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Old 09-25-2024, 02:42 PM   #1
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3 Way Fridge 12 Volt Wiring

I really appreciate this forum and all the helpful people here.

I'm installing my new Boondocker power panel and converter. I'm reading about wiring the Dometic RM360 Fridge. I need to know how big of a fuse to put it on.

It tells me that it needs two separate wires, one for positive and one for negative back to power source i.e. the power panel. Right now the trailer was only wired with one positive wire and grounded to the frame. So do I really need to run a wire for the ground?

Also, the fridge manual says "do not operate the refrigerator on 12 volt when the vehicle is parked. You will run out of battery in a rather short time."

So with the trailer battery and vehicle battery basically hooked together when towing, how much time do I have before this happens? The manual tells me that it is a 10 amp draw. Is it a constant 10 amps? Or does it cycle like my kitchen fridge does?
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Old 09-25-2024, 03:01 PM   #2
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Since it's a absorbsion fridge, it will most likely run full time.
Driving, you won't run down your batteries, 10 amps is easly handled thru a good charge line.

Parked, if drawing 10 amps, that's 10 AHs. Park for 3 hours and it uses 30 AHs. I don't know how many AHs your battery is but if your parked longer then that, switch it to propane.

Chassis ground can be unreliable, run a 2 conductor 12 gauge cord to the battery with a 15 amp fuse, near it.
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Old 09-25-2024, 03:01 PM   #3
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I'm not familiar with your power panel or exact model of fridge, but here are some general thoughts.

3-way fridges, in my experience, are iffy at best on the 12V setting. They don't cool very well, and use a lot of power.

You should be able to ground through the frame if the battery is. I don't like to, because it seems the grounding connections always get crappy.

Is there a manual for the fridge? If so, they should give installation instructions on wire gauge and fuse. If it draws 10A, I would probably use a 15A fuse, but make sure you have the wire gauge to support it.

A 10A draw will kill your battery quickly. Like a few hours. If I had to guess, it would be a constant draw when the fridge is on. Again, your manual may mention if there is a thermostat. But the controls on these fridges aren't very sophisticated.

In theory, if your truck and trailer wiring are correct, you can keep the camper battery somewhat charged while the TV is running. But it almost never works that way in the real world, especially with the big load of that fridge. You would need dedicated huge wire from the TV. Which is why many have a DC-DC charger: to give the extra voltage boost necessary to overcome wire resistance.

Edit: Oops, just saw Twinboat beat me to the reply with similar info.
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Old 09-25-2024, 03:03 PM   #4
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Quote:
Originally Posted by LaneG View Post
I really appreciate this forum and all the helpful people here.

I'm installing my new Boondocker power panel and converter. I'm reading about wiring the Dometic RM360 Fridge. I need to know how big of a fuse to put it on. Fuse for the wire size, if you choose larger wire for voltage drop fuse at no more than 20, although for your 15 amp load 15 would work so I would do that

It tells me that it needs two separate wires, one for positive and one for negative back to power source i.e. the power panel. Right now the trailer was only wired with one positive wire and grounded to the frame. So do I really need to run a wire for the ground? It's usually recommended, also as I mentioned if the fridge is not real close to the power source it's best to upsize the wires to 8's or even 6's for less voltage drop. You'll get a substantial voltage drop on a 10 amp resistive load on a 12 volt system.

Also, the fridge manual says "do not operate the refrigerator on 12 volt when the vehicle is parked. You will run out of battery in a rather short time."

So with the trailer battery and vehicle battery basically hooked together when towing, how much time do I have before this happens? The manual tells me that it is a 10 amp draw. Is it a constant 10 amps? Or does it cycle like my kitchen fridge does? Generally the 12 volt element is for travel use only and does not cycle on and off, it stays on the whole time. You shouldn't use it when parked because 10 amps at 12 volts will eat up your battery quick. With a 3 way you can switch to gas when parked, many do it automatically. With a trailer you may even lose ground on the battery unless the 7 way can provide more than 10 amps to the trailer
On my 3 way there was about 20 feet of #12 wire and I was only getting around 11 volts at the element with the engine running. I changed to #8 wire directly off the battery about 8 feet total and my voltage went to 13 gaining me something like 35 watts more power to the heating element.

(I just looked at your instructions and they recommend an ignition switched relay setup to cut the 12 volts when the ignition is switched off)

edit: I see I am a slow typer. Oh well 3 replies better than none I guess
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Old 09-25-2024, 03:59 PM   #5
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several great replies above.

I will echo the thought that the fridge works poorly at best on 12V electric.....in fact, most absorption fridges are 2 way and have been for many years (propane and 120V for when plugged in). As the others have said, running it on 12V off battery is going to kill most batteries in short order

Personally, I wouldn't run it on 12V and I'd simply run it on propane when not hooked to shore power.....my Norcold has an auto switchover and I run it continuously from April to October - 120V at the house and propane when driving or camping.


2 cents,
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Old 09-25-2024, 04:12 PM   #6
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Quote:
Originally Posted by twinboat View Post
Since it's a absorbsion fridge, it will most likely run full time.
Driving, you won't run down your batteries, 10 amps is easily handled thru a good charge line.

Parked, if drawing 10 amps, that's 10 AHs. Park for 3 hours and it uses 30 AHs. I don't know how many AHs your battery is but if your parked longer then that, switch it to propane.

Chassis ground can be unreliable, run a 2 conductor 12 gauge cord to the battery with a 15 amp fuse, near it.
Thanks for that. The more I read the more I feel like I should run two wires to it. So the battery should power the fridge for a few hours anyhow. Thanks again.
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Old 09-25-2024, 04:16 PM   #7
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Originally Posted by db4570 View Post
I'm not familiar with your power panel or exact model of fridge, but here are some general thoughts.

3-way fridges, in my experience, are iffy at best on the 12V setting. They don't cool very well, and use a lot of power.

You should be able to ground through the frame if the battery is. I don't like to, because it seems the grounding connections always get crappy.

Is there a manual for the fridge? If so, they should give installation instructions on wire gauge and fuse. If it draws 10A, I would probably use a 15A fuse, but make sure you have the wire gauge to support it.

A 10A draw will kill your battery quickly. Like a few hours. If I had to guess, it would be a constant draw when the fridge is on. Again, your manual may mention if there is a thermostat. But the controls on these fridges aren't very sophisticated.

In theory, if your truck and trailer wiring are correct, you can keep the camper battery somewhat charged while the TV is running. But it almost never works that way in the real world, especially with the big load of that fridge. You would need dedicated huge wire from the TV. Which is why many have a DC-DC charger: to give the extra voltage boost necessary to overcome wire resistance.

Edit: Oops, just saw Twinboat beat me to the reply with similar info.
Thanks for all you input. Yes I plan on running two wires, probably 10 or 8 gauge to be safe.

The fridge manual says to use 10 or 12 guage wire. But I think I'll over do it.
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Old 09-25-2024, 04:24 PM   #8
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Originally Posted by bigb56 View Post
On my 3 way there was about 20 feet of #12 wire and I was only getting around 11 volts at the element with the engine running. I changed to #8 wire directly off the battery about 8 feet total and my voltage went to 13 gaining me something like 35 watts more power to the heating element.

(I just looked at your instructions and they recommend an ignition switched relay setup to cut the 12 volts when the ignition is switched off)

edit: I see I am a slow typer. Oh well 3 replies better than none I guess
bigb56, I can appreciate why you're saying that about the bigger wire. The manual says to use 10 or 12 gauge, but I'm going to use 10 or 8 depending on what I've got on hand. May have to buy some. And I'll use a 15 amp fuse too. Good suggestion.

Yes I see what you're talking about with the ignition switched relay. I totally bypassed that in my initial reading. I'll think about putting one in. Not sure if I really need it though. I can't imagine being parked for long enough to warrant it before I switch it over to propane.
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Old 09-25-2024, 04:43 PM   #9
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Originally Posted by Dave Pelletier View Post
several great replies above.

I will echo the thought that the fridge works poorly at best on 12V electric.....in fact, most absorption fridges are 2 way and have been for many years (propane and 120V for when plugged in). As the others have said, running it on 12V off battery is going to kill most batteries in short order

Personally, I wouldn't run it on 12V and I'd simply run it on propane when not hooked to shore power.....my Norcold has an auto switchover and I run it continuously from April to October - 120V at the house and propane when driving or camping.

2 cents,
Dave

Yes I've realized that the 12v power for the fridge isn't the best. I'm planning on only using it for actual travelling. Your idea of travelling with propane is interesting. I hadn't thought of that. When I get this thing road worthy I'll have to try it out. I just thought that the air moving would blow out the flame. A test is in order for sure.

It would be nice to replace it with a newer one that automatically switched, but that isn't in the cards right now. This whole rig is el-cheapo as I can get-to!

So far I'm under 10k for this rig and just about done.


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Old 09-25-2024, 04:52 PM   #10
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Originally Posted by LaneG View Post
Yes I've realized that the 12v power for the fridge isn't the best. I'm planning on only using it for actual travelling. Your idea of travelling with propane is interesting. I hadn't thought of that. When I get this thing road worthy I'll have to try it out. I just thought that the air moving would blow out the flame. A test is in order for sure.

It would be nice to replace it with a newer one that automatically switched, but that isn't in the cards right now. This whole rig is el-cheapo as I can get-to!

So far I'm under 10k for this rig and just about done.


Attachment 428394

Nice work; my son is restoring a 1973 15' Scamp currently. I see now why you have a 3 way fridge. Many of us run with propane fridges on whilst traveling and have been without issue for many, many years. You need to be aware that you should shut it off before refueling, on ferries and in some places, when going through tunnels. Sometimes they blow out but we just relight when that happens. Our latest two trailers would blow out sometimes but I fabricated a wind screen for the burner and it hasn't blown out since.



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Old 09-25-2024, 05:09 PM   #11
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My previous 2 trailers had 2 way fridges (120 & propane), and I always traveled with them running off propane. The only concern I had was fueling up I made sure the trailer was as far from the pump as possible.
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Old 09-25-2024, 06:36 PM   #12
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Originally Posted by LaneG View Post
bigb56, I can appreciate why you're saying that about the bigger wire. The manual says to use 10 or 12 gauge, but I'm going to use 10 or 8 depending on what I've got on hand. May have to buy some. And I'll use a 15 amp fuse too. Good suggestion.

Yes I see what you're talking about with the ignition switched relay. I totally bypassed that in my initial reading. I'll think about putting one in. Not sure if I really need it though. I can't imagine being parked for long enough to warrant it before I switch it over to propane.
At least with only 120 watts there won't be much voltage drop if you follow the chart in the manual.
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Old 09-25-2024, 06:46 PM   #13
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Originally Posted by Brunnie View Post
My previous 2 trailers had 2 way fridges (120 & propane), and I always traveled with them running off propane. The only concern I had was fueling up I made sure the trailer was as far from the pump as possible.
Good point, thanks for that.
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Old 09-25-2024, 07:31 PM   #14
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Actually, I think the electrical bottleneck, so to speak, is the truck wiring to the trailer plug, more than the wiring inside the camper itself.

It would be interesting to know what wire gauge is typically used for most camper fridges. I doubt it's bigger than 12.

On a previous TV and camper I had a shop run a heavier gauge wire to the 7-pin to try to accomplish what the OP wants. I forget the gauge. Maybe 12. I never really ran it through its paces before I changed TV and camper.

With my current setup, I have an 8 amp DC-DC charger which is a perfect solution. I ran a heavy-gauge wire (10 ga., overkill) direct from the TV battery (with a fuse) to the back of the TV with enough extra length to reach the trailer tongue. That's where I installed the DC-DC charger, protected from the weather under the propane tank cover.

The purpose of the DC-DC is to raise the voltage to the camper battery, and also to limit the amps drawn from the TV system. We all think in terms of 12V, but a battery needs 13 or 14 volts to charge the battery. (I forget the exact voltage, and it depends on the battery chemistry.)

The voltage to a regular 7-pin, after traveling 25' or so of small gauge wire in the TV, can be too low to charge a battery much, if at all. The DC-DC is really a perfect solution. It doesn't care if its input voltage is a little low, and it boosts its output up to perfect battery-charging voltage.

I bought the Powerwerx 8 amp DC-DC for $70. (A great outfit, BTW, that I buy all kinds of DC electrical stuff from). It does this task being discussed perfectly for me.

I use it when towing for a full day. My battery might go down 60 ah or so in a day using the fridge and other stuff, so at 8 amps for 8 hours of towing I'm keeping up.

OP, with your 10 amp fridge draw, might need a bigger one to keep up.
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