AC Power System

A sailboat can get AC power from any or all of three main sources:

Shore Power

• The most common source when docked.
• You plug into a marina pedestal (usually 30A or 50A service in the US).
• Standard is 120 volts AC, 60 Hz.

Generator

• Some boats have a built-in diesel generator.
• Provides AC when away from the dock.
• Often used for high-load items like air conditioning.

Inverter

• Converts DC battery power (12V or 24V) into AC.
• Powers smaller loads when off-grid (outlets, computer, etc.).
• Limited by battery capacity.

Shore Power Inlet and Cord

• A heavy-duty marine shore power cord connects the dock pedestal to the boat.
• The inlet on the boat is usually twist-lock and weatherproof.
• Good setups include an ELCI (Equipment Leakage Circuit Interrupter) right after the inlet for shock protection.

Main AC Panel

Once power comes aboard, it goes to the AC distribution panel, which includes:

• Main breaker (disconnects all AC power)
• Branch breakers for individual circuits
• Volt and amp meters
• Polarity indicator (important—reversed polarity is bad)

Typical circuits:

• Outlets (but never plug large loads into an outlet; run a 12/3 extension cord ashore instead)
• Battery charger
• Water heater
• Air conditioning
• Microwave or toaster

Transfer Switching

If the boat has multiple AC sources (shore, generator, inverter), there must be a way to switch between them safely. This prevents "backfeeding" power into another source, which could be dangerous or destructive. Switching can be manual or automatic.

Safety Systems

Marine AC systems have extra protections compared to houses:

Grounding System

AC safety ground is connected to the boat's bonding system and underwater metals.

Galvanic Isolator

• Installed in the shore power ground line.
• Prevents stray DC currents from causing corrosion.

ELCI and GFCI

• ELCI protects the whole boat from leakage current (fire and shock risk).
• GFCI outlets protect individual circuits (like in kitchens or bathrooms).

Reverse Polarity Protection

Alerts you if dock wiring is backwards. That can be dangerous or destructive.

Typical Loads

AC is mainly used for high-power devices:

• Air conditioning (biggest load)
• Water heater
• Battery charger
• Microwave or induction cooktop
• Cabin outlets

Most always-on systems (lights, instruments, pumps) run on DC instead.

30A vs. 50A Service

• 30A is common on mid-size boats
  • Max ~3,600 watts
  • Must manage large loads (eg, AC + water heater may trip breaker)
• 50A on larger boats
  • Up to ~12,000 watts (split-phase)
  • Can run multiple heavy loads simultaneously

Key Differences from a House

• You are connected to an external power source (shore) with unknown wiring quality.
• Corrosion and stray current are constant concerns.
• Moisture and vibration demand marine-grade components.
• Must safely integrate with DC power and engine.

Practical Example

At the dock:

• Turn pedestal main breaker off, plug in shore power, turn pedestal main breaker on. Do this also when unplugging to prevent arcing (ie, sparks).
• Battery charger runs and might be always on.
• Water heater heats.
• You can run AC and outlets.

At anchor:

• No shore power.
• Use inverter only for light AC loads.
• Run generator for heavy loads.