Shore power is the simplest way to keep a boat's batteries topped up and to run high-load appliances that solar and wind can't realistically support — air conditioning, microwave, a battery charger running at full output. Most cruising sailors view shore power as a convenience: plug in when available, run on batteries and solar when not. That's a reasonable approach, but it understates the safety dimension. Shore power is also the way that marinas slowly destroy the underwater metals of boats that haven't taken the right precautions, and it's the source of the electric shock drowning hazard that has killed swimmers in marinas with faulty wiring.
Getting shore power right means understanding the physical system, the corrosion risk, the shock hazard, and what equipment provides genuine protection versus what provides a false sense of security.
The Shore Power System: Basics
US marina shore power comes in two common configurations. 30-amp, 120V single-phase is the standard for most slips designed for sailboats and smaller powerboats. The pedestal provides a 30A, 125V twist-lock outlet (NEMA L5-30 connector). The shore cord runs from the pedestal to the boat's shore power inlet, which is typically mounted on the stern or hull side. Inside the boat, shore power feeds a battery charger and possibly an AC distribution panel for direct use of 120V appliances.
50-amp, 240V service (two hot legs, each 120V, 50A) is available at some larger marinas and is standard for bigger power boats and live-aboard slips with heavy electrical loads. A 50A service provides twice the power-carrying capacity of 30A service and is required for air conditioning on larger boats or for fast battery charging with a high-output charger. Adapters exist to connect 30A inlets to 50A pedestals and vice versa, but they must be used carefully — adapting 50A service to a 30A inlet allows more current capacity than the inlet wiring is rated for, which is a fire hazard if you run high loads through the adapter.
The shore cord itself — the flexible cable with twist-lock connectors at each end — is typically 25 or 50 feet long. Power Cord (yellow or orange jacketed) is rated for marine use; residential extension cords are not. The connectors should be inspected for corrosion and heat discoloration before each use. A warm plug or inlet after use is a sign of a resistive connection that needs attention.
Galvanic Corrosion: How Shore Power Kills Underwater Metals
This is the hazard that most sailors underestimate until they see what a 6-month slip at an active marina does to an unprotected boat. The mechanism works as follows.
When a boat connects to shore power, the AC safety ground — the green wire in the shore cord — connects the boat's AC ground to the marina's grounding system, which connects to all other boats on the dock that are also plugged in. This creates a common electrical path through the water between all connected boats. Different metals underwater (bronze propellers, stainless steel shafts, aluminum trim tabs, lead keels) have different electrochemical potentials. When connected through the water via a common ground, these boats and their metals form a galvanic cell — exactly like a battery, but one where the "electrode" being consumed is your propeller or shaft.
The rate of corrosion depends on how much current is flowing and what metals are present. In a marina with many plugged-in boats, mixed metal types, and poor bonding practices on some boats, stray current can consume a propeller shaft zinc in days rather than months. Severe cases eat away bronze propellers, corrode stainless steel shaft couplings, and in the most extreme situations damage fiberglass hulls through electrolytic pitting.
The solution is to interrupt the DC galvanic current path while maintaining the AC safety ground path. Two devices accomplish this in different ways.
Galvanic Isolator vs. Isolation Transformer
Galvanic isolator: a galvanic isolator installs in series with the shore power safety ground wire inside the boat. It contains back-to-back diodes (or capacitors in more sophisticated versions) that block DC current — the galvanic current flowing through the ground connection — while passing 60Hz AC fault current, which must be able to flow for the GFCI and shore power protection to work. A quality galvanic isolator (Victron Galvanic Isolator VDI-16, Marinco, or Charles Industries units run $80–200) significantly reduces galvanic corrosion in most marina environments. It is not perfect — it does not completely eliminate the ground connection, and in marinas with significant stray AC current (as opposed to purely DC galvanic current) it provides limited protection.
A galvanic isolator is the minimum installation for any boat that spends meaningful time on shore power in a marina. It should be tested annually: a failed isolator typically fails short, offering no protection, and the failure is not visible without testing. Quality isolators have a built-in LED indicator that confirms they're blocking DC current.
Isolation transformer: an isolation transformer is the gold standard. It is a large, heavy transformer installed inside the boat between the shore power inlet and the distribution panel. The primary winding connects to shore power; the secondary winding generates 120V for the boat's internal AC system. Crucially, the two windings are not directly connected — the only coupling is magnetic. This completely breaks the electrical connection between the marina ground and the boat, eliminating galvanic current flow entirely. It also eliminates the reverse polarity hazard (the secondary winding is always correctly wired regardless of what the pedestal delivers), handles moderate variations in input voltage, and provides a degree of isolation from marina wiring faults.
The downsides of an isolation transformer: size and weight (a 4kVA transformer — appropriate for a 30A shore power system — weighs 20–30kg), cost ($600–1,500 depending on capacity and brand), and installation complexity. Victron's Isolation Transformer IT range and Charles Industries both offer quality marine units. For a boat that lives at a dock for extended periods in a marina with mixed electrical conditions, an isolation transformer is a worthwhile investment. For a boat that plugs in occasionally, a galvanic isolator is usually sufficient.
ELCI: The Safety Requirement You May Not Have
ABYC standard A-31 (Shore Power Systems) has required Equipment Leakage Circuit Interrupter (ELCI) protection at the shore power inlet since 2016. An ELCI is similar to a GFCI outlet but designed for the full shore power circuit — it monitors current imbalance between the hot and neutral conductors and trips if more than 30mA of current is leaking to ground (a level that can cause electrocution in water). The ELCI installs as the first circuit protection device after the shore power inlet, before the main panel breaker.
Boats built before 2016 typically do not have ELCI protection. This is a significant safety gap: electric shock drowning — where a person swimming near a boat or dock is exposed to AC voltage in the water from a ground fault — is a real hazard in marinas, and ELCI protection at the inlet is the most effective onboard measure. Retrofitting an ELCI breaker (Blue Sea Systems makes marine-rated units at $80–150) is a straightforward installation and should be on every boat's safety upgrade list. This is one of the few electrical upgrades where the cost-to-safety-value ratio is unambiguous.
Testing for Hot Marinas
Not every marina has significant galvanic or stray current problems, but some do — particularly older facilities with mixed-age pedestal wiring, boats with poor bonding practices, and aluminum docks. Before committing to a long stay in an unfamiliar marina, it's worth checking.
A simple test: use a digital multimeter set to AC volts with one lead in the water and one touching the boat's hull or a through-hull. A reading above 1–2V AC indicates significant stray AC current in the water — an electric shock drowning hazard and a sign that something in the marina's wiring is seriously wrong. Many cruisers also use a silver zinc galvanic meter (a reference electrode system that measures the electrochemical potential of the boat's bonding system) to detect elevated galvanic activity. If your zincs are disappearing much faster than usual within the first few weeks of a new marina, it's a sign of elevated galvanic activity. Investigate before you lose a propeller.
European Shore Power: The Voltage Problem
US boats sailing to Europe face a direct incompatibility: European marinas deliver 230V at 50Hz, not 120V at 60Hz. A US battery charger or inverter/charger designed for 120V input will not operate on 230V shore power, and connecting it without protection will destroy the unit. A US air conditioner designed for 60Hz compressor motors will run slowly and inefficiently on 50Hz even if the voltage is correct.
The practical solutions depend on what AC equipment you're running. For battery charging only — the most common need — a modern smart charger or inverter/charger with a wide-input-voltage range handles both 120V and 230V automatically. Victron's MultiPlus and MultiPlus-II series (described below) accept input from 90V to 265V and 45–65Hz, making them genuinely universal. Older chargers with fixed-input transformers require either a step-down transformer (230V in, 120V out) or replacement with a wide-input unit.
For 120V AC loads (appliances, tools, anything designed for US power), a step-down autotransformer or a separate isolation transformer rated for your load is required in European marinas. The frequency difference (50Hz vs 60Hz) is largely irrelevant for most electronics and appliances; it affects motors and some older devices but most modern equipment handles it transparently.
The Victron MultiPlus: One Box That Handles Most of This
The Victron MultiPlus (and the newer MultiPlus-II) has become the standard solution for boats that want to consolidate their inverter, battery charger, and transfer switch into a single well-integrated unit. It accepts shore power input and automatically transfers AC loads to shore power when connected, while simultaneously charging the battery bank at full rated current. When shore power is unavailable, it inverts from the battery bank to provide 120V AC. Switchover between shore power and inverter output is nearly instantaneous — fast enough that most electronics don't notice the transition.
The MultiPlus also includes PowerAssist: when shore power is available but limited (a 16A pedestal in a European marina, for example), PowerAssist draws from the battery bank to supplement shore power, allowing the boat to run loads that exceed the shore power limit without tripping the pedestal breaker. This is genuinely useful in European marinas where the available shore power is often lower than US standards.
The MultiPlus-II 12/3000/120-50 (a common choice for a 30–40 foot sailboat) handles 3,000W of inverter output, 120A of charging current, and 50A of shore power input — all in one unit, with VE.Bus communication to integrate with the Victron monitoring ecosystem. At $1,100–1,400 for the unit alone, it's not cheap, but it replaces several separate components and brings a coherent system architecture that is well-supported by a large service and parts network worldwide. For a bluewater cruiser planning extended sailing in both US and European waters, a Victron MultiPlus-II with wide-input capability is the practical answer to the voltage compatibility problem.