12V or 120V For Refrigerator

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12V or 120V For Refrigerator

Postby RunnerDuck » Tue Aug 30, 2016 9:11 am

I have a Dometic CF-50 12 Volt / 120 Volt refrigerator and I'm wondering what would be the most efficient, running it off a a 12 V to 120 V converter or straight off of 12 V. The spec sheet doesn't seem to distinguish between the power consumption from either source, it's always 7 AMP.

I'm getting ready to wire up my TD and just not sure if I should wire it up to always run off the converter or 120 V when on shore power and 12 V when on the battery or wire it up to always run 120 V off of the converter.
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Re: 12V or 120V For Refrigerator

Postby Camp4Life » Tue Aug 30, 2016 9:40 am

Both voltages should keep the bevies cold. But for efficiency on the power usage side, I would say the 12v is more efficient simply because inverters aren't 100% efficient and they use more juice than they put out, there's always some loss. Even with nothing plugged in, an inverter uses power when it's on.
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Re: 12V or 120V For Refrigerator

Postby yrock87 » Tue Aug 30, 2016 10:01 am

If you spent the money on a 12v/120v fridge I would use the 12v as well. I don't know that fridge in particular, but I do know you will end up with conversation loss running a converter. That loss can be anywhere from 20-40% depending on the size of the converter, quality, and the draw of the load. ie. A small 300-400 watt load running though a 3000 watt inverter will not run as efficiently as the same load running off a smaller 600watt inverter.

You need to see what your Power consumption is and see if you battery can keep up, regardless of if you choose to run 12v direct or off the inverter for 120v. Watts x volts = amps. # of amps x hours of use gives you amp hours. A group 24 has about 40 usable amp hours (@ 50% state of charge, any lower and you damage the battery) which means that your 7 amp fridge will run your battery empty in abiut 5.7 hours. Assuming it is running with a 100% duty cycle (always running the compressor) You can probubly get more time out of it if you start with a pre-cooled fridge as it won't have to work to get itself cooler down.

Sorry for the long response. There are a LOT of variables to consider with battery power and you really need to look at your requirements and compare that to your system capacity.
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Re: 12V or 120V For Refrigerator

Postby H.A. » Tue Aug 30, 2016 11:36 am

T :beer: ed.
Last edited by H.A. on Wed Sep 14, 2016 4:54 pm, edited 1 time in total.
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Re: 12V or 120V For Refrigerator

Postby GuitarPhotog » Tue Aug 30, 2016 4:04 pm

RunnerDuck wrote:I have a Dometic CF-50 12 Volt / 120 Volt refrigerator and I'm wondering what would be the most efficient, running it off a a 12 V to 120 V converter or straight off of 12 V. The spec sheet doesn't seem to distinguish between the power consumption from either source, it's always 7 AMP.

I'm getting ready to wire up my TD and just not sure if I should wire it up to always run off the converter or 120 V when on shore power and 12 V when on the battery or wire it up to always run 120 V off of the converter.


Wire it direct to the 12 VDC. But. 7A at 12 VDC is not the same as 7A at 120 VAC. At 12 VDC it's 84 W and at 120 VAC it's 840 W, quite a large difference! I would assume it's 850 W, reasonable for a compressor fridge, which means it will draw 70A at 12 VDC. You will need quite large wires for that. But you would need even larger wires on the input to the inverter because of the inverter losses.

It would make sense to me to set up a switch so you can run it on 120 VAC when you have shore power and 12 VDC when you don't.

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Re: 12V or 120V For Refrigerator

Postby RunnerDuck » Tue Aug 30, 2016 5:12 pm

Thanks for the great comments, I now have a direction. I'm going to hard wire 120V to the refrigerator and have 12V as a back up.

Now the question is, how heavy of wire should I use for the 12V to the refrigerator over fifteen feet? I currently have 14 AWG.
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Re: 12V or 120V For Refrigerator

Postby GuitarPhotog » Tue Aug 30, 2016 5:37 pm

This calculator says AWG 4 cable for <5% voltage drop over 15 ft at 70A

http://www.powerstream.com/Wire_Size.htm

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Re: 12V or 120V For Refrigerator

Postby RunnerDuck » Tue Aug 30, 2016 5:48 pm

Damn, I doubt that's going to happen. I still don't understand the 70 Amps in your calculation.

I was thinking with 7 Amps 10 AWG would be plenty.

By the way I was wrong about the amps at 120V. Here is the technical data sheet.
CF-50 Techincal Data.jpg
CF-50 Techincal Data.jpg (45.11 KiB) Viewed 340 times
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Re: 12V or 120V For Refrigerator

Postby Philip » Tue Aug 30, 2016 6:27 pm

7.0 amp draw. You could use a 16 gage wire. I would run 12 myself just for the added length to get you to the back of the trailer.
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Re: 12V or 120V For Refrigerator

Postby H.A. » Tue Aug 30, 2016 7:00 pm

Now the question is, how heavy of wire should I use for the 12V to the refrigerator over fifteen feet? I currently have 14 AWG.


Offhand, your length of 30-ish feet, I say 10awg. A voltage drop calculator can give you more precise.
Make good effort to minimise voltagedrop.
These refrigerators typically sensitive to low voltage & auto shutdown at most inconvienient times.
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Re: 12V or 120V For Refrigerator

Postby RunnerDuck » Tue Aug 30, 2016 8:48 pm

Only 15 feet, not 30. I know, two wires but....
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Re: 12V or 120V For Refrigerator

Postby aggie79 » Wed Aug 31, 2016 9:16 am

GuitarPhotog wrote:It would make sense to me to set up a switch so you can run it on 120 VAC when you have shore power and 12 VDC when you don't.

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I believe that most of these refrigerators "auto-detect" dual power sources and "auto-switch" to 120 vac if that power source is present.
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Re: 12V or 120V For Refrigerator

Postby RunnerDuck » Wed Aug 31, 2016 9:41 am

That's right and this one does as well. I'll run it off of shore power but when I unplug it will automatically switch over to run on 12V from the battery.
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Re: 12V or 120V For Refrigerator

Postby RunnerDuck » Thu Sep 01, 2016 8:44 am

Here's a mock-up of my wiring panel. The 120V breaker box will feed the outlets in the TD and a separate one for the refrigerator which will run the refrigerator when on shore power but when I unplug will automatically switch over to 12V. I do believe this will work! Now I just have a butt load of wiring to do.

From the front of my tongue box:
wiring layout.jpg
wiring layout.jpg (60.67 KiB) Viewed 251 times


From the back:
wiring layout back.jpg
wiring layout back.jpg (50.15 KiB) Viewed 251 times


Let me know if you see anything that's glaringly bad as this is my first time doing this.

Thanks,

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Re: 12V or 120V For Refrigerator

Postby Camp4Life » Thu Sep 01, 2016 2:32 pm

FYI, that inverter looks a lot like a Black and Decker 400w inverter that I had, and it runs very hot. Mine ended up destroying my laptop's AC adapter and then quit working. Not sure if it overheated and malfunctioned or what, but I couldn't get it going again anyways and it ended up in the recycling depot.
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