New to 12 Volt..can we "piggyback" off running light circuit

Only if you give the cooler its own fused wiring branch so that it's not on the same wires and fuses as the lights, you don't want to add 4 more amps to the old lighting wiring.

And only if the wiring harness from the truck is big enough to carry the extra amps. That's a function of wire gauge and length, look up on an ampacity table and compare with your truck's wires and the distance from the alternator to the camper. And how many amps the lights are already drawing, a regular old 1156-mount light bulb takes 2 amps in my camper.

My guess though is that another four amps will be fine if you're pulling a small trailer without a laser light show (or A/C) on it.

OK, I was talking about the truck's wiring harness being big enough, because I was assuming you were hooking into a "factory" camper wiring harness built to power roof/interior lights as well as taillights. That Amazon harness is 18 gauge and is only meant (and wired) for taillights, not running lights along the outside roof line, or a camper's interior lights. It's more for boat and utility trailers than campers. I wouldn't use it to power my cooler (or anything but taillights, for that matter). Besides taillights, what lights are in the camper? How many outside and inside? In my camper, just one 12v interior light bulb (standard old 1156 bayonet mount) takes 2 amps.

The wire-sizing calculator I use wants 12 gauge wire to carry 10 amps 20 feet (my guess at the distance between truck alternator and camper) in order to keep the voltage drop to less than 3%, and ten amps ain't much when you're talking about light bulbs... www.bulkwire.com/wireresistance.asp

Just taking the 4 amps of the cooler 20 feet takes 16 gauge. 18 gauge, according to the calculator, is only good for 2 amps over the 25-foot length of that harness. If you must use that harness for some reason, run a separate, large wire for the cooler, and give it its own fuse.

I'm amazed that little wire can supply all those bulbs. Are they as bright as the truck lights?

I wouldn't even look for a pre-built harness. I'd buy the heaviest four-wire connector I could find and a bunch of wire and do it myself. I'd probably run a fairly large hot wire from the connector into the trailer, then run it into a fuse block to split it into the hot wires for the interior lights, the running lights, and the cooler.

Someone else may know of heavier pre-built harnesses, but I'm too cheap to buy one so I've never looked. And if I found one I probably wouldn't think their wires were big enough.

Excellent advice above.

I'm thinking your new harness will run the towing lights if nothing is added. Many of my trailers have used 18 ga wire originally. That being said, a 71 has a lot more lights than a mid 60s trailer. Try the lighting after dark so you can see it they are dim or bright. If all are about the same, you might be OK with the new harness.

BUT: there's always a catch. In order to run anything inside the trailer on the road, you will need a 7 way cord and a 7 way receptacle on the vehicle. They are universal and parts readily available, don't settle for a 6 way.

Run a fused hot wire from the truck battery to the back of the vehicle. The 7 way receptacle with have color labeled posts stamped in the plastic. The hot wire connects to Black post.

White = Ground
Black = 12 volt Hot/charge wire
Blue = Brakes which should be connected
Green = Tail lights and Marker Lights
Brown = Right Turn
Red = Left Turn
Yellow = Aux, not often used (can be back up lights)

NOTE THAT THE ABOVE 7 WAY CODE IS DIFFERENT THAN THE 4 WAY CODE. I will be glad to help if you need it.

Place a 2 gang junction box in a convenient location near the tongue. Mine are usually located under the trailer. The 7 way cord is securely clamped to the box, which serves as a safe enclosure to connect the tow wiring and a 12ga. hot line. The hot line goes into the trailer to a fuse block. Run the fused hot wire to power the 12 volt cigar plug for your refrigerator. A ground from the trailer frame goes into the trailer as well. I use a ground bracket inside and then run from there to the cigar plug.

Parts:



Diagram:



What it looks like:



Close up of J box:



Hope this helps.

vikx, do you have a diagram like that but without hooking up a battery? I think I would like to wire my trailer with 30Amp.

Please note the above is for Tow Wiring and a possible 12 Volt System inside the trailer.

It is not 110, so changing to a 30 amp 110 system does not apply to the above photos and information. Many trailers have 3 separate systems: 110 for lights and outlets, an inside 12 Volt system, and Tow wiring to run tail and marker lights. Tho separate, the 3 systems do interact; Charge line to 12 volt system hot, 110 to power a battery maintainer.

Inside 12 Volt System:

Scgal, it appears the photos are showing an inside 12 volt fuse block. On the top are the white grounds on a separate terminal block. Tow Wiring is not normally run thru a fuse block. I'm guessing this is not tow wiring, but runs certain items inside the trailer? (12 volt lights, water pump, cigar plug, etc)

I need more information about your trailer. Does it have 12 volt lighting? A water pump?

Tow Wiring:

If the fuse block is for Tow Wiring, the wire colors are not in line with either a 4 way or 7 way color code. That doesn't mean they are incorrect, just that somebody rewired with the pictured colors.

To get back to basics: You MUST have a 7 way cord on the trailer to provide hot 12 volts while traveling. It appears the system is already there. A battery is important because it acts as a buffer for direct 12 volts and also powers the 12 volt system when disconnected from shore power.

If your trailer is mid 70s, I'm thinking the two systems are already there, just not quite working right now...

You're welcome. You are doing the right thing with the new 7 way. I missed your comment that both vehicles were wired for 7 way and one had been changed. So that's good.