This article describes how to install a 30 Amp shore power system on a small boat. A 30 Amp 120 volt service gives enough power for battery charging, a receptacle or two, and a water heater, or maybe even a small air conditioner. Larger boats with multiple appliances and air conditioners probably would need to look at a 50 amp system.
Starting at the dock end you will need a new shore power cord for 120 volt 30 amps and you will need a new Deck Inlet on board the boat to connect it to. The examples below are what would be appropriate, the length of shore cord of course depends on the length of the boat and whether you dock it bow first or stern first. You normally want to locate the deck inlet in an area that is not too exposed and where the cables wont get in the way. I have often seen deck inlets located near the bow of the boat or even in the anchor locker and they usually fail due to water intrusion, its just too wet up there!
Once inside the boat the wiring for 30 Amps 120 Volts need to be 10 awg three conductor. The ABYC allows a maximum of 10 feet between the deck inlet and the main circuit breaker. Since this design is for a small boat we will just assume that it is 10 feet or less to the main panel. For 30 Amp service the wire before the main breaker needs to be Triplex 10/3 awg
Before going to the main breaker panel you should seriously consider a galvanic isolator. This is what interrupts any galvanic currents between your boat and the dock, isolating you from problems in the marina caused by other boats or the dock and reducing the loss of zinc anodes. Its especially important for boats with aluminum outdrives, outboards or saildrives but is a valuable addition to any boat that plugs into shore power on a regular basis and will save money in the long run. If you decide to install the galvanic isolator then test it first in accordance with the instructions provided and be sure to test it at intervals throughout its life. Its easier to do this the first time when it is in your hands than it is when hanging upside down in a cockpit locker.
Next we come to the main breaker panel. To meet current ABYC standards this needs to have a 30 Amp double pole main breaker with a 30 mA ELCI trip. The 30 Amps is for overcurrent protection, the ELCI provides protection from stray currents that could cause electrocution, especially to swimmers in the water around the boat*. A typical circuit breaker panel for this would be the Blue Sea 8101 120 Volt AC Circuit Breaker Panel with ELCI Main It has the requirements above and also has two 15 Amp circuit breakers for distribution circuits as well as three additional spaces for more circuit breakers. Another option would be the Blue Sea 8102 ELCI Panel with 30A Double Pole Main + 2 Positions with Voltmeter which is the same thing with the addition of a voltmeter. If the back of these panels is accessible in any way they are required to have a back cover such as the 4027 Panel Back Cover
The rest of your distribution wiring can be done with a smaller wire gauge, I normally use 12 awg so I dont have to do any calculations, it will be good for anything up to 20 Amps no matter if it goes in the engine room or how many wires are in the bundle. If you are installing AC receptacles make the first one in the series a GFI receptacle and then it will protect the rest of the chain. These GFI receptacles have a 5mA trip and protect the user from electric shock, they are especially needed in wet areas like galleys and heads but in my opinion should be used throughout.
All the usual ABYC standards apply to wiring of course, it should be harnessed in place every 18" or run in conduit, all the receptacles have to be mounted in a box, any connections need to be clamped and protected, use locking crimp connectors such as rings or forks with captive ends. The AC Ground must be connected to the engine block. Connecting the boats ground to the AC safety ground makes sure that if any AC Hot wire comes in contact with any metal part of the boat it will trip the AC main breaker.
When AC and DC conductors are run together, the AC conductors need to be sheathed, bundled, or otherwise kept separate from the DC conductors, using Triplex wire as I have shown meets the requirements of that part of the standard. Keep wires out of the bilge as much as possible, any connections in the bilge need to be watertight. Keep wires away from hot exhausts or moving parts. All electrical appliances and equipment designed for permanent installation must be securely mounted to the boat's structure.
Ignition protection: Assume that nothing is ignition protected unless it specifically says it is. The only item described in this article that is ignition protected is the Galvanic Isolator. Equipment not listed as ignition protected is not suitable for use in the engine compartments of gasoline powered boats.
Here is the official ABYC wiring diagram for a simple system like this:
Below is a marked up photo showing how the 8101 Panel is wired:
On completion make sure all your connections are tight. Test your galvanic isolater if you used one. Test your AC polarity with a polarity tester. Test your GFIs'
*Never swim around boats in a marina, its just too dangerous.