Trailer Lights and Wires

On new trailer builds, as well as on existing rigs, the trailer lights and wires must all function.  Here are some tips on the subject.  If you find the electrical a little confusing or intimidating, read on. This article is all about wiring up your trailer for success on the road.

Trailer Lights and Wires InstalledThere are lots of great options in trailer lights these days, but little has really changed in wiring.  Still it can be a little confusing, and the consequences of getting something wrong?  Well we don’t want that, so hopefully we can unravel some concepts and make it easy to do it yourself.

Yeah, sometimes dealing with electrical things can be a little daunting.  So many choices, so many ways to do it.  Oh, and I don’t want to get shocked!  Well, it’s fairly straightforward when you understand the basics.  Let’s look at wiring now for trailers.

Trailer Lights

With current products, there is almost no reason to go with anything but LED lights.  Even if you’re replacing one on an existing trailer, it’s very likely you can find a replacement to fit in the same form and bolt pattern, but in LED.  (LED is Light Emitting Diode, in case you wondered.  It’s just a lower energy way of creating brighter light.  They are also cheaper, and much more durable, which is nice for trailers.)

The technology with LED’s has come of age.  For new trailer builds, you can get some really slim and very bright lights.  We recommend sealed and submersible for everything because they’re only a couple bucks more, and it potentially solves many issues.  Of course your trailer may never go underwater, but if the lights are submersible, then you’ll never have to worry about rain, corrosion or internal moisture damage that can make lights quit working.

Do be careful, however, when choosing lights.  There are some really cheap ones (and I don’t me just inexpensive).  Yes, cost is always a factor, but for just a few bucks more, buy the mid-range and the quality is usually better.

Connectors – Hitch to Trailer

Connectors for Trailer Lights and WiresOh boy!  There are a ton of different styles of trailer electrical connectors.  From 4-Pins to many.  4- & 5-Pin flat and 7-Pin RV style are the most common for non-commercial applications, and I’d say if you have the choice, go with one of these.  That said, consider your tow vehicle, and make the connector match.

Our Trailer Wiring Diagram page has all the pins and color codes if you want them.  That can make the trailer lights and wiring go much easier.

Connectors are more than just a lump dangling on the end of some wires.  The trailer lights and wires depend on that connector, so make sure wires are soundly attached internally.  Connect each wire securely, and button them all up well.  Also, test, test, test all the connections.

Protect the wires going from the trailer to the tow vehicle.  Use a wire jacket or something else around the wires.  Split loom is common, or heavy heat shrink.  I like using rubber hose anchored at the trailer frame.   Because these dangle, and are handled frequently, protection will avoid headaches down the road.  Also, give it some strain relief.  Make sure the attachment to the trailer will not pinch or fatigue as you use it.

For heavier trailers, don’t forget wiring the Breakaway Kit too (if your local laws require it).

Button It Up?

T-Splice Electrical Connectors
T-Splice Type Connector

Yeah, Button it.  Connecting wires for each light is often a hassle.  Here are some tips (some controversial) for success.  Use what you like, discard the rest.

  1. When connecting wires to lugs, like in a 7-pin connector, spray a little PAM or WD-40 in AFTER to keep moisture at bay.
  2. Loose Wires can connect in many ways.  Guillotine ‘T’ connectors (shown blue) are popular, though some have trouble after a few years.  I prefer solder, then heat shrink.  Crimp Connectors also work with the right tools.  Heat shrink over crimps too.  See this tutorial for marine grade crimp connectors.

    Butt Splice Crimp Electrical Connector
    Butt Splice Crimp Connector
  3. For loose wire connections, use electrical tape.  Are You Kidding??  Don’t EVER use that – at least not by itself.  Truth is, electrical tape makes a reasonable DIY shield around joints AFTER connecting the wires another way.  So, connect the wires, then wrap the connection with electrical tape, and finish with zip ties around the ends of the electrical tape.
    NOTE:  Electrical tape is DIY belts and suspenders.  Heat shrink or marine grade connectors are better, for sure.  However, electrical tape applied tight and generously OVER another connector, with zip-ties or other fastening, is a reasonable extra shield.  It’s not waterproof or a substitute for a good mechanical connection, so only use it  1)  with another connecting means,  2)  for non-submersible connections.
  4. Stagger the wire connections.  When multiple connections are in the same area, make them different lengths so the connections stagger.  (See the photo above.)  Wrap them separately (not done in the photo) for isolation, then wrap them all together for final shielding.
  5. Use flexible conduit, rubber hose, or some other flexible protection over the wires going from the trailer to the tow vehicle.
  6. Secure all wires so they can’t snag or dangle.

Wiring Diagrams

Typical Trailer Wiring Diagram and SchematicSo, how do we wire all this stuff onto the trailer?  Let’s start with a map, and let’s choose which functions we need, and which we don’t.

A small version of a typical trailer wiring diagram is shown here.  It works for most trailers.  Click for the complete Trailer Wiring Diagrams article where you’ll find the full-size diagrams and a ton of other information about trailer lights, main connectors, and wires.  That page also includes a bunch of info about choosing the wires.

We recommend one of 3 standards for wiring.  They are all discussed in the Trailer Wiring Diagram article.  Follow the standards.  Don’t just pass off the trailer lights and wires thinking that you’ll be the only one that ever uses it.  Sure shooting, you’ll want to pull some other trailer or you’ll get a new vehicle and have to mess with all the wires again.  In this case, standards are good.

From the wiring diagram, use the wires that are applicable, and just leave out the rest.  It doesn’t hurt to have empty slots in the connector, or unused wires in the bundle on the trailer.

Once you know what functions to include, just lay out the lights and wires.  Starting at the front, measure the lengths of wire, then add for some extra, and get the parts.  A spool of string and some tape can help measure the length so you can buy the right length of wires.  It can also help you think through routing so the wires are easier to run.

Get the Needed Trailer Lights and Wires

All the regular trailer electrical parts are easily available at your local trailer supply store or online.  For lights, I’ve found that some of the online sources have a much wider variety than the local shops.  Try eTrailer.com for instance, and do a search for the size and type of lights you want.  Other places like JohnsonTrailerParts.com can usually get what you need.

For other supplies, I find that the local stores carry the right pieces most of the time.  Things like brake controllers, for instance, because they can help you find just the right things for your vehicle.

The wires themselves are conveniently bundled with the right colors and come in a long strand.  They are easily available.  Make sure you get more than you need.  For connectors and for attaching clips, I find local hardware stores, like Ace or True Value tend to carry a good selection.  Same with heat shrink.

Tips For Building

When building a new trailer, dry fit all the trailer lights and wires to the trailer frame before painting.  Add any holes for access or for mounting before the paint.  This keeps you from needing to cut or drill into the beautiful new paint.

Follow the guide for color.  If you always connect the same colors as defined, you won’t worry about misconnections.  What if wire colors don’t match for the lights I buy?  Many lights don’t care about polarity, so both wires might be black (or other colors).  If the instructions don’t say otherwise, it’s easy because you can hook them up either way.

P-Clips for Trailer Lights and WiresHang wires from the decking (if it’s wood) rather than the trailer frame.  Why?  It avoids having to drill or tap into the metal frame.  Also, it’s simple and easy to use a wood screw into the underside of the decking.

For a more advanced look and sometimes better protection, use conduit or tube to house the wires.  (Sometimes, because you must keep water out, and ends of tubes offer a location for wire damage.  Do it with care, and it can be wonderful.

Or another option is a lot of wire hangers — like this rubber wrap P-Clip.  (aka Adel Clamp).  They come in metal, plastic and sometimes rubber.  This image is looking up from under the trailer showing a routing support clip on the underside of the deck.

How do you route wires around cross members?  A groove in the underside of the decking allows the wires to easily pass over the cross members without pinching.  It’s a protected slot just for the wires.  P-Clips like this one secure the wires in place.  (Just don’t forget to disconnect all the clips before replacing the trailer deck!)

Good luck with your Trailer Lights and Wires!

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