Wired for the World

For no-hassle AC power anywhere, especially for U.S. boats going foreign, it’s necessary to think globally



More of today’s cruising boats provide all the comforts of home, and with luxury comes an increased demand for AC power, or alternating current. It seems like every galley now has a microwave oven; television sets with a VCR or DVD player are common. And how about a laptop computer or electric hair dryer? Or a central air-conditioning system? In no small degree, AC power on board has become a way of life for many cruisers. The trouble is, the AC electrical service that we have in the United States differs in voltage and frequency from systems found in most parts of the world. So how do we equip our boat if we’re planning an extended cruise outside of the United States?

The Hertz Factor
A primary consideration when dealing with onboard AC power systems and equipment is the operating frequency. Every AC-powered device is clearly labeled with its rated operating voltage and frequency. In the United States, alternating current is delivered at 60 hertz, meaning that it alternates 60 times per second. But only about 20 percent of the power available in the world is delivered at 60 hertz; in the other 80 percent of the world, power is delivered at 50 hertz.
Fortunately for American cruisers, not all electrical equipment is frequency sensitive. Take laptop computers. According to the Underwriter’s Laboratory label on my laptop, it’s designed to operate at either 50 or 60 hertz and within a voltage ranging from 100 to 240 VAC. In other words, it should work just fine anywhere in the world, provided that I’ve got the right adapters to plug in. The television in my office, however, is specifically designed to run at 120 volts and 60 hertz.

Microwave ovens are also quite sensitive to frequency.
Any AC device with a motor is frequency sensitive, but it might still operate on more or less than the design frequency. For example, if you try using your 120-volt electric hair dryer rated for 60 hertz in Barbados, which has 115-volt/50-hertz power, you might not notice any difference in performance. The actual output from the dryer’s heating element will only be 15 percent to 20 percent less than it would be at home. The real problem lies within the windings for the dryer’s fan motor. Running on 50-hertz power, the internal current or amperage for the device has increased 15 to 20 percent. The higher current generates excessive heat within the device, which might cause the insulation of the internal wiring to deteriorate more quickly. Once the insulation starts to go, the equipment will fail, and an electrical fire becomes a real possibility.


Depending on the quality of the product and of the wiring insulation, these problems might crop up within a day or not for several months. The only way to be safe is to carefully design your onboard electrical system and to select the appropriate appliances. Clearly, the best appliances for cruising are those that operate fine at either 50 or 60 hertz. The next question is what voltage to use–120 volts, the household current in the United States, or the more universal 240 volts.

The Onboard Supply
Boat size will to some degree dictate how you tackle the global AC-power problem. Space is limited on boats under 40 feet, so one potential solution–installing an AC generator–might not be practical. Even if you have the space, a noisy generator probably won’t endear you to your slip neighbors. The generator is best left to supply AC needs when you’re away from a dock.

Let’s say that you’re wintering in a marina in Greece; how can your AC-power needs be satisfied there? Greece delivers 220-volt AC at 50 hertz. The best solution is to employ what’s known as an isolation transformer (see Figure 1, page 64). To maximize your global potential, the transformer should be specified to accept a 220- to 240-volt input. Remember: Eighty percent of the power distributed globally is delivered at these voltage potentials, and most major marinas here in the United States will offer this output at all or some of their docks (see “Battling the Demons of Shore Power,” page 70).


Using this higher voltage as a starting point offers many benefits. One plus is that the boat side of the transformer (the secondary side) can be split to supply both 240-volt and 120-volt AC. This will enable you to have some appliances running at 240 volts (as many air conditioners require) and some at 120 volts. Another advantage is that this configuration will accommodate the voltage supplied throughout most of the world. There’s a less obvious yet more important reason for using an isolation transformer as your onboard AC source: safety.

One of the common means of providing an earth ground system in countries outside the United States is what is known as the “terra-terra” method, whereby a shore-power system utilizes a grounding rod for all of the dock outlets (see Figure 2). The only electrical link between the dock ground and the AC source ground (the utility transformer) is via terra firma. This arrangement can be dubious at best because it really relies on the quality of the rods, their connections, and the actual conductivity of the earth at the marina. In theory, all of this is quite acceptable, but in practice, the quality of these grounds is often questionable.

In the United States, statistics tell us that the situation isn’t much better. Here, we rely on a third wire (green insulated) that’s supposed to connect each receptacle to the source of power, either at the utility transformer or at a nearby grounding bus, where it’s joined with the neutral conductor and grounded (see Figure 3). This also works just fine, so long as the green wire running from each receptacle has full continuity back to the utility transformer and is capable of handling the current it might carry.


No matter which of these grounding methods is used, any interruption of continuity in the ground circuit can pose a serious shock hazard to people on board. If the grounding system is deficient, a short circuit inside an AC-powered appliance could cause a fatal accident.

Assuming that all of your onboard AC circuits have adequate overcurrent protection (including the all-important dual-pole breakers at the primary side) and, where needed, are also protected with a ground fault circuit interrupt (GFCI), a properly installed isolation transformer will eliminate this danger. The last bonus of an isolation transformer is that it’s self-polarizing, so polarity at the dock is no longer an issue. No matter what polarity you have at the dockside service receptacle, the isolation transformer ensures that the polarity on board is always correct.

Amperage Issues
Now that we’ve decided that a 240-volt isolation transformer is the best way to deliver onboard shore power, we still have to resolve the frequency issue. It’s important to remember that a transformer will deliver AC on its secondary side at the same frequency as it’s receiving on its primary side. So let’s go back to our dock in Greece, where we’ve managed to hunt down the requisite shore-power adapter we’ll need to plug in. We’ve got our 240 volts, but the frequency is still only 50 hertz. What about all this equipment we want to use that requires 60 hertz? Tough luck. The bottom line is that any appliances that we try to run with this fairly simple setup must have a dual 50-hertz/60-hertz rating if we expect trouble-free operation wherever we roam. And frequency isn’t our only concern.
Dockside amperage ratings can also be an issue in some places. In the United States, dockside service is usually either 30 or 50 amps. In Europe, service amps are commonly available in 16- , 32- , and 63-amp configurations. A boat with high AC power loads (multiple refrigerators or air-conditioning units, for example) might require two or more shore-power inlets to deliver the amperage for peak loads. In this case, one of the boat’s service inlets is often dedicated to air-conditioning and refrigeration, and the second is dedicated to all other AC loads. Keep in mind that if this is the final decision, you’ll need a separate transformer and dual-pole breaker for each of your inlets.


For most global cruising boats in the 40- to 60-foot range with moderate to high AC power demands, a single isolation transformer rated for 50-amp service should be sufficient. You would want the transformer to accept input power of 240 volts at either 50 or 60 hertz and to be able to deliver both 120 and 240 volts on board. (Onboard frequency will be the same as the input frequency.) The optimal output would depend on your needs, but somewhere between 3.5 and 12 kilowatts should be sufficient. Such a setup would serve you fine in the United States and abroad, as long as all of your appliances can operate on either 50 or 60 hertz.

Bigger-Boat Options
But what about a Europe-bound boat that’s already equipped with some costly equipment that operates only on 60 hertz? How can we use these gadgets in our Greek marina? On slightly larger cruising boats, the global AC system can be designed with a bit more flexibility and a higher degree of sophistication. As we already mentioned, a 60-hertz AC generator is a viable solution, but one better suited for use at anchor or offshore. If your finances allow, a high-output DC-to-AC inverter, which converts DC voltage from the battery into AC voltage, would be more suitable for dockside living. High-output inverters are commonly combined with chargers, and a good charger will accept input voltage at either 50 or 60 hertz. With such a unit, replenishing your batteries dockside anywhere in the world is no problem.

Now all that’s needed is to choose an inverter with enough output to run the 60-hertz-only loads you’ll need in Europe. You could either dedicate some receptacles on board for your 60-hertz equipment, or you could hardwire the 60-hertz-only appliances to the inverter output circuitry. Now your North American appliances will run anywhere, regardless of the shore-power input frequency. Just be sure to select an inverter/charger that will operate with inputs of either 50 or 60 hertz and 240 volts. Today, inverters are commonly available with up to 5,000-watt continuous-output ratings, which is certainly adequate to run all small appliances, including refrigeration systems and an air conditioner with enough power to cool a 40- to 50-foot sailboat.

Conversion Quandary
Trying to reconcile assorted industry standards and wiring philosophies can lead to a variety of problems, even when an experienced technician is in charge. An electrician or builder who primarily equips U.S. boats for domestic U.S. use might make assumptions regarding shore power that won’t apply in Europe. Another common scenario is that a foreign boat built for 220-volt service winds up in the hands of an American owner who wants to install some 120-volt equipment or fully convert to the 120-volt system. I’ve seen more than a few of these boats “Americanized” by simply swapping out the electrical receptacles and shore-power inlet assemblies. This is an extremely dangerous move.

One of the compelling reasons to wire a boat for 220- to 240-volt service is that the required wire size, or gauge, is half of that needed to safely carry 120 volts. So to convert a 240-volt Euro-boat to accommodate your 120-volt appliances or plug into 120-volt shore power, you’ll likely need to upsize all the boat’s wiring. Conversely, if you’re transforming your existing American-built 120-volt boat for 220- to 240-volt service, the change won’t require any wholesale rewiring; the wires will actually be oversized–not a bad thing in the corrosive marine environment.

Whatever your own expectations may be, the ship’s AC power system is no place for guesswork or half measures. The risk of shock or fire is just too great. Once you have a good idea of your electrical needs and the equipment you want on board, let a qualified professional ensure that your boat’s wired for the world.

Ed Sherman is the electronics editor for Cruising World.