I have a new (to me) 2006 Chevy 2500 and a camper to go with it. This truck has a factory tow package but the red wire "battery feed" on the factory plug at the bumper is dead, even with the truck running. Instead of trying to track down where that red wire goes under the hood I want to run another wire from the truck battery back to the camper and was wondering what amp fuse I should use to protect this circuit?I am ordering this isolator switch Smart Dual Battery isolator which is rated for 140 Amp. Was thinking about a 200 amp fuse so that it wont blow when there is a low battery condition in the camper, coupled with say, lights and the DC fridge running starts drawing a lot of amps from the alternator. Thanks for any advice!

Take a look at the owners manual if it is still with the truck. Should be a place in the large under hood fuse box for a fuse to protect that line. Maybe the old owner never plugged one into it.

The under-hood fusebox has a fuse called stud#1. It's 40A and feeds the trailer plug. There's no isolator other than pulling that fuse.If you run your own wire, the fuse should be sized to prevent overloading that wire.If you want to add an isolator to the existing wire, it's connected to a post (stud) just outside the fusebox and you can put the isolator in series there.

Fuses are sized to protect the WIRE, so the wire gauge will determine the correct fuse.It will be in the range of 10-30 amp depending on wire gauge It will be in the line from isolator (right at the isolator)to the camper.Be sure to run a comparable gauge ground/negative wire from camper to truck frame.

Hi,If you have #4 wire going all the way to the trailer, then a 100 amp fuse is the max size that should be used. You will not get more than 50 amps to travel between the two 12 volt batteries. You will need a very oversized connector, like a forklift battery connector to get more than a few amps from the truck to the trailer. Look for a 50 amp rated 'anderson connector'. It will handle 75 amps without getting warm, and is rated at 50 amps when plugging in and out all the time without a problem. The wires going through the factory wiring harness are probably #12 wire - rated around 20 amps, and with that distance, probably will never see more than about 10 amps going into the trailer battery. By connecting a wire directly to the alternator, you will get a full 14.2 volts, while the battery probably only has about 13.5 volts. Good luck,Fred.

Thanks so much! this info you have given has been so valuable to me. I opened up the fuse box under the hood (Nice I didn't have to stand on my head to look under the dash!) and found where Stud#1 is. Appears to be just a red piece of plastic plugged into it. Am heading to NAPA now to get a 40A.. Maybe we'll have power to the camper for the trip we got planned for Monday! If so I wont bother running another wire back to the camper as I just want the batt back there to charge up after a day's driving. Not really needing a lot of power for this little 8' cabover..Thanks again.

What amp fuse for charging circuit from truck Alt?

time2roll
Nomad
Mar 16, 2015
I would sooner pull wire down from solar panels before I rewired my truck.
pianotuna
Nomad III
Mar 15, 2015
Hi Arktikos,

Thanks.

Do consider #8 or even #6 wire for the charging path. 20 amps won't do much in the way of recharging.
Arktikos
Explorer
Mar 15, 2015
pianotuna wrote:
Hi Arktikos,

Maximum measure load on the DC side for my inverter was 276 amps. That happened by accident.

That sounds like an impressive system!
pianotuna
Nomad III
Mar 15, 2015
Hi Arktikos,

Maximum measure load on the DC side for my inverter was 276 amps. That happened by accident.
Arktikos
Explorer
Mar 15, 2015
pianotuna wrote:
Hi smk,

No, I don't mind automatic in some cases. My charging solenoids are automatic, but I do have manual control over them. What I don't like is the 12.9 volt cut out and the 13.4 volt turn on.

I don't think I'd install wire that was only capable of 20 amps. I changed over to 50 amp automatic circuit breakers after blowing 2 60 amp fuses on the OEM charging path. (I admit I'm a power hog and I push my system HARD).

I have seen 12.7 volts on my chassis system, for example when running the water heater on the electric setting. I admit that most folks would not do that.

Are you talking about running an electric water heater through an inverter off the 12V battery? If so that is a lot of amps! If it is a 1500W element, that is 12.5 amps@120v. And at 12v is 120 amps plus whatever the inverter uses. My little 8 foot cabover TC doesn't even have a water heater so I expect very little load other than the furnace.
Anyway, thanks for your perspective.
pianotuna
Nomad III
Mar 15, 2015
Hi smk,

No, I don't mind automatic in some cases. My charging solenoids are automatic, but I do have manual control over them. What I don't like is the 12.9 volt cut out and the 13.4 volt turn on.

I don't think I'd install wire that was only capable of 20 amps. I changed over to 50 amp automatic circuit breakers after blowing 2 60 amp fuses on the OEM charging path. (I admit I'm a power hog and I push my system HARD).

I have seen 12.7 volts on my chassis system, for example when running the water heater on the electric setting. I admit that most folks would not do that.
time2roll
Nomad
Mar 15, 2015
The relay will work fine. For his own reasons pianotuna tries to stay away from automatic things. I like the relay and I prefer things automatic. It would have to be very odd circumstances where the relay remained open when you expected it to be closed.
Arktikos
Explorer
Mar 15, 2015
pianotuna wrote:
Hi,

The 12.9 cut out and the 13.4 connect are why I don't think the "smart" relay is a good idea. Particularly if an inverter is in use the battery voltage may drop. At least one converter goes to 13.2 when it is in "float" mode. That means the "smart" relay will not maintain the chassis battery.

Those are some of the reasons I recommended a simple inexpensive continuous duty solenoid relay.

I don't quite understand. How this isolator works is it makes a connection between the aux and main battery if either battery reaches 13.4v. If I am driving somewhere the main battery will be over 13.4 (more like 14.4V) and there will be a connection, charging the rv battery. If there is something in the coach drawing so much power that the truck charging system wont handle it, dragging down the alternator to under 12.9v then the connection will be cut, but the 20 amp fuse on the aux circuit to the camper will have blown anyway so that will never happen.
kaydeejay
Explorer
Mar 15, 2015
wa8yxm wrote:
wolfe10 wrote:
Fuses are sized to protect the WIRE, so the wire gauge will determine the correct fuse.

THIS is the correct and proper answer... LISTEN to this man

Putting a 40 amp fuse on a 12 ga wire is a good way to show of the last RV fire I saw.. This was a TT.. The TT had smoke damage to the outside, A good bath (perhaps with a bit of Clorox) is all it needed.

The Truck Pulling it was TOAST (Do not know why, Did not stop since Fire and Friend were both on scene already so no need for me to stop)

But putting a big fuse on a small wire is one way to do it.
The correct fuse for the trailer feed on GM trucks IS 40A. The OEM wiring is sized correctly.
wa8yxm
Explorer III
Mar 15, 2015
wolfe10 wrote:
Fuses are sized to protect the WIRE, so the wire gauge will determine the correct fuse.

THIS is the correct and proper answer... LISTEN to this man

Putting a 40 amp fuse on a 12 ga wire is a good way to show of the last RV fire I saw.. This was a TT.. The TT had smoke damage to the outside, A good bath (perhaps with a bit of Clorox) is all it needed.

The Truck Pulling it was TOAST (Do not know why, Did not stop since Fire and Friend were both on scene already so no need for me to stop)

But putting a big fuse on a small wire is one way to do it.

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What amp fuse for charging circuit from truck Alt?

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