How does shore power work on a boat?
I am new to cruising and my boat has a shore power socket on the transom. How exactly does the system work, what cables do I need, and how do I safely connect it without frying my batteries or causing corrosion?
Answer
Shore power brings 230V AC mains electricity from the marina pontoon directly into your boat to run domestic appliances, heat water and charge your batteries. It relies on a waterproof cable, an onboard consumer unit with safety trip switches and a galvanic isolator to protect your underwater metals from stray currents.
The cable and pontoon connection
The standard European and UK marina connection is a blue 16-amp 3-pin industrial plug, commonly called a Commando plug. The cable should be a 3-core 2.5mm² flexible marine wire. Manufacturers often coat this in yellow or blue Arctic-grade PVC so it stays flexible in freezing winter temperatures. Expect to pay £45 to £70 for a 15-metre or 25-metre lead. Your boat will have a matching male inlet socket mounted in the cockpit, on the transom or inside the anchor locker. Many modern boats use a twist-lock inlet from brands like Marinco or Hubbell to provide a better waterproof seal. Inspect these plug pins annually. If you see black scorch marks, the plug is arcing due to a loose connection and needs immediate replacement before it melts.
The onboard consumer unit
Once power enters the boat, it must go straight to a marine consumer unit located within 3 metres of the inlet. This box looks identical to a domestic fuse board and contains two safety devices. The first is the RCD (Residual Current Device), usually rated to trip at 30mA. This monitors the live and neutral wires. If power leaks to earth, it cuts the circuit in milliseconds to prevent fatal electric shocks. The second components are the MCBs (Miniature Circuit Breakers). These protect your boat wiring from overheating if you overload a circuit. You will typically find a 16A main breaker split into 10A circuits for sockets and a 6A circuit for the battery charger. If you cruise the Mediterranean, fit a reverse polarity indicator. Many older European marinas wire their bollards backwards, making the neutral live.
Stopping galvanic corrosion
When you plug into shore power, your boat earth wire connects physically to the marina earth. This links your underwater metals to every other boat on the pontoon. If another vessel has an electrical fault or a bronze propeller while you have an aluminium saildrive, a low-voltage DC current travels through the water and up your earth wire. This will destroy your sacrificial anodes and eventually your propeller. To stop this, fit a galvanic isolator into the shore power earth wire immediately after the boat inlet. Units from brands like Safeshore Marine cost £100 to £150. They block the low-voltage DC current but still allow a 230V AC fault current to pass safely to earth. Larger yachts over 40 feet use an isolation transformer from Victron or Mastervolt instead. This entirely severs the electrical connection to the shore.