A sailboat will have a DC electrical system and, except in small craft, a separate AC electrical system. The DC system usually operates at 12 volts. Some boats have 24 volts DC because by doubling the DC voltage they can use wire only one-quarter the size. The AC system relies on a shore power hookup unless the boat is equipped with a generator or an inverter.
Shore
power
A sailboat can get household 120-volt AC power at 60 Hz—Edison’s finest—from any of three main sources. The most common is shore power, available only when docked and plugged into a marina pedestal. The pedestal supplies 30- or 50-amp service (in the US) or sometimes both. Cord plugs for the two are different. Either will twist-lock to make waterproof connections at each end. A 30-amp circuit can carry up to 3,600 watts intermittently or 80% of that peak continuously. In a 50-amp circuit, those upper limits double.Main
AC panel
The main AC panel is usually right beside the DC panel and color coded. It will have a main breaker switch that disconnects all AC power. Also branch breakers for each individual circuit. Most have voltage and amperage meters and a polarity light that shows when the hot and neutral wires are reversed. The low voltage DC side, usually on the left, is always on. The high voltage AC side has power only when plugged into shore power, while an inverter is on, or while the genset is running.Genset
Some boats have a built-in diesel generator, sometimes called a genset, typically used when away from the dock and mainly for air conditioning, microwave cooking, or perhaps a water maker. It is basically a second, somewhat smaller diesel engine and is more common on power boats.Portable gensets that are fueled by gasoline produce rough AC that they then convert to DC and back to the cleaner AC needed by electronics. Besides being less efficient owing to the conversions, they are not popular on boats because of the gas and carbon monoxide, risk of theft, and difficult handling in rough weather. Typical built-in marine gensets from the likes of Fisher Panda or Northern Lights produce AC directly without inverting it to DC and back.
Inverter
An inverter is a device that converts 12- or 24-volt DC battery power into 120-volt AC. Limited by battery capacity, it can power only smaller loads such as a coffee pot, computer or phone charger, or a small low-wattage household appliance (eg, probably not a toaster or hair dryer and certainly not a heater).Most often, on sailboats, the inverter also charges the batteries and is called an inverter/charger. Inexpensive, high-capacity LiFePO4 batteries—the only kind of lithium batteries that should be used on boats—are making sailboat gensets obsolete. Companies like Victron and Mastervolt sell powerful inverter/chargers that can be supplemented with a solar panel. Coupled with a high-output alternator driven by the engine, they can supply near household levels of power and in fact are used in many off-grid homes.
If a boat has multiple AC sources (shore power, generator, inverter), there must be a safe way to switch between them either manually or automatically. This prevents backfeeding power from one source into another, which could be dangerous or destructive.
Galvanic
isolator
A galvanic isolator in the shore-power green ground wire blocks stray current from the dock. It uses diodes arranged in pairs to block low-voltage corrosion currents while still allowing safety-critical fault currents to pass. Small DC voltages—often less than 1.5 volts—can exist between a boat’s bonding system and shore ground. Those tiny voltages drive galvanic corrosion currents through the shore power ground wire. A galvanic isolator interrupts that path, but only for low voltages.Two diodes are wired in parallel but oriented in opposite directions. That pair is then duplicated in series. A single silicon diode has a forward voltage drop of about 0.6–0.7 volts and two in series drop about 1.2 to 1.4 volts. Because they face both directions, the device works for either polarity of current.
In a dangerous fault (ie, a hot wire touching the boat’s metal), voltage jumps well above diode threshold, the diodes turn on, and the path to ground is restored. Fault current flows back to shore tripping the pedestal breaker, as it should to preserve safety grounding. Having multiple diodes raises the blocking threshold above typical galvanic voltages and adds redundancy to meet marine safety standards.
Modern so-called fail-safe isolators often include capacitors, to pass AC faults instantly, and monitoring circuits to detect diode failure. If a diode fails shorted, corrosion protection is lost. If it fails open, safety grounding might be lost.
A simple analogy is a hose with a valve that opens at a certain pressure. At low pressure the valve stays closed and there is no flow (hence no corrosion). At high pressure the valve opens to allow flow (for safe grounding).
ELCI
and GFCI
Marine AC systems have extra shock protections not always found in houses. A ground fault circuit interrupter (GFCI) monitors the difference between the ground and neutral wires and shuts off the power instantly if more than 4 to 6 mA goes missing. This protects people from shock. One installed within an outlet will serve the entire branch circuit. Test it monthly: press the red TEST button and the RESET button will pop out; press that to restore power in the branch.An equipment leakage circuit interrupter (ELCI) detects leakage current. Like a GFCI, it too monitors the difference between the ground and neutral wires while being more tolerant of the stray currents often found in complex wiring. It cuts power if more than about 30 mA of such leakage is seen. This protects sensitive equipment, such as electronics. One installed near a shore power entry point can serve the entire boat and help prevent electroshock drowning of swimmers in a marina.