How To Wire Trailer Brakes

For electric trailer brakes, correct wiring is absolutely essential to having them work right.  But trailer brakes are a system, not just the actuators at the ends of the axle.  Several pieces must work together as we wire trailer brakes into a functioning system.

The wiring is only part of the system.  We also need the tow vehicle connector, brake controller, and breakaway kit.  Together, these components carry the braking signal from the tow vehicle to the electric brakes on the trailer.

In this article, we will look at the whole system, starting with the trailer brake wiring diagram, then follow the connections through the brake wiring, breakaway kit, trailer connector, and tow vehicle brake controller.  Along the way, we will cover wire size, grounding, and a few other practical trailer wiring tips.

Start With The Trailer Brake Wiring Diagram

There is nothing super difficult about wiring trailer brakes, but it can be intimidating anyway – because electricity can be a little mysterious.

Let’s start with a high level view.  This image is our trailer wiring diagram, and it includes the brakes.  The colored lines represent wires, and the boxes represent parts in the system.  (Also known as a wiring schematic.)

For trailer brakes, we are primarily interested in the Blue and White wires.  If we add a breakaway kit, then we also need the Red wire.  More about that below.

The White wire in the wiring diagram is called the “Ground” or “Common”, and it connects as a return path to the (-) Negative side of the tow vehicle battery.  The Blue is a (+) Positive signal that comes from the brake controller in the tow vehicle, not directly from the battery.

Notice the colored circles where wires intersect.  Those indicate electrical connections.  For the brakes, the Blue wire branches to every brake actuator, and the White wire does the same.

When we wire trailer brakes, we simply make wiring connections for each branch in the diagram.  The actual wires can be long or short, up, over, around, and through the trailer as needed.  They certainly will not look as neat and straight as the lines in a wiring diagram.

Also, the physical location of each branch connection does not matter.  In other words, the Blue-dot connection points can be located wherever it makes sense on the trailer, as long as the wires go to the wheels.

The image shows a tandem axle trailer, but it is very similar for a single axle or a triple axle setup.

Connect The Electric Trailer Brakes

Now let’s move from the wiring diagram to the actual electric brakes on the axles.

Once we wrap our minds around the wiring diagram, the wiring portion is mostly just connecting the dots, electrically, so to speak.  The skills to wire the trailer brakes are fairly simple.

Each brake has 2 wires coming from the electromagnet inside the brake assembly.  They must connect, one to the blue wire, and one to the white.

One big question we hear:  Does wire polarity matter with trailer brakes?  Answer:  No, for standard electric trailer brakes.  Both wires must connect, of course, but the polarity does not matter.

The internal electromagnet works the same way no matter which way the electric current flows.  That makes the 2 wires interchangeable – which is why both wires are usually the same color.  Either way works.

Connect one wire to the brake power feed (Blue brake controller wire) and the other to your ground (White wire).  (These colors are consistent in both the SAE Wiring and in the RV Style wiring.)

In the image, A is the electromagnet, sometimes called the brake magnet or coil.  B shows the 2 wires that connect to the electromagnet, then go out through a hole in the back plate so we can connect them.  Notice that both wires are the same color.

If you are interested in what happens inside, here is a video from Dexter about How Electric Trailer Brakes Work.

Always wire electric trailer brakes in parallel.  That means connect every brake actuator directly to the two main wires, one wire to the Blue and one to the White (ground).  This is true for a single axle, or multiple axles – every brake actuator.

Parallel wiring gives every brake direct access to the brake controller power and to ground.  Do not wire trailer brakes in series.

Importance of a Ground Wire

A poor “Ground” or “Common” wire is a frequent cause of trailer brake problems.  For that matter, it is a regular problem with all trailer wiring.

To avoid these problems, we recommend connecting each item (lights, brakes, etc.) directly to a ground wire rather than allowing the trailer frame to be the ground.  In theory, the metal trailer frame can serve this purpose, but in practice there are many weird voltage and connection issues that can happen when the frame is used instead of a good ground wire.  Rust, weather, and other problems can also create faulty ground issues.

I won’t go into detail here, but trust me, a floating ground, odd resistance, or intermittent connection is not pleasant to diagnose.  One of the easiest ways to avoid future problems is to simply connect every electrical item directly to a dedicated ground (White) wire.

Choose The Right Brake Wire Size

We normally recommend 16 gauge wire for lights, but brake wire size depends on the situation.  Length of the wires and electrical current requirements both affect the wire size to choose.  Here are several factors to consider.

• Electricity flowing in a long wire creates a voltage drop.  We want to minimize this effect so the trailer brakes get the power they need.
  • The longer the wire, the greater the voltage drop.
  • A smaller diameter wire has a greater voltage drop, while a larger diameter wire has less.  (It can be helpful to think of it as easier for the larger wire to carry more electricity.)
  • So, for a long trailer with multiple axles, we want larger diameter wires.
• Larger brakes will typically draw more current.  It is not a lot more, but as we add axles, the total electrical current requirement goes up.
  • For Example:
    • Brakes on a 2,000 lb axle may draw 3.0 – 3.2 amps per side, or 6.0 – 6.4 amps for the axle.
    • Brakes on a 7,000 lb axle may draw 3.2 – 4.0 amps per side, or 6.4 – 8.0 amps per axle.
    • While the above is not a huge difference, if there are 2 axles, the total is 12.8 – 16.0 amps for the tandem axles.
  • Triple axles up to 24 amps!
  • To put this into perspective, most house circuits are limited to 15 amps.  (Of course, the voltage is very different, but it gives some perspective on the amount of current involved.)
  • Electrical current values vary between brake manufacturers.  Check the specifications for your brakes.

As A General Guide:

• For a small single axle trailer, 14 gauge wire is usually good.  Some manufacturers say 12 gauge (or AWG).
• For tandem axles, use 12 gauge wire.
• For a long, heavy trailer with triple axles, 10 gauge wire is probably better.
  • Wires at each brake do not need to be 10 gauge because each magnet only carries 4-ish amps, but the main wiring must carry the full amount.
  • One way to reduce the brake wire size is to have 2 separate runs from a junction box, one down each side of the trailer.  Each side feeds 3 brake actuators, so 12 gauge wire works.
  • You can apply this logic to tandem or even single axles if it helps.
• For all these situations, if the wire length from the tow vehicle connector to the brakes is longer than 30 ft, go up (larger diameter) one wire size.  For instance, 14 gauge to 12 gauge wire for a long single axle trailer;  12 gauge to 10 gauge for a long tandem axle trailer; etc.  When in doubt, use a larger wire diameter.  That is always OK.

Wire size is important.  A too-small wire can cause excessive voltage drop, give weak brake performance, and cause overheating.  So Do not skimp on wire size for your brakes.  I have seen undersized brake wire get hot enough to melt the insulation and short out under a trailer.

Practical Trailer Brake Wiring Tips

A trailer is a tough environment for wiring.  It moves, flexes, vibrates, gets wet, and endures road debris.  Here are a few practical tips to make trailer brake wiring more reliable.

1. Give wires sufficient length.  There is a fine line between wires that are loose and floppy, and wires that are neat with no stress.  We also don’t want wires that have stress from being too tight.
2. Use a service loop.  The term “service loop” can mean an actual loop, or it can mean a generous area so the wires do not flex much when going to parts that move.  We never want tight wires going to the axles, because axles move.  That means the wires must have sufficient length so they do not pull, or twist, or flex a lot when the suspension moves.  Wires that flex repeatedly are most likely to break.  So, make the wires flex as little as possible.  A generous loop or fold of wire can make that happen.
   
3. Don’t allow wires to sag where they can snag.  Make sure loops are in a protected area.
4. Run a dedicated ground wire.  As discussed above, do not depend on the trailer frame as the electrical ground.  Run the White ground wire directly to each brake.
5. Make connections secure.  Use crimp connectors, soldered connections, terminal blocks, and other proper connection methods.  Do not simply twist wires together and cover them with tape.
6. Keep connections weather-tight.  Heat-shrink connectors or heat-shrink tubing are excellent where practical.  Electrical tape can help protect and insulate a connection when applied properly.  However, DO NOT use electrical tape to hold a connection.
7. Protect wires everywhere you can.  Pay particular attention where wires pass through holes, around sharp edges, or near moving parts.  Use grommets, loom, conduit, clamps, or other protection where appropriate.  Vibration can wear through insulation surprisingly quick.

Wire The Breakaway Kit

We will not spend too much time here since we have an article all about the Breakaway Kit already.

With reference to the wire colors above, connect the breakaway kit to the White wire (for ground), the Blue wire (for brake actuation), and the Red (or Black) aux power wire (which keeps the breakaway battery charged).  Don’t rely on the wire colors, however.  If your wire colors are different, that is OK.  Simply check your exact kit and the wiring polarity it needs, then connect it into your trailer wires with the functions it shows.

Wiring To The Tow Vehicle Connector

Every trailer needs an electrical connection to the tow vehicle.  There are several styles of standard connectors which we cover in our Trailer Wiring Diagram article.

For electric trailer brakes, the connector needs a dedicated circuit from the tow vehicle brake controller.  While there are several connector configurations that can accommodate electric brakes, we recommend one like the 7-pin because it is the common choice.  It also provides connections for auxiliary power to charge the breakaway battery as well as a circuit for trailer brakes.

There are 2 common ways to connect the tow vehicle plug to the trailer wiring:

1. Connect the plug harness directly to the trailer wires.  This approach is simple and very common.  I did this for many years on all my trailers.
2. Connect the plug harness to a junction box, then connect the junction box to the wires in the trailer.  This requires another part, the box, but it gives some useful advantages.

Reasons for a Junction Box

The most likely group of wires to be damaged on the trailer is the tow vehicle connector harness.  Yes, the one that hangs out, gets twisted around, sometimes tripped over, or even dragged on the ground (if someone forgets to connect it).  I have spliced in more than one new tow vehicle connector harness.

A junction box has some nice advantages even beyond making the tow vehicle connector harness easier to replace.  As far as the electric trailer brakes are concerned, here are some things to think about:

1. If you ever need to troubleshoot the circuit on your trailer, the junction box gives easy access to measure continuity, voltage, or resistance.
   • You can test the tow vehicle connection, the connector harness, and the individual trailer circuits from convenient points inside the box.
   • It is especially useful to verify that the correct power or signal is reaching the trailer before chasing wires underneath and through the trailer.
   • If you separate the brake wiring into multiple runs, such as one down each side of the trailer, the junction box also makes it easy to isolate a problem to one side, the other side, or the connector harness.
2. Since the wires going to the tow vehicle have more exposure to damage, the connector harness can use heavy-duty cable with a thick protective jacket.  At the junction box, each trailer circuit can then continue with the wire size appropriate for its electrical load.
3. Electric trailer brakes need a larger wire size than lights, especially LED lights.  A junction box makes it easy to use the proper wire gauge for each circuit and to split the brake wiring into separate runs when appropriate.

For a little extra cost and wiring effort, a junction box can make the trailer brake wiring cleaner, easier to service, and much easier to troubleshoot later.

The Tow Vehicle Side, And The Brake Controller

Fortunately, many modern SUVs and trucks have a tow package of some type wired in from the factory.  Some include an integrated tow vehicle brake controller and the tow vehicle connector near the hitch, while other provide wiring provisions for one.  Cars are less likely to have one built in, but still it is often a factory option.

If your vehicle does not have one, a vehicle brake controller is not a standard one-size-fits-all part.  Each vehicle manufacturer has their own way of doing things, so find the one that fits your vehicle, then follow the instructions.  The same is true for adding a tow vehicle connector near the hitch.  To some people, messing with the tow vehicle wiring is the most confusing and sometimes frustrating part of wiring electric trailer brakes.

We recommend that you get factory specifications for your specific vehicle if you need to install one.  If you are not comfortable working on your tow vehicle’s electrical system, this is a reasonable part of the job to have professionally installed.

Output from the tow vehicle brake controller is the braking signal from the vehicle.  It is the Blue brake wire that goes to the trailer connector.

Test The Trailer Brakes

After everything is connected, test the complete trailer brake system before heading down the road.  Make sure the brake controller activates the trailer brakes, and verify that the brakes respond at each wheel.  To test it, jack up the wheels, spin them, and have someone press the tow vehicle brake pedal.  (You can test one side at a time if you don’t have the ability to jack up the full trailer.)

Also test the breakaway system to confirm that it applies the trailer brakes independent of the tow vehicle.  Do this by First disconnecting the trailer electrical connector from the tow vehicle.  (Testing it while connected may damage the brake controller.)  Second, pull the safety pin from the breakaway switch.  All brakes should apply firmly.  (Please Note:  Don’t leave the breakaway pin out too long or it will draw down the battery.)

Finally, inspect the wiring around the trailer.  Make sure wires are secure, protected from rubbing or snagging, and have enough flexibility for full suspension movement.

This is also a good time to adjust the brake controller gain and the individual brakes as needed.  (See the manufacturer’s instructions.)  Good wiring is essential, but the complete system is both electrical and mechanical.  It must all work together properly.

Putting The Trailer Brake Wiring All Together

Wiring electric trailer brakes is not especially complicated once you understand how the pieces connect and interact.  Start with the trailer brake wiring diagram, then follow the circuit from the brake controller through the tow vehicle connector and trailer wiring to each brake actuator.

The important details are what make the system reliable.  Wire all of the brakes in parallel, provide a dedicated ground wire, choose wire large enough for the electrical current and wire length, protect the wiring from movement and damage, and make every connection secure and weather-resistant.  The breakaway kit and tow vehicle brake controller are also parts of the system, and each should be connected according to its own instructions.

Take your time with the wiring, then test the complete system before towing.  Good trailer brake wiring should not just work when you finish the installation – it should continue working reliably through years of vibration, weather, suspension movement, and use.