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Old 05-21-2012, 07:22 AM   #1
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Default 240V AC supply into a 120V boat?

Hey Yawl!

Our boat was built for 120 volt AC.

We're in a 240 volt country.

All of my appliances (TV, DVD, computer, battery charger, etc) will happily run on either voltage... any Hz.... everything except my water heater.

Is it safe to plug into 240 volt shore power?

Will it fry my water heater element?

I know the wires for a 120 volt system are bigger and can easily handle 240 volt supply.

Am I missing something?

Thanks,

Kirk
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Old 05-21-2012, 04:38 PM   #2
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1. Power=Current squared x Resistance,
2. Voltage= Current x Resistance,

Lots of things allow a range of voltage input and you'll see that in the rating on the item. However, resistive devices are a little different. In you water heater, resistance stays the same. But, Voltage is doubled. So, that means current just got doubled (per 2nd formula) and power increased by that NEW current squared. Right? So, you'll exceed the power rating of your hot water heater. You'll blow a fuse or melt something perhaps.
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Old 05-21-2012, 07:29 PM   #3
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As an experiment, take an old piece of 12v gear. Hook it up to a 24 volt source. (You can make one by serializing two 12v batteries.)

See what happens.

Really, be certain about your power source. Most of the 240v stuff in the world is 50 cycle rather than 60 cycle. So between the serious difference in energy and the difference in cycle rate, only the crudest electrical gear will withstand the difference.

I believe you can buy a converter. Even then be careful about what is output by the converter. Is it really sine wave, 120v AC? Maybe you can get someone with a silly-scope to take a look at it.

Another option: Run your house off batteries. Get a local battery charger. Run only that off your shore power. You can set up side by side charging systems so that you can switch between them as you travel.

Remember the KISS principle.
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Old 05-21-2012, 07:40 PM   #4
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I just re-read your post. I see that it is only your water heater you are concerned about. I read your post, walked away, came back and wrote later. Oh, the shame!

I don't know what you have as a heater, but perhaps you can buy a 240v element for it. I bet the manufacturer would take your email and give you details. Or tell you not to do it...

I remember that the heating elements are not the pricy part nor are they difficult to change. Unless you are going back and forth regularly, this might be a simple solution.
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Old 05-21-2012, 09:08 PM   #5
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A simple step down transformer will give you what you need, 240 to 110. They come in a variety of output ranges. A 1000va transformer will handle an 80 amp draw. So work out how many watts or amps you need and go from there. Step down transformers are available in Australia from Dick Smith or JayCar Electronics.
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Old 05-23-2012, 10:32 AM   #6
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Most electronic devices, as you have found out, work with either 220 or 110V because they have their own power supply that converts the AC voltage to the voltage needed by their electronics.

A small number of devices such as clocks are cycle dependent and if they are rated for 60 cycles (Hz) will not work properly with 50 even if they are provided with their rated voltage. The easiest solution for these devices on the boat is to get a Washington, DC to 110AC 60 Hz sine converter. This is not a very efficient usage but most cycle dependent devices are low power and the conversion inefficiency is not significant.

Resistive devices, like the water heater in your case, are not cycle dependent but are power hungry. The ideal solution here is to replace the heating element from a 110 to a 220 or 240 V, as Coyote sugests. But it may be difficult to find such a replacement element for your heater model. The second best is to buy a step down transformer as Lexx has suggested. Step up (110 to 220V) transformers are very inefficient but step down (220 to 110V) are less so.

Plugging in your 110V water heater to 240V will burn its fuse at best or at worse its heating element. It is not a solution.
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