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Old 04-20-2019, 09:49 AM   #1
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Wire Gauge Significance in 4-pin SUV to 7-pin A-Frame Connection

(2018 Flagstaff T21TBHW; 2018 Kia Sorento 6cyl,AWD)

I had an auto-electric shop wire up my 4-pin Kia to 7-pin to connect to the a-frame. They used 12-gauge wire for the power wire to the car battery, I found. Is this enough to trickle-charge the a-frame battery and run the fridge while driving or should they have used 10-gauge? Is the voltage loss that significant, ya think? Thx
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Old 04-20-2019, 10:04 AM   #2
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What refrigerator? Guessing an absorption LP/115vAC/12vDC version. Running on 12v provides the least cold. Run it on LP on the road and don't worry about the 12v power needs. Tow vehicle alternator voltage regulator is only intended to maintain the vehicle battery and will merely trickle change the trailer battery especially with the voltage drop in the long wire between the alternator and trailer battery.

10 gauge is better. Are all 7 poles in the adapter powered? If they are and are not controlled by a relay to the ignition the 12v refrigerator and other trailer loads will suck the tow vehicle battery dry when the engine isn't running.

-- Chuck
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Old 04-20-2019, 11:06 AM   #3
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Originally Posted by Chuck_S View Post
What refrigerator? Guessing an absorption LP/115vAC/12vDC version. Running on 12v provides the least cold. Run it on LP on the road and don't worry about the 12v power needs. Tow vehicle alternator voltage regulator is only intended to maintain the vehicle battery and will merely trickle change the trailer battery especially with the voltage drop in the long wire between the alternator and trailer battery.

10 gauge is better. Are all 7 poles in the adapter powered? If they are and are not controlled by a relay to the ignition the 12v refrigerator and other trailer loads will suck the tow vehicle battery dry when the engine isn't running.
-- Chuck
Chuck, the stock 4-pin harness is connected to a standard 7-pin adapter. The remaining wires on the 7-pin are as follows:

The black (power wire) is spliced to a 12-gauge wire which goes under the vehicle, through a 40-amp inline circuit-breaker to the battery.

The yellow wire is for 12v power.

The white (ground) wire is grounded to the chassis.

The blue (brake) wire is unconnected (I use the Curt Echo wireless brake controller which doesn't require the blue wire connected; it senses brake action through the brake-lights, as I understand it).

No ignition relay, but I can't see how the trailer could drain the vehicle battery via the controller (which fits between the vehicle and trailer 7-pin connectors and is powered by the vehicle only; so when the vehicle is off, the controller is off). Thanks.
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Old 04-20-2019, 02:38 PM   #4
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Assuming a maximum of 20 feet from the vehicle battery to the trailer connection would allow 10 Amperes using 12 AWG wire. If your run is shorter (likely) then that value increases based on the length so you are OK. A 10 Ampere current availability is more than enough to trickle charge your trailer battery, probably ten times as much needed if the battery has a fresh charge before you leave. You'll have to check on your refrigerator ratings to see what it draws on 12V operation to factor that in but I would recommend using gas operation instead as other posters have stated. The refrigerator takes a very tiny amount of gas to run while traveling and works much better than 12V.
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Old 04-20-2019, 03:18 PM   #5
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Do you have a standard or a High Wall A-frame? The fridges are very different. On a standard A-frame the fridge does not autolight. And the low height between bottom and top vents reduces the chimney effect. All of which means it is very likely that the fridge flame will be blown out by the wind as you are driving on a standard height A-frame. Which means you have to run on DC while towing or do without the fridge. That's what happened to me with my A122.

I forget the exact wattage but both the AC and DC coils are the same wattage. Because of the extra air flow the fridge cools better at highway speeds.

Unless you install a fan - I used a 12V computer case fan tie wrapped to the top vent - cooling is very hit or miss when not being towed at highway speeds. Using a battery-powered wireless outside thermometer. ($10 at Wal-Mart) I found that the fridge temp was down in the 20s after a few hours towing at highway speeds. I would then turn the fridge off until temps were back to 40 to give my TV (minivan) a chance to top off the batteries.

My new High Wall A-frame has a very different fridge that autolight and will happily run on propane while towing.

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Old 04-20-2019, 03:55 PM   #6
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I was not aware of the fact that some refrigerators do not have the autolight feature. This is my fourth trailer (but first A frame) and they all had autolighting. Mine is a high-wall so no worries but it is good to know that some do not. Something to watch for if you ever need to replace one.

If the AC and DC coil both use the same power rating in Watts then you can easily calculate the current difference required. For practical purposes since 12V is one tenth of 120V then the current will be ten times that at 120V for 12V. For example, 120 Watts at 120 V would be 1A and therefore the same power would be 10A at 12V. (Power is equal to the Voltage times the Current as a simple calculation...don't get me started on resistance vs impedance and phase angle/power factor because I can talk your ear off and bore you to death. Just ask my wife. You don't really want the actual calculations but this simple approximation is more than adequate for your use here.)
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Old 04-20-2019, 05:22 PM   #7
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Originally Posted by pgandw View Post
Do you have a standard or a High Wall A-frame? The fridges are very different. On a standard A-frame the fridge does not autolight. And the low height between bottom and top vents reduces the chimney effect. All of which means it is very likely that the fridge flame will be blown out by the wind as you are driving on a standard height A-frame. Which means you have to run on DC while towing or do without the fridge. That's what happened to me with my A122.

I forget the exact wattage but both the AC and DC coils are the same wattage. Because of the extra air flow the fridge cools better at highway speeds.

Unless you install a fan - I used a 12V computer case fan tie wrapped to the top vent - cooling is very hit or miss when not being towed at highway speeds. Using a battery-powered wireless outside thermometer. ($10 at Wal-Mart) I found that the fridge temp was down in the 20s after a few hours towing at highway speeds. I would then turn the fridge off until temps were back to 40 to give my TV (minivan) a chance to top off the batteries.

My new High Wall A-frame has a very different fridge that autolight and will happily run on propane while towing.

Fred W
Prev Rockwood A122 A-frame
Now Flagstaff T21TBHW A-frame
2008 Hyundai Entourage minivan
I have the same model HW as you (2018) so the same fridge, I assume -- Dometic 2354. It's nice to know I can run it on propane w/o the flame blowing out. I will get one of those thermometers -- great idea -- to see exactly what's going on; I'm very curious now. Thanks!
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Old 04-20-2019, 05:31 PM   #8
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Yes, that is the same fridge as in my 2019.
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Old 04-21-2019, 01:59 PM   #9
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Kia?

Forget about the wire, just what are you towing with a Kia that requires a 7 pin connector?
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Old 04-21-2019, 02:37 PM   #10
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Forget about the wire, just what are you towing with a Kia that requires a 7 pin connector?
2,600 lb Flagstaff T21TBHW. Has brakes and so needs a 7-pin.
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Old 04-21-2019, 03:28 PM   #11
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Do you have a wire carrying current to the trailer brakes? You mentioned one of the new wires is a ground; the original 4 pin connector already has a ground wire. I suggest using a circuit tester and verifying all 7 pins on the new connector.
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Old 04-21-2019, 03:53 PM   #12
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Do you have a wire carrying current to the trailer brakes? You mentioned one of the new wires is a ground; the original 4 pin connector already has a ground wire. I suggest using a circuit tester and verifying all 7 pins on the new connector.
We're all good; tested by Dakota Battery & Electric, the the auto-electric shop I took it to. The wireless brake controller (Curt Echo, like the better known Tekonsha Prodigy) does not need a wire to the trailer brakes; it gets its signal from the wire to the brake lights.

The Echo, shaped like a hotdog, sits between the TV and Trailer 7-pin plugs. It's controlled by your smart phone via an app that you download. You set up whatever profile you want from the app, then the Echo uses that as your default, and then you don't need the smart phone anymore, unless of course you want to change the default at a later time.

Funny, but the shop guys had never seen one before and were intrigued by it, so they hooked up a trailer to watch it work. They were happy it did.
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Old 04-22-2019, 11:37 AM   #13
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I owned a Lance slide-in pickup camper that had a 3-way fridge. Even with heavy 6-gauge wires, my truck wouldn’t keep the camper’s battery charged when the fridge was on DC. Although the wire and alternator had the capacity to fully supply the fridge on DC, as mentioned by Chuck S, the tow vehicle’s battery influences the alternator’s output and wouldn’t allow it to provide enough power for the fridge when on DC. Since a refrigerator doesn’t consume much LP, I simply left it on LP when traveling.

Although it would be a hassle, I recommend you stop a couple of hours into your trip and check the trailer’s battery voltage to see how well your tow vehicle is keeping it charged. On a long trip, I’d check it every few hours. If you have a problem, switch to LP if you have that option. If the trailer battery keeps charged, then you know you’re okay and can rely on your setup in the future without the fuss.

If your tow vehicle doesn’t disconnect the 12 VDC power going to the trailer when the ignition is off, then I recommend you disconnect your trailer from your tow vehicle when camping. Anything in your trailer that draws power will also deplete your tow vehicle’s battery. Check pin 4 on your tow vehicle’s 7-pin connector when running and when shut off for 12 VDC.
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Old 04-22-2019, 12:33 PM   #14
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I would be willing to bet that your trailer is only using 12 gauge wiring at most for the fridge power. Can't imagine that GMC used 10 gauge wire when they when the constant power pin in my truck. I wouldn't worry too much about it.
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Old 04-22-2019, 05:17 PM   #15
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I agree. I know for fact that my old 1998 Expedition with the factory towing package used 14 AWG on the power pin in the 7 pin connector because the fuse block broke at one point and had to be replaced; the wire passed through that block.

My 2002 Expedition had 12 AWG and a slightly different path to the connector.

I'm not sure on my 2010 Taco but the last time I looked at it it seemed to be similar and definitely NOT anywhere near 10 AWG.
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