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Old 12-03-2013, 06:21 AM   #1
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How many watts needed for a 31ft Land Yacht

My 1973 31ft Land Yacht is now safely here in the UK and is in almost completely original condition thanks to its careful one owner. I am planning to use a step down transformer from 240v to 120v to run it of UK mains. It has an electric/gas fridge and air conditioning - does any one know how many watts it will draw with everything on? I will get a bespoke transformer made which is 3 times the maximum draw to avoid it tripping or overheating.

Many thanks
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Old 12-03-2013, 06:29 AM   #2
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My 1973 31ft Land Yacht is now safely here in the UK and is in almost completely original condition thanks to its careful one owner. I am planning to use a step down transformer from 240v to 120v to run it of UK mains. It has an electric/gas fridge and air conditioning - does any one know how many watts it will draw with everything on? I will get a bespoke transformer made which is 3 times the maximum draw to avoid it tripping or overheating.

Many thanks
You can tell by looking at the shore power cable. Three-prong is 30 amps single-phase 120vAC. Four-prong is 50 amps three-phase 120vAC. That gives you a target value to aim for.

It may not be quite that simple, though. How many cycles does the UK power grid provide? My memory wants to say 50 Hertz, but my memory lies to me incessantly. US appliances are designed for 60 Hertz.
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Old 12-03-2013, 06:38 AM   #3
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In the US you would need around 5,000 watts at 120 volts AC to power everything effectively. Protagonist is correct about the cycles too, in the US it is 60 cycles. From my dim memory I believe that motors and compressors are the most likely to be affected by the difference in cycles, some will work on different cycles others won't. Need to talk to a good electrician and check out ALL of the data plates. I am sure someone over the pond there has figured it all out before, there are quite a few Airstreams over there now.

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Old 12-03-2013, 06:55 AM   #4
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In the US you would need around 5,000 watts at 120 volts AC to power everything effectively.
Thanks. I put everything in my response BUT the watts, which is what the OP specifically asked for. Good thing you were more paying more attention than I was…
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Old 12-03-2013, 07:30 AM   #5
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You can tell by looking at the shore power cable. Three-prong is 30 amps single-phase 120vAC. Four-prong is 50 amps three-phase 120vAC. That gives you a target value to aim for.
Actually, this has been discussed before almost to extreme, but RV's in the US with a four-prong plug are 50 amps 240 VAC split phase, not three-phase.
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Old 12-03-2013, 07:34 AM   #6
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Actually, this has been discussed before almost to extreme, but RV's in the US with a four-prong plug are 50 amps 240 VAC split phase, not three-phase.
Thank you. Senioritis strikes AGAIN! Darn it!
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Old 12-03-2013, 08:38 AM   #7
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All '73 Airstreams came with 30 amp, 120 volt service. Watts = volts x amps. So 120 x 30 = 3600 watts.

The refrigerator will take about 300 watts, but does not have a motor in it, so it does not care about frequency, which in the UK is 50 vs. US 60. The Air Conditioner motors were designed for 60 hz and so may not run well or at all, but they are the largest load. The original Univolt converter/charger (which everyone will recommend be replaced anyway) has a large special transformer in it, which may or may not work on 50 hz.
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Old 12-03-2013, 09:46 AM   #8
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Thanks for the help, I have temporarily set it up with a 110v transformer to see what works. The battery charger makes a constant hum which may improve when 120v is put in or it may be because it is 50hz in the UK and not 60. The fridge and AC are not working which they did when in the US but I think 110v is too low for them?
Many people over here swap to 240v but I want to keep it as it was built with all the original appliances.
I should probably call an electrician!!
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Old 12-03-2013, 10:08 AM   #9
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The fridge has a heating element in it, I don't think 50 vs 60 cycles will matter with that. 110 volt should be fine with the fridge also, unless it has been replaced with a new one with a control board. Then all bets are off using 50 cycle power instead of 60 cycle power.
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Old 12-03-2013, 10:20 AM   #10
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The control boards us 12 volt DC to operate, so there should be no problems with a newer refrigerator.
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