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Old 04-07-2016, 13:39   #1
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Correct wattage size stepdown transformer 220v -> 110v

Fridge specs:
"Norcold TEK II Dual Voltage Ice Box Conversion System, 120V AC and 12/24V DC, Model: SCQT-4407 (40W)" (copied from Owner's Guide)

I need to buy a stepdown transformer (not an inverter) to power my new Norcold fridge from 220V shore power.

I would appreciate to know what wattage transformer would be appropriate please.

Local shop has 100W, 200W, 300W and 500W (and higher) transformers, and I reckon one of those should be ok. The fridge specs say it is 40W.

Am I correct to assume that this is same case as with inverters, wherein one should get a transformer 3 or 4 times the wattage of the appliance powered, due to higher start-up draw? And I guess one should not run the transformer at or near it's max wattage?

Therefore,
100W transformer is too small?
200W transformer maybe ok?
300W better?
500W too big?

As this transformer will be used only from shorepower, the total amp draw is of no significance. (at sea the fridge is run from 12V DC)

At the moment I can not think of any other use for this transformer than to power the Norcold fridge when in marina and at shore power. I do not have any other 110V appliances or power tools so I can not think of a reason to "buy larger capacity for future needs". Any AC appliances/tools are and will be 220V.

However, what I would prefer is a safe and reliable transformer generating as little heat as possible.
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Old 05-07-2016, 02:07   #2
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Re: Correct wattage size stepdown transformer 220v -> 110v

why not just let it run off 12v? let the battery charger do it's thing.

are you 220v 50hz or 60z? if you are trying to feed it 120v 50hz from a step down it may not like that.
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Old 05-07-2016, 03:32   #3
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Re: Correct wattage size stepdown transformer 220v -> 110v

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Originally Posted by smac999 View Post
why not just let it run off 12v? let the battery charger do it's thing.

are you 220v 50hz or 60z? if you are trying to feed it 120v 50hz from a step down it may not like that.
Spot on smac. This morning I had no replies so I did some more research and found out that regarding solely the wattage "be sure to buy a converter with a wattage rating that is two to three times higher than the device you plan to operate in order to safely convert." (to answer my original question)

How Do I Choose a Voltage Converter?

Then, however, I also found out that transformers do NOT convert the frequency cycle (Hz). "All voltage converters only convert the voltage and not the cycle, however most appliances and electronics will function properly with them." (but I don't think I want to try my luck..)

https://voltage-converter-transforme...pages/faq-html

So, like you said, with a stepdown transformer my output would be 120V at 50Hz, NOT at 60Hz. Therefore, I guess I better do like you suggest and forget about this stepdown transformer plan and just run the fridge from the batteries and the battery charger.
I can see the convenience factor there in USA though when you could just plug in your boat fridge and forget about the batteries.
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Old 05-07-2016, 13:15   #4
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Re: Correct wattage size stepdown transformer 220v -> 110v

Does your battery charger handle 220v input? If not you'd need the stepdown for it. Also, many 120v60hz devices work fine, if a bit inefficiently, on 50hz.
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Old 05-07-2016, 13:33   #5
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Re: Correct wattage size stepdown transformer 220v -> 110v

Quote:
Originally Posted by ErikFinn View Post
I can see the convenience factor there in USA though when you could just plug in your boat fridge and forget about the batteries.
Glad smac helped you think this through, but not all fridges are 12V & 120V. So...if they are only 12V, even those of us in the 120V world use the 12V, right?
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Old 05-07-2016, 13:35   #6
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Re: Correct wattage size stepdown transformer 220v -> 110v

Quote:
Originally Posted by ErikFinn View Post
Fridge specs:
"Norcold TEK II Dual Voltage Ice Box Conversion System, 120V AC and 12/24V DC, Model: SCQT-4407 (40W)" (copied from Owner's Guide)

I need to buy a stepdown transformer (not an inverter) to power my new Norcold fridge from 220V shore power.

I would appreciate to know what wattage transformer would be appropriate please.

Local shop has 100W, 200W, 300W and 500W (and higher) transformers, and I reckon one of those should be ok. The fridge specs say it is 40W.

Am I correct to assume that this is same case as with inverters, wherein one should get a transformer 3 or 4 times the wattage of the appliance powered, due to higher start-up draw? And I guess one should not run the transformer at or near it's max wattage?

Therefore,
100W transformer is too small?
200W transformer maybe ok?
300W better?
500W too big?

As this transformer will be used only from shorepower, the total amp draw is of no significance. (at sea the fridge is run from 12V DC)

At the moment I can not think of any other use for this transformer than to power the Norcold fridge when in marina and at shore power. I do not have any other 110V appliances or power tools so I can not think of a reason to "buy larger capacity for future needs". Any AC appliances/tools are and will be 220V.

However, what I would prefer is a safe and reliable transformer generating as little heat as possible.
Transformers can handle the in-rush at startup, the 100w transformer would work. Any quality transformer will run at capacity 24/7, a bigger one may be less noisy.
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Old 05-07-2016, 16:35   #7
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Re: Correct wattage size stepdown transformer 220v -> 110v

Many devices designed for 60 hz do run fine on 50hz. Resistance heaters, step down dc transformers and the like. Synchronous AC motors designed to work on 50 hz can be safely run at 60 hz, They will just spin a bit faster but the motor will not overheat. However, "non-inverter duty" 60hz motors (generally all the motors on boats less then about 100 feet) will run hotter if run at 50 hz. That may cause the motor to trip it's high limit safety on high winding temperature.

Safest thing to do, when you already have a 120V 60hz / 12V fridge is to always run the fridge at 12V with a boat wired for 220V 50hz .
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