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 how to wire a trailer.  If you find the electrical part a little confusing, or slightly intimidating, read on. This article is all about wiring up your trailer for success on the road.

Trailer Lights and Wires InstalledThere are a lot of great options in trailer lights these days, but little has really changed with wires.  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 confusion and make it easy to do it yourself.

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

Trailer Lights

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

Technology with LED is awesome.  For new trailer builds, you can get some really slim and very bright lights.  We recommend sealed and submersible for everything because they are only a few dollars more, and they potentially solve many issues.  Of course the trailer lights and wires may never go underwater, but if they are submersible, then you will never have to worry about rain, corrosion or internal moisture to make lights quit working.

Do be careful, however, when choosing lights.  There are some really cheap ones (and I don’t mean just inexpensive).  Yes, cost is always a factor, but for just a few dollars more, buy the mid-range, then 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-Pin & 5-Pin flat – as well as the 7-Pin Blade style connectors are the most common for non-commercial applications, and I will say if you have the choice, go with one of these.  That said, consider your tow vehicle, and make the trailer electrical 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.  In addition, we also have an article that discusses the difference between the Traditional SAE standard versus the newer “RV Standard” for wiring.

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 for trailer lights and wires (some controversial) for success.  Use what you like, then 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. Electrical tape?  Are You Kidding??  Some people say “Don’t EVER use that!”  I say, “Not by itself”.  Truth is, electrical tape is 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, is a reasonable extra shield.  It is not waterproof, or a substitute for a good mechanical connection, so use it only  1)  with another connecting means,  2)  on 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

Trailer Wiring Diagram and SchematicSo, how do we wire all this stuff onto the trailer?  Start with a map, and 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.  See the complete Trailer Wiring Diagrams article where you will find the full-size diagrams and a ton of other information about trailer lights, wires, and connectors.  That page also includes a bunch of info about choosing the wires.

We recommend following a wiring standard as discussed in the Trailer Wiring Diagram article.  Don’t just pass off the trailer lights and wires thinking you will be the only one to ever use it.  Sure shooting, you will want to pull some other trailer or you will get a new vehicle and have a mess with the trailer lights and 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 does not 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 have 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 the trailer lights and wires.  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, fit all the trailer lights and wires to the trailer frame before painting.  Add holes for access and mounting before the paint.  This keeps you from needing to cut or drill into beautiful new paint.

Follow the color guide.  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 another color).  If the instructions don’t say, you can probably hook them up either way.  If you wonder, hook it to a battery and try it.

P-Clips for Trailer Lights and WiresHang wires from the decking (if it is wood) rather than the trailer frame.  Why?  It avoids having to drill or tap into the metal frame.  Also, it is 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.  So, 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 is a protected slot just for the wires.  P-Clips like this one secure the wires in place.  (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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