run 24 volt systems. It saves copper, saves weight, and eases the voltage-drop headache on long runs.
Walk into a hardware store and the wire on the shelf is almost always solid or stranded bare copper meant for a dry house. That wire has no business on a boat. Marine-grade wire differs in two ways that matter enormously in a salt environment. First, it is finely stranded, with many small strands instead of a few large ones or a single solid conductor, so it flexes with the constant motion of a hull without work-hardening and cracking. Second, and this is the big one, each individual strand is tinned, coated in a thin layer of tin that resists the corrosion that eats bare copper alive in salt air.
Bare copper wick corrodes from the cut ends and from any nick in the insulation. Moisture wicks up between the strands by capillary action, and green copper oxide creeps along inside the insulation where you cannot see it. That corrosion increases resistance, which increases voltage drop and heat, which accelerates the failure. Tinned wire resists all of this. It costs more up front and pays for itself many times over in connections that still work a decade later. On a boat, marine tinned wire is not a luxury, it is the baseline.
Connectors follow the same logic. Use tinned copper ring terminals and butt connectors with adhesive-lined heat-shrink, crimped with a proper ratcheting crimper, not the cheap stamped tool that comes in a bargain kit. A good marine crimp is gas-tight, meaning it excludes air and moisture from the joint, and the adhesive-lined shrink seals the wire entry so water cannot wick in. Ring terminals are preferred over spade or hook terminals on anything that vibrates, because a ring cannot fall off if the screw backs out. Wire nuts, the twist-on connectors from house wiring, are prohibited on boats. They trap air, invite corrosion, and shake loose with vibration.
ABYC requires wire to be supported at least every eighteen inches so it does not sag, chafe, or fatigue. Runs should be kept out of the bilge where they can sit in water, protected wherever they pass through a bulkhead with a grommet or chafe guard, and kept away from hot engine components and moving parts. Good routing is invisible when done right and obvious when done wrong, because a boat that has been rewired by an amateur usually shows its history in the tangle of wires draped over the engine and pooling in the bilge.
Here is a principle that trips up a lot of newcomers: a fuse or breaker protects the wire, not the device. The overcurrent protection is sized to the ampacity of the smallest conductor in the circuit, so that if a wire shorts or is overloaded, the fuse blows before the wire overheats and starts a fire. The device on the end has its own concerns, but the fuse exists so the wire never becomes the fuse.
Every ungrounded conductor, meaning every positive wire in a DC system, needs overcurrent protection at its source, at the point where it connects to the battery or the bus. The only common exception is the heavy cable from the battery to the engine starter, which is allowed to be unprotected because the surge current of cranking would blow any reasonably sized fuse, though it must be kept short and well protected from chafe. Everything else gets a fuse or breaker sized at or below the ampacity of the wire it feeds.
Fuse types matter. A standard blade fuse is fine for a cabin light circuit, but a battery main needs a fuse with a high enough interrupt rating to safely break the enormous short-circuit current a battery bank can deliver. Class T fuses and ANL fuses are used for large DC feeds and inverter circuits because they can interrupt thousands of amps without arcing over. A cheap fuse in that spot can literally weld itself closed under a dead short and fail to protect anything, which defeats the entire point.
Circuit breakers do the same job as fuses but reset instead of needing replacement, which is why they populate distribution panels. A good marine breaker is ignition-protected where required, meaning it will not spark in a way that could ignite fumes, and it is sized to the circuit wire. Resist the temptation to fit a bigger breaker to stop nuisance trips. A breaker that keeps tripping is telling you something, and the answer is almost never to give the circuit permission to draw more current than its wire can safely carry.
Grounding and bonding are the parts of marine electrical work that owners understand least and that cause the most argument at the dock. The terms get used loosely, so it helps to define them. Grounding, in the DC sense, is the negative return path that carries current back to the battery. Bonding is a separate green-wire system that connects the underwater metals, the engine, and the AC safety ground together so they all sit at the same electrical potential. The AC grounding conductor is the safety wire that gives a fault current a path back to trip a breaker. These systems interconnect at defined points, and getting those points right is what keeps a boat safe.
The bonding system ties together the underwater metal fittings, through-hulls, the propeller shaft, the engine, metal fuel tanks, and other large metal masses with a heavy green wire, usually 8 AWG or larger. The purpose is threefold. It keeps all these metals at one potential so stray currents have nowhere interesting to flow, it provides a controlled path for lightning energy on boats with a lightning protection system, and it lets sacrificial anodes protect all the bonded metal at once. A bonding system is not universal, and some builders deliberately leave certain fittings unbonded, but where it exists it must be continuous and well connected.
Anodes, commonly called zincs even when they are made of aluminum or magnesium, are blocks of a more reactive metal bolted to the hull, shaft, or bonding system. They corrode preferentially so that your propeller, shaft, and through-hulls do not. Zinc is used in salt water, aluminum in brackish water, and magnesium in fresh water, because the water's conductivity changes which metal protects best. Anodes are consumable and need inspection and replacement, and an anode that never wears down is a warning sign that it is not electrically connected to what it is supposed to protect.
Corrosion on a boat comes in two flavors that people constantly confuse, and telling them apart is the first step to fixing either one. Galvanic corrosion is natural and slow. Stray-current corrosion is a wiring fault and fast.
Galvanic corrosion happens whenever two dissimilar metals sit in the same electrolyte, and seawater is an excellent electrolyte, connected by a conductor. The more reactive metal gives up ions to the less reactive one and slowly dissolves. This is exactly the effect anodes exploit on purpose. The trouble at a marina is that shore power's green safety ground connects your boat's underwater metal to every other boat's underwater metal through the dock wiring. Your bronze through-hull can end up wired, through the shore ground, to your neighbor's aluminum outdrive, and the galvanic couple eats whichever metal is more reactive. Your anodes then try to protect the entire dock and disappear alarmingly fast.
Stray-current corrosion is far more aggressive and comes from a wiring fault that leaks DC current into the water through the boat's metal. A chafed positive wire touching a bonded fitting, a bilge pump with a compromised connection sitting in bilge water, or moisture bridging a terminal can send current out through a through-hull and into the sea. This kind of corrosion can destroy a bronze fitting in weeks rather than years. The signature is dramatic, localized loss of metal, and the cure is finding and fixing the leak, which is exactly the kind of job where a marine electrician with the right meters earns their fee.
When you plug into shore power, you connect your boat's AC system to the dock's, and with it the safety ground that ties every boat on the pier together electrically. That shared ground is a genuine safety feature, because it gives a fault current somewhere to go so a breaker trips instead of leaving a hull energized. It is also the pathway that lets galvanic corrosion spread from boat to boat, which is the problem a galvanic isolator solves.
A galvanic isolator is installed in the shore-power grounding conductor. It blocks the small DC voltages, under about one and a half volts, that drive galvanic corrosion, while still passing AC fault current freely so the safety function is preserved. It does this with pairs of diodes that will not conduct until the voltage across them exceeds their threshold, which galvanic voltages never do but a real AC fault easily does. A fail-safe galvanic isolator is designed to short closed if it fails, so that a failure defaults to the safe condition of a solid ground rather than an open one. For boats that spend a lot of time on shore power, a galvanic isolator is often the cheapest insurance against a haul-out full of eaten fittings.
The next level up is an isolation transformer, which magnetically couples shore power into the boat without any direct metallic connection to the dock ground at all. This completely breaks the galvanic path, gives the strongest protection against stray current from the dock, and adds a layer of shock safety. Isolation transformers are heavy and expensive, so they show up more on larger yachts and boats kept permanently at a dock, but where corrosion problems are severe they are the definitive answer.
AC shore power connections must be checked for correct polarity, because a miswired pedestal can put the hot and neutral backward and leave metal parts energized even when a switch is off. A reverse-polarity indicator on the AC panel warns of this, and a modern approach adds an ELCI, an equipment leakage circuit interrupter, which is a boat-scale version of the GFCI in your bathroom that trips the whole AC system if it detects current leaking to ground. ABYC now calls for ELCI protection on the main AC feed, and it has prevented a number of the in-water electrocutions that used to happen around docks and swim platforms.
The American Boat and Yacht Council publishes the consensus standards that define what good marine electrical work looks like in the United States, and much of the world follows them or something very close. ABYC standards are not law in most places for a private boat, but they are what insurers, surveyors, and courts treat as the standard of care, and any competent marine electrician builds to them. Several ABYC standards touch electrical work. E-11 covers AC and DC electrical systems, A-31 covers battery chargers and inverters, TE-4 addresses lightning protection, and E-13 addresses lithium battery installations.
The value of ABYC to an owner is that it turns vague folklore into specific numbers. It tells you the voltage-drop limits, the wire-support spacing, the overcurrent-protection distances, the required disconnect switches, and the connector types that are allowed. When you hire an electrician, asking whether the work will meet ABYC E-11 is a fair and useful question, and a professional will not blink at it. When you sell the boat, a survey that finds ABYC-compliant wiring is worth real money because the buyer's insurer is far happier. Building to the standard is simply the cheapest way to end up with a boat that is safe and insurable.
Most electrical faults on a boat come down to a handful of causes, and a cheap digital multimeter finds the majority of them. The three measurements you use constantly are DC voltage, resistance or continuity, and, with a clamp meter, DC current. Learning to read them turns a mysterious dead circuit into a logical hunt.
Set the meter to DC volts and you can check the health of the battery, confirm power is reaching a device, and find voltage drop. A rested, fully charged 12 volt battery reads around 12.6 to 12.7 volts. Under 12.2 volts it is roughly half discharged, and under 12.0 it is nearly flat. To find voltage drop, measure the voltage right at the battery, then measure it at the device while the device is running. The difference is what you are losing in the wire and connections. A large drop points to undersized wire, a corroded connection, or a bad ground.
Switch the meter to the continuity or ohms setting, with the circuit powered off, and you can check whether a wire is intact, whether a switch actually closes, and whether a fuse is good. A good fuse reads near zero ohms. An open fuse reads infinite. This is also how you chase a broken wire inside a loom, by checking continuity from one end to the other and wiggling the run to find an intermittent break.
A clamp meter that reads DC current lets you find parasitic drains and leaks without breaking any connections. Clamp it around a wire and it reads the current flowing through. Clamped around the shore-power cord's grounding conductor, unusual DC current there points to a galvanic or stray-current problem. Used on individual circuits with everything switched off, it reveals the phantom draw that flattens a battery over a week at the mooring.
| Symptom | Likely Cause | First Check |
|---|---|---|
| Device is completely dead | Blown fuse, tripped breaker, broken wire, or bad connection | Check fuse continuity, then voltage at the device |
| Lights dim or pump runs weak | Voltage drop from undersized wire or corroded terminal | Measure voltage at battery vs at the load while running |
| Battery flat after a few days idle | Parasitic drain or self-discharging battery | Clamp-meter each circuit with everything switched off |
| Anodes disappear very fast | Galvanic coupling through shore ground or stray current | Test shore-ground DC current; inspect for wiring faults in bilge |
| Reverse-polarity light on at the dock | Miswired shore pedestal or boat inlet | Unplug, try another pedestal, inspect the inlet wiring |
| Electronics reset or misbehave | Loose or corroded negative bus connection | Inspect and clean the DC negative bus and grounds |
| Breaker trips repeatedly | Overloaded circuit or a short to ground | Disconnect loads one at a time to isolate the fault |
| Corroded green wire terminals | Moisture intrusion and bare or non-tinned wire | Cut back to clean copper, re-terminate with tinned, sealed connectors |
The same errors appear on boat after boat, and knowing them helps you spot trouble before it costs you. The most common is using house wire instead of marine tinned wire, which corrodes from the inside and fails years later where no one can see it. Next is undersized wire that ignores voltage drop, leaving circuits that work but underperform and run warm. Wire nuts and electrical tape splices are a constant find, both of which trap moisture and shake loose. Missing or oversized fuses are dangerous in the other direction, leaving wire unprotected against a short.
Poor grounding shows up as flickering lights and confused electronics, usually traced to a loose negative bus or a return path that shares a wire it should not. Unsupported wire that sags into the bilge chafes through and shorts or corrodes in standing water. On the AC side, the classic mistakes are a missing galvanic isolator on a boat kept at a dock, a bonding wire connected to the AC neutral instead of only the ground, and no reverse-polarity protection. Any single one of these can be the reason a boat has a corrosion problem, a fire risk, or a shock hazard, and a survey usually finds several at once on a neglected vessel.
Plenty of marine electrical work is well within reach of a careful owner. Replacing a cabin light, adding a USB outlet on its own fused circuit, running a new wire to a fishfinder, swapping a bilge pump, and cleaning up corroded terminals are all reasonable weekend jobs if you use marine tinned wire, proper crimps, correct fuses, and follow the sizing rules in this guide. Learning to do this work makes you self-sufficient far from a marina and saves real money over a season.
Some work belongs with a professional, and the dividing line is mostly about risk. Anything involving 120 or 240 volt AC shore power deserves an expert, because a mistake there can kill someone. Installing or reconfiguring a battery bank, especially lithium with its management system and high charge currents, rewards professional design. Diagnosing corrosion, chasing a stray-current leak, integrating a generator or inverter into the AC panel, and installing a galvanic isolator or isolation transformer all call for the meters, experience, and standards knowledge a marine electrician brings. If a job involves shore power, underwater metals, or a battery bank large enough to weld with, hiring a pro is the smart call.
A good marine electrician also documents the work, labels the circuits, and builds to ABYC, which pays off at survey and resale time. For a deeper look at choosing and working with these pros, see our guide to marine electrical contractors. If your project touches charging and power conversion, our guide to marine battery chargers and inverters covers that side in detail, and for one of the most common DIY wiring jobs afloat, see our bilge pump sizing, wiring, and cost guide.
Costs vary by region, boat size, and how much wiring already exists, but 2026 figures give you a planning range. Marine electrician labor typically runs from 95 to 175 dollars per hour in most North American markets, with high-cost coastal areas and superyacht specialists charging more. A service call to diagnose a single fault often lands between 150 and 400 dollars depending on how long the hunt takes.
Adding a simple new circuit, such as an outlet or an accessory on its own breaker, commonly costs 200 to 600 dollars including parts and labor. Installing a galvanic isolator runs roughly 300 to 700 dollars installed, while an isolation transformer, being heavy and complex, can reach 2500 to 6000 dollars or more depending on size. A new battery charger or inverter install ranges widely, from a few hundred dollars for a small unit to several thousand for a large inverter-charger with the cabling and protection it needs.
The big one is a full rewire. Stripping out old, corroded, non-tinned wiring and replacing the DC and AC systems on a mid-sized cruiser is a multi-week job that commonly runs from 8000 to 30000 dollars or more, driven mostly by labor and by how hard the boat is to access. It sounds steep until you weigh it against a fire, a sinking from a corroded through-hull, or an insurance claim denied because the wiring did not meet standard. Materials are a small fraction of the total, so this is never the place to save money by buying cheap wire.
Marine wire is finely stranded so it flexes with the boat without cracking, and every strand is tinned to resist corrosion in salt air. Regular house wire is coarser stranded or solid and uses bare copper, which corrodes from the ends and from any nick in the insulation, wicking moisture up inside where you cannot see it. On a boat, only marine-grade tinned wire holds up over the long term, which is why ABYC and every professional insist on it.
Size for two things and take the larger result. First, the wire must carry the current without overheating, which is its ampacity. Second, the wire must hold voltage drop within limits over the round-trip length of the run, three percent for critical circuits like nav lights and electronics, and up to ten percent for general loads. On boats, voltage drop usually dictates a bigger wire than ampacity alone would, especially on long runs at 12 volts. Use the table in this guide as a starting point and confirm against current ABYC values.
Fast anode loss almost always means one of two things. Either galvanic coupling through the shore-power ground is connecting your underwater metal to other boats at the dock, in which case a galvanic isolator or isolation transformer is the fix, or a stray-current leak from a wiring fault is pushing DC into the water through your fittings, which is more aggressive and needs to be found and repaired. A marine electrician can test the shore ground and the bilge wiring to tell which one you have.
If your boat spends significant time plugged into shore power, yes, a galvanic isolator is strongly recommended and often the cheapest protection you can buy against corrosion. It blocks the small DC voltages that drive galvanic corrosion between boats while still passing AC fault current so the safety ground keeps working. Boats kept permanently at a dock, or those already showing corrosion, may benefit from stepping up to an isolation transformer.
Much DC work is reasonable for a careful owner, including replacing lights, adding fused accessory circuits, running new wire to electronics, and cleaning corroded connections, as long as you use tinned marine wire, proper crimps, and correct fusing. Leave AC shore power, battery bank redesign, corrosion diagnosis, generator and inverter integration, and galvanic isolation to a professional, because the risk of a serious mistake there is high and the consequences can be fire, sinking, or electrocution.
ABYC E-11 is the standard for AC and DC electrical systems on boats. It specifies wire sizing and voltage-drop limits, overcurrent protection placement, required disconnect switches, approved connector types, wire support spacing, and grounding requirements. It is the standard of care that surveyors and insurers use to judge a boat's wiring, so building to E-11 keeps your boat safe, insurable, and worth more at resale even though it is not usually a legal requirement for a private vessel.
A full rewire of a mid-sized cruiser commonly runs from 8000 to 30000 dollars or more, with labor being the largest share and access difficulty driving much of the variation. Smaller jobs are far cheaper, a new circuit runs a few hundred dollars and a galvanic isolator install a few hundred more. Because materials are a small part of the cost, using proper marine tinned wire and quality connectors adds little to the total and is always worth it.
A boat's electrical system rewards the owner who understands it. Once you can trace power from the battery through the switch, the bus, and the panel out to a load and back, once you size wire for both heat and voltage drop, and once you grasp why tinned wire, proper fusing, solid grounding, and galvanic isolation all matter, the mystery falls away and the boat becomes far safer and more reliable. Do the DC work you are comfortable with, use the right materials every time, and bring in a qualified marine electrician for the AC, the battery banks, and the corrosion puzzles. That combination of informed owner and skilled professional is what keeps a marine electrical system working season after season, in the one environment on earth designed to tear it apart.