April 8

Understanding Marine Electrics: Converting a 230V SHORE POWER system to 120V

In the USA, when you plug into an outlet in your house or on a boat you are plugging into a 120-volt circuit. In Europe, when you do the same thing, you are plugging into a 230-volt circuit.

Equal Power, Different Amps

Let’s say we are plugging in a microwave with a power rating of 1000 watts (W). Watts are a function of volts multiplied by amps. We can turn this around to say the amps required to power the microwave are determined by dividing the supply voltage into its watt rating.

You are probably wondering where I am going with this! Well, that 1,000-watt microwave is going to pull 1,000W/120V = 8.33 amps in the USA whereas in Europe what is effectively the same microwave will pull 1,000W/230V = 4.35 amps. Basically, half the amps.

The AC wiring on US-build boats (black, white, green) typically has a larger cross-sectional area than the AC wiring on European-build boats (blue, brown green-yellow)

When we wire a circuit the size of the conductors is in part determined by the maximum number of amps that will flow through the circuit. For a given power level, a U.S. boat requires conductors that are considerably larger than a European boat wired for the same power level.

European-built and American-built boats are, in fact, wired for very similar power levels. The most common U.S. shorepower connection is for a 30-amp circuit @ 120V; 30A x 120V =3,600W. The most common European connection is for a 16-amp circuit @ 230V; 16A x 230V =3,680W. The two boats will be able to power devices that require the same level of power, but the European boat can do it with considerably smaller conductors.

Using our Wire Sizer App for Cable Sizing

To make sure that the conductors are sized appropriately for the lower voltage, use our Wire Sizer Tool. This free app is considered the best in the marketplace, and it helps boat owners ensure that they have the right-sized conductors for any given task.

If we now bring this European boat with a 16A shorepower circuit to the U.S. and plug it into a 30A shorepower circuit and run this at the full rated 3,600 watts, the onboard wiring will be substantially undersized. It is likely to melt down and start a fire.

But if we take the U.S. boat with a 30A shorepower circuit to Europe and plug it into a 16A shorepower circuit and run this at the full rated 3,680 watts there will be no problem. The wiring is oversized but that’s OK.

You cannot bring a European-wired boat to the U.S. and expect to plug it in unless the boatbuilder specifically upsizes the wiring for the lower U.S. voltage.

You can, on the other hand, take a U.S. wired boat to Europe and plug it in without any overheating issues. However, you will destroy the onboard equipment with the higher voltage unless you step the voltage down to 120 volts with a transformer.

Frequency Issues

Even then you will run into another entirely different problem: frequency incompatibility. Shorepower is known as alternating current, or AC for short. The voltage and amperage cycle back and forth from positive to negative in relation to the earth. The rate at which they cycle (known as Hertz, or Hz) differs between Europe and North America. In Europe it is 50 times a second and in North America 60 times a second.

Let’s say we have taken our U.S wired boat to Europe and we have a 230V to 120V transformer, so the conductors are adequately sized, and we have the correct voltage for our U.S. equipment. There will still be a frequency mismatch. Some of our U.S. equipment won’t care, some of it will be designed for both frequencies, some of it will run but not well, and some of it will be wrecked!

This all gets especially complicated in the Caribbean because some ex-British Islands have North American 120V/60Hz power (e.g., the British Virgin Islands), and some British (i.e., European) 230V/50Hz power (e.g., Bequia). It is really important to check before plugging in. And then, of course, there are a wide variety of outlet configurations and plugs, but that’s a whole additional story!

The Universal AC Setup

There is a simple and economical workaround to these issues for both the European and U.S. boat. If all the AC equipment on the boat is powered by a DC-to-AC inverter, the only piece of equipment that needs to be connected to shorepower is a battery charger. The battery charger will feed the DC input side of the inverter, and the inverter will power the AC circuits. You can buy marine battery chargers that will accept as the AC input anything from 90V to 270V, and anything from 45Hz to 65Hz - i.e., a ‘universal’ AC input. So long as your shorepower cord is sized to handle the lower U.S. voltage, you can plug in anywhere in the world and use all the onboard AC equipment. I have done this for years. It works like a charm.

If you want to learn everything you need to know about AC shorepower, you should have a look at our Advanced Marine Electrics course, containing an entire module on this important topic.

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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