How To Wire A Trailer . . . Trailer Wiring Diagrams . . . And More

So many wires, so many colors, so many kinds of trailer wiring connectors.  Yikes!  Where do I start?  I need a trailer wiring diagram.  And, a little more information to make sure I get it right!


Trailer Wire ColorsThere are several standards for trailer wires, and if you search, you’ll find a different Trailer Wiring Diagram for each.  Each standard has it’s different purpose, so please don’t just make it up as you go.  Don’t get caught with a weird wired trailer.  Follow these guidelines and make it right!

The approach to use depends on your electrical needs.  To start, every trailer needs lights — brake lights, turn signals, and tail lights.  Some also have side markers and running lights.  Brakes probably need electricity too — to actuate electric brakes, or to disable hydraulic brakes when backing up.

The following trailer wiring diagram(s) and explanations are a cross between an electrical schematic and wiring on a trailer.  We recommend these standards because they are pretty universal.  That said, for specific situations, there are industrial standards with different connectors and wire arrangements.  It can get confusing, so if you don’t already have a specific standard in mind, follow these.

 

Trailer Wires 4-Pin Connector

4-Pin Flat Connector

At a minimum, all trailers need at least these 4 functions:  Tail lights, Brake lights, Left & Right signals.  4 wires can provide these functions, so the simplest scheme is a 4-pin connector.

The most common 4 wire connector is the 4-Pin Flat Connector as shown here.  Trailers that use this are usually fairly light weight and don’t have brakes or other power accessories.  It’s the most common style for “consumer” type trailers.  Smaller utility trailers, light boat trailers, small campers, off-road trailers and many more use a 4-Pin Flat connector.

Lighter Duty Trailer (No Brakes) = Use a 4-Pin Connector.

1.  White = Ground  (See White Wire Notes below.)
2.  Brown = Tail Lights, Side Markers and Running Lights  (See Brown Wire Notes below.)
3.  Yellow = Left Turn Signal & Left Brake Light
4.  Green = Right Turn Signal & Right Brake Light

Using the Trailer Wiring Diagram and Connector Application Chart below, the 4-Pin connector only has the first 4.  The rest you can ignore.

 

Trailer Wiring 5-Pin Connector

5-Pin Flat Connector

(Round style 5-Pin Connectors also exist.)

Trailers with capacity over 3000# Total Gross Trailer Weight should have brakes.  That’s not mandatory everywhere, but it’s a good idea.

If a trailer has brakes, then it needs a connector with at least 5 pins.  The 5th pin, a blue wire, gives power to operate (or disable) the trailer brakes.

Traditional Trailer + with Brakes = Use a 5-Pin Connector

1-4  Wire the first 4 pins (White, Brown, Yellow, Green) just like the 4-pin connector above.
5.  Blue = Electric Brakes or Hydraulic Reverse Disable (See Blue Wire Notes below.)

In the Trailer Wiring Diagram and Connector Application Chart below, use only the first 5 pins, and ignore the rest.

If your truck has a built-in 7-pin socket, but you only need 5 pins, use a 7-pin connector anyway (see below), and just leave out the last 2 wires.  It accomplishes the same thing for 5 wires, but with a connector compatible with your truck.  The 5-Pin flat connector above is nice for ease of handling, but if your vehicle already has a 7-pin, just use it.  It’s OK, within the standards, to customize for your situation.

 

Trailer Wire Connectors - 7-Pin RV Style

7-Pin Connector

For trailers that have a little more going on electrically, we recommend 7-pin connectors.  The 2 added pins are for Auxiliary Power and Back-up Lights.

Expanded Use Trailer + with Brakes, Aux Power & Back-up Lights = 7-Pin Connector.  (6-Pin Connectors also exist, but they are less common.)

1-4  Wire the first 4 pins just like above, and the 5th line goes to the brakes.
5.  Blue = Electric Brakes or Hydraulic Reverse Disable (See Blue Wire Notes below.)
6.  Red (or Black) = 12V Auxiliary Power (See Red Wire Notes below.)
7.  Purple = Back-up Lights (Sometimes another color.)

7-Pin Connectors like the one pictured here are very common for RV’s.  Other styles exist — though the pin-outs are often different.  Several industrial styles are similar and definately use different pins.

It is OK to leave a pin or two blank (unused and unconnected).  If you want Auxiliary Power, for instance, but don’t have and don’t want back-up lights, then just leave that wire out.  A blank spot (unconnected pin) doesn’t hurt anything.

 

The Trailer Wiring Diagram and Connector Application Chart

 

Trailer Wire Color Codes  –  Colors Coordinate With Trailer Wiring Diagram
Connector Style Pin Function Color Description
7-Pin 6-Pin 5-Pin 4-Pin 1 Ground White Ground for all trailer electrical functions.
2 Tail Lamps
Running Lights
Side Markers
Brown Power for all normally ON lamps.
Tail, Running & Side marker lights.
3 Left Brake Light
Left Turn Signal
Yellow Multi-function signal for the
Left Side Rear Tail Lamp
4 Right Brake Light
Right Turn Signal
Green Multi-function signal for the
Right Side Rear Tail Lamp
5 Brake Blue Electric Brakes Control Power
or Hydraulic Brake Disengagement (5-Pin only)
6 Battery Red
(or Black)
Vehicle Power +12V
For trailer battery charging and accessories.
7 Back Up Purple Back Up Lamps on Trailer Tail Lights
/ Hydraulic Brake Disengagement

Typical Trailer Wiring Diagram and Schematic

Typical Trailer Wiring Diagram and Schematic

These 2 wiring diagrams fit the needs for most trailers.  The image above shows a single axle trailer, and next image shows Tandem Axles.  Only the brake wiring is different.

Use only the wires that are needed, ignore the others.  For example, if you don’t need Auxiliary Power, leave it off — just leave that pin blank, but don’t change the pin numbers because one of the functions are not used.

Trailer Wires For Tandem Axle Trailers

Trailer Wiring Diagram Notes:


 

Wire Size Notes:

Many different sizes of wires are available.  Typically wire sizes list by “Gauge” — a smaller number is a thicker wire.  We recommend 16 gage and larger for lighting.  Then, for power hungry things like brakes, use a thicker wire size, like 14 gauge or 12 gauge.  Same for Auxiliary Power.

Lighting circuits with low power lights like LED’s have low power requirements, so even with a lot of lights, they don’t use much power.  For lights, a relatively small wire gage works.  We recommend 16 gage and larger, not so much because of the power requirements, but because they are stronger, more robust, and have more surface for splice connections.  Again, it’s worth the small additional expense.

 

Lights:

We recommend sealed and submersible LED lights for just about everything.  Yeah, most trailers are never submersed, but almost all get very wet like in heavy rain or when washing.  Pay the extra dollar or two and get the higher quality lights.  They are worth it.

 

White Wire Notes:

The White Wire is the “Ground” or the negative wire connecting to the vehicle battery “minus” side.  The trailer wiring diagram shows this wire going to all the lights and brakes.  And, it must connect with things (if included) that use the Aux Power and Back-up lights too.

Some trailer builders just connect this wire to the frame, then connect the ground from all the other lights and accessories to the frame as well.  While this usually works, the ground portion of the circuit is often the root of trailer electrical problems.  To avoid some of those issues we recommend running the white wire with the others and connecting the ground from each light directly to the White.  It is a little more work, but it can save big headaches later.

We also recommend connecting the white wire directly to the trailer frame (in addition).

Size:  This wire should be at least as big as the largest wire in your harness.  If the lights are LED (low power), then the white wire can also be small, but if you have electric brakes or auxiliary power, this wire must be larger.

 

Brown Wire Notes:

The Brown Wire goes to the lights that are always ON as you travel.  These are the running lights, the low intensity portion of the tail lights, side markers, corner markers, and if used, the sets of 3 lights central in front and back of the trailer.  Check local laws for requirements on lights.

The typical sets of 3 lights central in the trailer are not in the above diagrams as they are not normal for low or utility type trailers.  If you need them or want them, the brown wire feeds them (and the white for negative).  Tiny Houses may or may not need the 3 lights, but again, check local laws.

Size:  The Brown wire only feeds power to lights, so it should be sized for the power requirements of your lights.  For a utility trailer, that is probably not much power, so a smaller gage is OK.  For a large enclosed trailer with lots of running lights, consider a larger gage.

 

Blue Wire Notes:

Some places label the 5th pin for “Reverse Lights”.  Certainly that works, but make sure to note it on the trailer because Blue is the color for brakes.  Also, some trailers with hydraulic brakes use this 5th pin to disable the brakes when the vehicle is reversing.  (If you do this, connect the blue wire to the reverse lights on the vehicle side.  And, be sure to note what you’ve done.)

Please note the 5th pin is not as standard as the first 4.  Be careful when using a 5-pin connector.  Be sure the car wires match functions of the trailer.

On the vehicle side, for electric brakes the blue wire goes to the brake controller.  Many styles are available, so find one that works for your vehicle.

Case Example:  To solve issues with tow vehicle wiring that is different from my trailer (for instance when a friend wants to borrow it), I simply have a short adapter that connects the 5-pin harness to a 4-pin vehicle and the trailer goes without brakes.  It works because the trailer is not big or heavy — and lightly loaded it does not require brakes.  I just tell the person borrowing it that the load capacity is 3000# (even though I know that’s without brakes because the true load capacity is 5000#.)  Another way is to have an adapter that goes from the trailer 5-pin to a standard 7-pin (with 2 wires left blank).  That way the trailer brakes are ready, if the tow vehicle has a 7-pin connector.

Size:  Don’t skimp on wire size for your brakes.  For a single axle, 14 gage is good, but for tandem axles, use 12 gage wire.

 

Red Wire Notes:

The pin for Aux Power is usually with a Red Wire, but sometimes it’s different like Black.  Sometimes we call it ‘Aux Power’ or ‘Auxiliary Power’ or ‘Accessory Power’ or ‘Batt 12V+’.  Whatever the name it provides a tap for access to the tow vehicle positive power.  Typically, auxiliary power is for charging RV batteries, interior lights, power for accessories, etc.  The routing is not on the above Trailer Wiring Diagram because it is optional and different for every trailer.

If you don’t need separate power on the trailer, just leave that pin out.  If you do use it, then be sure you protect the vehicle electric system from shorts and from completely draining the tow vehicle battery.

Size:  Use a wire size appropriate for the power demands.

 

Wire Routing Notes:

The diagrams above give one example of routing direction – starting at the tongue connector and wrapping around the trailer.  Other people suggest splitting the wires near the tongue and routing down both sides with Right and Left specific.  Either approach is fine.

I like the wrap around approach a little more because it makes a ‘trunk’ that distributes power as it goes.  It also keeps the wires all in one group as they traverse along the tongue — which makes them easier to protect.  The amount of wire is almost identical for both.

We do recommend protection on the wires — a covering of some sort.  Flexible conduit, plastic conduit, or other approaches are great.  The routing covering does not need to be watertight, but do consider weather protection when splicing into the wires for each light.

 


Another really good source of information with a trailer wiring diagram comparing different styles of connectors is at etrailer.com  You can get most trailer electrical items at any trailer parts store, or online.  Etrailer and JohnsonTrailerParts.com are good sources.