Wiring Your Boat for Shore Power

This diagram shows a typical onboard shore power setup. Illustration by Alastair Garrod

Installing shore power on a cruising boat is an easy and relatively inexpensive project, as long as you have basic DIY skills, can read a manual and are realistic about your needs. If you’re just planning to live aboard your boat in a marina and want to run appliances like a heater, a fan, a TV and a blender (hey—why not?), then you can get by with a simple installation that will set you back just a few hundred bucks if you do the work yourself.

Let’s say you’ve bought an older cruising boat that you plan to live aboard and therefore need to be able to hook up to AC power. Plenty of boats built before the early 1990s never had shore power in the first place, and even if they did, it’s likely that the system is ripe for replacement.  Such was the case on our old Norlin 34 project boat: it had been wired for AC power in the distant past, but at some point all the wiring had been removed. All that was left was a corroded inlet plug, an ancient AC breaker panel and a single outlet by the galley. All three had seen much better days, so I scrapped the lot and started from scratch.
The old switch panel was in poor shape

I’ve been on many boats loaded to the gunwales with power-hungry accessories like air conditioning, washing machines, microwaves and so on. Mine would be a KISS-worthy installation: I wanted to use lights profligately, run the fridge without guilt, keep the laptop and phone charged up, use the stereo as much as I liked, and run essential 120V appliances (i.e., a blender and power tools). A 30-amp shore power system would be plenty adequate for that. To add power-hungry appliances like air-conditioning, I’d have had to double up the 30-amp inputs or go to a 50-amp system. One difference between the two is that the 50-amp cables have to handle more current, so they’re a heavier gauge and therefore cost more. They may also turn you into one of those marina power-stand hogs who deploy two cables to run all their equipment, thereby displacing and annoying those with more modest needs.

Find a suitable place for the inlet

Because the lights and fridge run off our boat’s DC system, the only 120V loads I had to consider were the battery charger and any AC appliances in use. Worst-case that would be a ceramic heater early and late in the season, and perhaps a TV; though an iPad or laptop serves that function well enough. Say 10 amps for the heater and another generous 10 for everything else—that wouldn’t tax a 30-amp system at all. The two individual breakers on the Blue Sea panel I purchased are rated at 15 and 20 amps.


Had I started from scratch, I’d have drawn a scale diagram of the boat to plan out cable lengths and the location of outlets, etc., and a list of stuff I’d have to buy. Since I knew where everything would be and had determined that I would need only 15ft of cable, I went straight to the list.

The shore power inlet: This was a no-brainer. A 30-amp inlet from Marinco was duly purchased.

The galvanic isolator is easy to install

A galvanic isolator: This required some thought. While not essential in the strictest sense of the word, a galvanic isolator (Galvanic Isolater, see below) would provide some peace of mind.

Master circuit breaker: This will be sized to trip in the event of a current surge and also used to disconnect the shore power feed to the distribution panel. (Note the American Boat and Yacht Council (ABYC) stipulates that if the master circuit breaker is located more than 10ft from the inlet, a second breaker must be installed within 10ft of the master breaker.) If you are replacing or adding a master circuit breaker, the new one should be an ELCI type.

A minimalist AC breaker panel And here’s the other side of it

An AC distribution panel: This should contain as many breakers as you need for individual loads. My panel incorporates the master circuit breaker and has a reverse polarity light. For simplicity’s sake, and because there wasn’t room for a bigger panel where I wanted to locate it, I settled for two circuits—one for the battery charger and one for the outlets. I’m now regretting not having a spare, as I could have used that for the water heater.

One ground fault circuit interrupter (GFCI) outlet: If I want, I can add a normal (non-GFCI) outlet downstream, as it will be protected by the first outlet.

A single GFCI outlet

Fifteen feet of 10 gauge electrical cable: This has three wires, black (live), white (neutral) and green (ground). I only needed 10ft, but I’d rather have a bit extra than run short anywhere. As it turned out, it was barely long enough. Note that you must only use stranded wire on a boat.

Cable terminals: Crimped ring connectors were used as far as possible, with butt connectors in case the shore power cable needed to be lengthened. Never, ever use wire nuts!

A shore power cable: These are available ready-made in 25ft or 50ft lengths, or you can buy as much of the cable as you need and make it up yourself with the end fittings.


I didn’t have to cut a hole for the shore power inlet, just remove the old one and replace it with the new one. Were I doing it from scratch, I’d locate it somewhere with no trim behind it and where the cable wouldn’t get in the way of anyone using the cockpit—probably on the transom. As it was, I was stuck with a previous owner’s idea of where it should be—almost, but not quite, in the way. The alternative would have been to plug up the old hole and cut a new one, which was way too much work. If there’s one thing you learn working on old boats, it’s to not make needless work for yourself. The photos show how the inlet is installed.

The green AC ground wire is connected to the boat’s common DC ground bus bar, which is connected to the engine’s ground point

There’s only so much you can do on a small 34-footer with a crowded engine compartment and nowhere to hide anything. The Guest galvanic isolator was mounted on a bulkhead in the quarterberth, where recumbent crew can admire its handsome sculpted lines and fetching blue anodizing. I found a home away from errant elbows and boots beneath the chart table for the GFCI outlet, and installed the Blue Sea distribution panel under the battery charger I had installed a year earlier in anticipation of this shore power project. The original AC breaker panel had been much larger, so I had to make a new main panel from G10 fiberglass sheet covered with white Formica to house the AC panel as well as the new DC breaker panels I was installing at the same time.


After consulting a wire-sizing chart I used 10 gauge Ancor UL 1426 tinned triplex cable from West Marine for the run from the inlet to the distribution panel (the shorter runs from the panel to the loads were 12 gauge). It costs a couple of bucks a foot and meets ABYC standards. The various crimps were made with great care, using correctly sized Ancor marine grade ring connector terminals and a good pair of crimping pliers. I gave each terminal a hefty yank after crimping it to ensure the connection was tight. I also made sure the terminals were the correct size, both for the lugs on the AC equipment and for the wire.

the new breaker panel incorporates the master circuit breaker, a reverse polarity light and two 15 amp breakers

I should have used heat shrink for the terminations, but since I was doing the job on the mooring I didn’t have access to a heat gun. I made do with blasting CT-11 anti-corrosion spray over all the connections.

Cordset 25ft$60
Galvanic Isoloator$90+
Outlet, cables$25

I sited the galvanic isolator hard by the shore power inlet where it was easy to split the green grounding wire. Then I hooked up the black (live), white (neutral) and green) grounding wires to the breaker panel. This is where a reverse polarity light comes in handy; if for example the black and white wires are connected wrong inside the dock pedestal, this renders the grounding wire useless. That’s when the reverse polarity light comes on. It’s a signal to disconnect from shore power immediately, as appliances you thought were turned off may now be live.

After connecting the hot wires from the AC loads (GFCI outlet and battery charger) to their breakers, I connected their neutral and green ground wires to bus bars as per Blue Sea’s instructions, and then connected the bus bars to the breaker panel.

The ABYC recommends you connect the green grounding wire directly from the AC panel to the grounding terminal on the engine instead of to the boat’s common DC negative bus bar. That way, a fault in the AC system can’t leak current through DC wiring to create a shock hazard on the boat or in the surrounding water.

All that remained was to plug the shore power cable into the 120V supply, then into the boat’s shore power inlet, switch the power on ashore, and then flick the breaker switch on my new AC panel. The “on” light glowed red, the reverse polarity light stayed off. Success…

The newly installed panel is clean and clear

Now that you’ve installed your own shore power system, take the time to educate yourself thoroughly about the intricacies of AC power in a marine environment. Don Casey’s Sailboat Electrics Simplified is an excellent resource. A simple project like this is well within the capabilities of a practically minded boat owner, provided he or she follows the relevant safety guidelines. Shore power can kill, so if you’re in any doubt about your skills, have a professional do the job or at least check your work over.

Photos by Peter Nielsen


ELCI Breaker

Where a GFCI protects you from being electrocuted by a wet appliance, an ELCI—Electric Leakage Circuit Interruptor—protects the boat and its crew from shocks due to faulty wiring, either ground faults (current leaks from hot or neutral wires to the ground wire) or faulty grounds (a loose or broken ground wire) that could leak current into appliances or the water around the boat, electrocuting people on board or swimming near the boat.

An ELCI will trip at 30mA, where a CFCI will trip at 5mA, so there is a place on board for both types of protection. The ABYC now requires ELCIs in all new shorepower installations. They must be mounted within 10 feet of both the shore power inlet and master circuit breaker, but if you’re retrofitting shore power from scratch then you may as well buy a switch panel where the ELCI also serves as the master circuit breaker.

Is it worth buying an ELCI if you seldom plug into shore power? It would be hard to justify the price—upwards of $150—if you are only an occasional marina-dweller, but if you’re living aboard or keep your boat in a slip with shore power always connected, it’s cheap insurance against what could be a tragic accident.


Galvanic Isolator

Take a marina slip, add a boat and connect the two with a shorepower cord and you have the ingredients for a cocktail of galvanic corrosion. Any time dissimilar metals come into contact in an electrolyte—for example, saltwater—they act as a battery and one will eat away at the other. Because the shore power system’s green grounding wire is connected to a boat’s DC ground, it effectively connects all the boats in the marina, allowing DC current to pass between the boats and corrode their least noble underwater metals as if in a giant battery.

A galvanic isolator breaks this circuit while allowing AC current to pass through it to the shore. It contains diodes that will only conduct a voltage greater than 1.2 volts. Because most DC voltages are lower than this, DC current will be blocked while dangerous AC current can pass across the diodes. The shorepower grounding wire seldom carries AC current, except in the event of a fault, but the isolator must still be able to cope with full current without overheating. Choose an isolator that also has a capacitor as a backup so that DC current will still be blocked in the event of AC current passing through the isolator.


Most Read on Sail

Also Popular on Sail


Leave a Reply