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Twelve-Volt RV Circuits 101: Down To The Wire

Pondering adding a 12-volt device or outlet to your camper?  John Wells gives us an overview on power sources, safety, wiring, and choosing a 12-volt socket type in preparation for your project.


There are so many things that we can do wirelessly now.  We can send messages, photographs, texts, music, and even stupid jokes.  What we can’t do is send usable quantities of electrons from point A to point B without using good old-fashioned copper wire.

As truck camper owners, we often need an extra 12-volt light, outlet, or appliance in a location the manufacturer never anticipated.  Without sacrificing safety or aesthetics, we want to add more 12-volt convenience and utility to our camper electrical systems.  For these articles, I’ll explore some of the underlying principles, as well as practical step-by-step solutions, to achieve this goal.

12-Volt Power Sources

Once you have decided that you would like to herd some 12-volt electrons to a particular point in your truck camper, there are several basic considerations that need to be addressed first.  Specifically, you need to consider what 12-volt power sources are available in your truck camper.  Here are the most common options:


Above: Typical truck camper battery showing positive and negative connections.  The smaller gauge wires may very well be from a solar charging system.

1. Batteries.  You can connect a new 12-volt line directly to the 12-volt batteries in your camper.  This is especially useful for circuits that will draw heavy loads, or provide charging.  For example, a solar panel charging system is often directly connected to the battery for efficient charging.  Safety note: When connecting a new circuit directly to the battery, you must include an appropriate in-line fuse in the hot (+/positive) lead, and have it located for easy future access.


Above: The orange circle in the photograph shows the distribution buss in a Chalet S100F.  The red circles are the positive (white) and negative (black) connections.  Notice the smaller gauge wires, that’s an added circuit for a 375W inverter.  The green circle shows the proper use of an inline fuse in the hot (+/positive) lead to the relay running the inverter.

2. Distribution Bus.  Some truck campers have a distribution bus located separately from the battery with heavy gauge connections to a distribution panel, generator, or other major connections.  This distribution bus can also be an acceptable location to add a 12-volt circuit if it has an inline fuse in an accessible location.


Above: Here’s an opportunistic tap of exposed wiring inside the wardrobe of a truck camper.  Note the two blue Posi-taps harvesting electrons to feed an added LED dinette light fixture just on the other side of that wardrobe wall.

3. Existing Circuits.  You can also redirect power from an existing circuit with wiring that is convenient and accessible to the desired final location of our new 12-volt outlet or device.  This connection would be most useful for devices that have a low current draw (5 amps or less) such as a cell phone charger, LED light, or a small fan.  This approach requires knowledge of the fuse size protecting the circuit as well as the draw of the intended addition.


12-volt-empty distribution slot

Above: Front and rear view of an empty distribution slot (see red circles in the photographs)

4. Empty Distribution Panel Slots.  Finally, you can add a 12-volt circuit to the existing distribution panel if there are empty slots available.   The fuse for this circuit will be installed in the panel.  We’ll use this approach in one of our future projects.

12-Volt Safety

Another important basic consideration is safety.  It’s critical to match the intended circuit load to the wire gauge and fuse.

If you crowd too many electrons (circuit load) into a tiny space (wire gauge and fuse) they get cranky (heat up) and lose their temper (heat up some more).  If we try to push too much current through too small a wire, that wire can overheat to the point of causing a fire.

To figure out what gauge wire and fuse is needed for a safe installation, consult a table that matches copper wire gauges with intended loads.  Here’s an example:

Above: An example of appropriate wire for many routine applications.  Photo taken from

In general, most low draw items in a truck camper will do well with a 14-16 gauge wire.  With wires, the larger the number the lower the size.  I don’t recommend using anything thinner (higher in number) than 18 gauge for 12-volt truck camper applications.


Above: Typical appliance label with suggested circuit requirement.  This will determine wire gauge and fuse size for your project.

If you’re in doubt, please use the table link above.  If you have an appliance that pulls 20 amps or more, please contact the appliance manufacturer or installation manual for the suggested wire gauge.  It’s your responsibility to remain within the stated safety guidelines.  The fuse size is likewise determined by the anticipated load of the device or circuit in question.

Before you start your 12-volt project, you must also make sure that you are going into a filtered DC circuit (like a radio/TV circuit), especially if your camper is more than a few years old.  You need filtered DC so that you don’t fry your electronic devices.  Check your converter’s owner’s manual for more information.

Running the 12-Volt Wire

There are two ways to run 12-volt wire through a truck camper; hidden wire, and surface wire.  Let’s examine these two options.


Above: The wiring for this solar charge controller is hidden in the hollow wall beside the refrigerator.  Note: The refrigerator cooling tower is a great place to run wires down from the roof; for example: solar, antennas, etc.

1. Hidden wire.  If you have access to usable space (interior hollow wall, basement, or storage cubby that runs from your power source to the desired 12-volt outlet destination), you can hide your new wiring.  Camper exterior walls are not hollow and, therefore, it’s often difficult or impossible to run hidden wire through exterior walls.

If you have ducted air conditioning in the ceiling of your camper, wire could be fished through those ducts.  Do not run wires through heating ducts.  Heat is not our friend when dealing with electricity.

12-volt-Wilson Cell Phone Signal Booster

Above: This attractive white flat plastic molding conceals the cable for the Wilson Cell Phone Signal Booster’s indoor antenna.

2. Surface wire.  Sometimes it’s not possible to hide wires inside or behind walls, basements, or cabinetry.  When hidden wiring is not possible, surface wiring is necessary.  The good news is that very nice self adhesive wire moldings exist that can be cut to length (complete with elbows, Ts, and straight runs) to give your new 12-volt circuit installation a professional appearance.

Choosing a 12-Volt Outlet Type

The last consideration for your 12-volt circuit is choosing between a permanently wired 12-volt device (such as a light fixture or fan) and a 12-volt outlet.

12-volt-375 watt inverter

Above: Here’s a 375 watt inverter hard-wired in the basement of the camper.  The circuit includes a 30 amp relay and a remote switch with LED indicator upstairs in the living space.  In this example, the inverter powers two televisions and a DVD player.

If your new circuit will be permanently wired to a device such as a 12-volt light fixture, it can be hardwired directly to your new 12-volt circuit.


Above: Just one example of many 12-volt outlets available.  These look essentially like the ones in a car or truck.  Photo taken from

If, however, you want the versatility of being able to plug and unplug a 12-volt device, you will need to select the appropriate 12-volt outlet for the devices you intend to use.  For example, most 12-volt devices have a cigarette-lighter type plug.  Other 12-volt devices, including many smart phones, MP3 players, Bluetooth devices, and other portable electronics, have a USB plug.  Inspect the gizmos you need to plug into your new 12-volt outlet, and choose your socket accordingly.

Above: Photo of a Charging Center taken from

An increasingly popular option is a dual 12-volt standard and 5-volt USB outlet.  If you need both cigarette-lighter and USB outlets, and you have the space in the mounting location, these versatile dual-outlets are definitely worth your consideration.

Above: Engel socket and mounting plate, Photo of an Engel socket and mounting plate on


Above: The Engel plug and socket is especially suited to higher current (up to 20 Amps) due to its large flat blades and the securing shroud that actually screws into the outlet after insertion.  This plug cannot pull out accidentally.  Photo from Engel plug on

There are other 12-volt outlet types to be aware of.  The Engel 12-volt refrigerator-freezer outlet is an Australian design with a 12-volt plug developed to be especially secure for appliances.  Some 12-volt appliances, including 12-volt refrigerator-freezers, require high current.  The Engel plug resists accidental unplugging through vibration or inadvertent bumping.  We will be installing an Engel refrigerator-freezer for an upcoming 12-volt project.
Photo from Flush mount Merit Socket on

Only slightly less secure than the Engel is the Hella or Merit socket.  Lots of motorcycle-designed accessories use Hella’s well-designed socket and plug combination and there’s no reason the Hella socket couldn’t be used in a RV if you have an accessory with a Hella plug.


Above: Here are all three plug types side-by-side for comparison and identification. Photo from


Above: In addition to the 12-volt wire, and chosen termination (device or outlet), you will need a short list of tools.  I’ve assembled some important tools above; cordless drill with hole saw, cable ties, bullet connectors, wire nuts, spade terminal, butt splice connectors, posi tap, posi-lock, electrical tape, scotch lock tap, digital multi-meter, and wire cutters.

You’ll see some common (and not so common) connectors as well.  Keep your eye on those blue ones on the tape roll.  I like to call them the Electron Pirates.  They’re my favorites, and you’ll see them featured in part 2!

Installing a 12-Volt Circuit

Once you have determined (1) a 12-volt power source, (2) the appropriate gauge 12-volt wire for safety, (3) hidden versus surface wiring, and (4) a termination type (permanently wired 12-volt device, or a 12-volt outlet), you’re almost ready to install a 12-volt circuit.


Above: Riddle me this, How will we install a lowly 12-volt cigarette outlet in yonder range hood?  Stay tuned…

Next Up: Installing a 12-Volt Outlet

Now that you’ve studied for your TCEE (Truck Camper Electrical Engineering) degree,  it’s time to go for your first practical exam; installing a 12-volt outlet.  For 12-Volt RV Circuits 102, I’ll take you through the step-by-step 12-volt outlet installation process using my own truck camper.

Photo from Wolo 345 Animal House 12-volt Electric Horn on

After that, I may even take requests.  Want a Wolo 345 Animal House 12-volt Electric Horn and PA system with sixty-nine different tunes and nine animal noises?  Of course you do.  We can do that, in addition to 12-volt LED lights, 12-volt fans, and more 12-volt outlets.

Welcome to the new 12-volt series I like to call, “Doctor John and the Electron Pirates!”  Until then, give me an “ERRRR!” for Electrical RRRResistance!

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