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.
I don’t understand. Do I need a transformer to step the EU voltage down to US when bringing my US boat to the EU, then run AC loads with the inverter/charger on invert mode? Does the inverter/charger continue to charge the battery in this mode?
Blue Sea does not seem to have a transformer, so I must be wrong.
Rich,
It’s a bit more complicated than this! We have two issues: voltage and frequency (the rate at which the alternating waveform in AC power switches back and forth between positive and negative). You can change voltage easily with a transformer, but frequency is much harder.
Even with the necessary transformer, if you plug a EU inverter into U.S. shorepower, or a U.S. inverter into EU shorepower, you will destroy the inverter. So, you can’t plug in, in which case you can’t use the inverter as a battery charger. You will need a separate battery charger which accepts the available shorepower. Note that some can be plugged into both U.S. and EU shorepower, but check the small print.
You can still use the inverter to supply onboard AC power, but you will need that separate charger to recharge the batteries when shorepower is available.
Nigel
Thank you. I’m leaning toward a Victron IP43 charger, a separate Phoenix inverter to run AC loads on the boat. Not all of the loads on our boat can tolerate 50Hz.
Rich,
That’s an excellent way to go. Separate the shorepower/dockside charging side from the onboard inverter-based AC side. That way, so long as you have a suitable charger you can plug into shorepower anywhere.
Nigel
Question about selectiing OCP. 4/0 cable has ampacity of 445A. A 400A Class T fuse does not trip until the current exceeds 150% of rated, well above the ampacity. Does this get taken into account? Would a 300A fuse be required to protect the cable?
Rich,
The fuse rating is based on the conductor ampacity regardless of slow blow, fast blow, etc. However, the fact it takes so many more amps to blow the fuse than its nominal rating reinforces the point we make repeatedly that it is better to be conservative than it is to push the technical limits of the ABYC and ISO standards.
Nigel
Understand. That clears it up. Thanks.
Can I ask about paralleling LFPs?
Two 460Ah, 12.8V, and off the pos of ea, I have a Class T, going to a batt sw, then to the busbar that parallels them. On the neg side, cables to another busbar. How identical does the resistance need to be between pos and neg busbars?
Rich,
The key thing is to have the same conductor lengths, etc, on the individual sides (pos; neg) but they can be different between the two sides (pos; neg).
Nigel
Thank you very much. Installation constraints require that the main DC bus bar that parallels the batteries, and the battery switches, need to be 10 ft away from the batteries. I have 0AWG parallel cables from the batteries to the switch, with 110A Class T fuses, and 1AWG cable from the switches to the bus bar. The cable difference is due to engine compartment. The battery switch is rated at 350A.
Is it necessary to separate AC sources using a rotary selector switch? We have a Genset, and shore power, and an inverter. The tolerance on the genset frequency is much higher than for the inverter.
Rich,
Unless there is a frequency synchronization function (which the inverter may have but the generator almost certainly will not) if you do not isolate one power source before connecting another there likely will be a big ‘bang’ and substantial damage!! I once saw the crankshaft on a large diesel engine snapped in half by improper synchronization.
So yes, you absolutely need some kind of a switch (typically a rotary switch) which disconnects one AC power source from the boat’s circuits before connecting another power source. The one exception being many modern marine inverters (notably the Victron Multiplus and Quattro inverters) which have the capability of synchronizing their AC output with the frequency of a shorepower or generator AC input source, in which case they can operate in parallel with these sources.
Nigel
Planning on using a phoenix inverter. The charger is separate so we can use EU shore power.
Thanks.