How To Wire A Trailer . . . Trailer Wiring Diagram . . . And More
So many wires . . . So many colors . . . And, 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 all the wires right!
There are several standards for trailer wires, and if you search, you’ll find a different Trailer Wiring Diagram for each. Each standard has its purpose, so please don’t just make it up as you go. Don’t make a weird wired trailer. Follow these guidelines and make it right!
The wiring approach for you depends on your electrical needs. To start, every trailer needs lights – brake lights, turn signals, and tail lights. Some also need side markers and running lights. Some brakes 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 for different connector styles and wire arrangements. It can get confusing, so if you don’t already have a specific standard in mind, follow these.
4-Pin Flat Connector
At a minimum, all trailers need at least 4 functions: Tail lights, Brake lights, Left & Right signals. 4 wires will give 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. Small utility trailers, light boat trailers, little campers, off-road trailers and many more use this traditional 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
Please see the Trailer Wiring Diagram and Connector Application Chart below. The 4-Pin connector only has the first 4 items listed. The rest you can ignore.
5-Pin Flat Connector
(Round style 5-Pin Connectors also exist.) Typically, the 5th wire (blue) is for electric trailer brakes, however, that is not always true. The 5th pin is not as standard as the first 4.
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 wires. The 5th pin, a blue wire, gives power to operate the trailer brakes. (The 5th wire can also be used to disable hydraulic brakes when in reverse.)
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 the first 5 pins, and ignore the rest.
If your truck has a built-in 7-pin socket, but you only need 5 of the pins. Use the 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 that’s compatible with your truck. The 5-Pin flat connector above is nice for easy handling, but if your vehicle already has a 7-pin, just use it. It’s OK, within the standards, to leave out wires for your custom situation.
Also, worth noting, When Does My Trailer Need Brakes?
For trailers that have a little more going on electrically, we recommend 7-pin connectors. The 2 added pins are typically 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 are very common for RV’s and other bigger-ish trailers. This is the style we recommend. Other styles exist — though the pin-outs are often different. Several industrial styles are similar and definitely use different pins.
It is OK to leave a pin or two blank (unused and unconnected). For instance, looking at the trailer wiring diagram, if you want Auxiliary Power, but don’t have back-up lights, then just leave the purple 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|
|7-Pin||6-Pin||5-Pin||4-Pin||1||Ground||White||Ground for all trailer electrical functions.|
|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)
|Vehicle Power +12V
For trailer battery charging and accessories.
|7||Back Up||Purple||Back Up Lamps on Trailer Tail Lights
/ Hydraulic Brake Disengagement
Single Axle Trailer Wiring
Tandem Axle Trailer Wiring
Typical Trailer Wiring Diagram and Schematic
The 2 above wire diagrams fit the needs of most trailers. The first image shows a single axle trailer, and the second, wiring for Tandem Axles. Only the (blue) brake and (white) ground wires are different. You can expand the same conditions for more axles.
Use only the needed wires, and ignore the others. For example, if you don’t need Auxiliary Power, just leave it out. If the axles do not have brakes, then no need for that. Don’t change pin numbers or wire positions if a function is not used — just leave the pin blank (not connected).
Three Center Marker Lights
The above trailer wire diagrams don’t show the triple set of marker lights central on the front and back. Some trailers need them, and some do not. Check local ordinances for requirements.
Some trailers require 3 center marker lights – located central on the back, maybe low (on the bumper) and/or high, and maybe on the front. Check legal requirements to see if they are required in your country or jurisdiction.
For trailers in the USA: Typically a red 3 light set is required on the back, if the trailer is 80″ or wider – or – if over 10,000 lbs GVWR. Also, near the top in the back if taller than a certain amount. An amber 3 light set is required near the top in the front, if taller than a certain amount (usually some amount over the height of the tow vehicle). Again, check regional requirements.
Typically the 3 center marker lights are at a high point on the trailer — like above the back doors for an enclosed cargo trailer. They are fine on the back bumper of a flatbed trailer, even when the load is much higher. There are lots of extras in the laws (like top corner markings), so find out what you need for your specific trailer.
If you need the more marker lights, connect them on the Brown and White wires just like the side marker lights. (See the partial trailer wiring diagram.) These do not require additional connections at the hitch, just more wiring within the trailer. These lights should be ‘on’ basically all the time.
Side Note: Reflectors
In addition to the three center marker lights, most trailers over 80″ width require reflectors or reflective tape in alternating red and white on the sides and back. There are a lot of regulations here for height, and GVWR, especially when trailers are longer than 20′. I’m not sure about requirements outside of the USA.
Check your local jurisdiction so you can mark and light your trailer properly. Reflectors and reflector types change by jurisdiction. To some, this is overkill, but even if it is, making it right can save you a ton of legal hassle and trouble.
Trailer Breakaway Wiring Diagram
Many trailers are required to have a Breakaway System on board. Basically, this is a way of applying the trailer brakes if the trailer comes disconnected from the tow vehicle. In many parts of the USA, trailers over 3000 lbs GVWR need a breakaway kit, so check your local laws.
If you have electric brakes (or electric over hydraulic or some others), then it will involve the trailer wiring. Here is a partial wiring diagram to include your trailer breakaway system. Since there is a lot to discuss, we have an entire article about breakaway kits with lots more information. In the meantime, use this diagram to guide the wiring of the system. Superimpose this on the images above to see how it all comes together.
The breakaway system usually resides in, on, or under the front part of the trailer. The pin pull switch is near the hitch. The system hooks into the electrical system by connecting Auxiliary Power (Red wire +12 VDC) to keep the battery charged, the Brakes (Blue wire) to actuate the brakes, and Ground (White wire) to complete the circuit. Again, please see the article about breakaway systems for a lot more information.
Where do the wires go? Now that we have the trailer wiring diagram and some definition for connectors, where do the wires actually go?
Nestle the wires into and around the frame where practical for protection. We do recommend protecting the wires with a covering of some sort. The cover is not in the trailer wiring diagram, but flexible conduit, plastic conduit, or other approaches are great. A covering does not need to be watertight, but do consider weather protection when splicing into the wires. For tips on wiring, splicing, routing and protecting, see our post on trailer lights and wires. See more in the Wire Routing Notes below.
This photo shows an ideal way to handle trailer wires. While the flexible sealed conduit nestles in and secures to the frame, it protects the wires from snags and from weather. Great job on this one.
Trailer Wiring Diagram Notes
Many different sizes of wires are available. Typically wire sizes are 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 still recommend 16 gage and larger, not so much because of the power requirements, but because the wires are stronger, more robust, and have more surface area for splice connections. It’s worth the small additional expense.
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. Trouble free operation with higher quality lights make them worth it.
White Wire Notes:
The White Wire is the “Ground” or “Negative” wire connecting to the vehicle battery “minus” side. The trailer wiring diagram shows this wire going to all the lights and brakes. Also, 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 all 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 only lights are in the circuit, and the lights are LED (low power), then a small white wire is acceptable. However, 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, and corner markers. Also, if used, the sets of 3 lights central in front and back of the trailer. Check local laws for requirements on which lights your trailer needs.
While the typical sets of 3 lights central in the trailer are not in the above trailer wiring diagram, they are important in some situations. They are not normal for smallish DIY utility type trailers. However, if you need them or want them, the brown wire feeds them too (and the white for ground). 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 size it 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.
Yellow and Green Wire Notes:
The two wires, Yellow and Green, handle double duty for function. They activate lights for both Turn Signals and for Brakes. It’s the same physical “light” at the rear of the trailer, but it serves 2 functions. The wiring in the vehicle sorts out how those signals are sent.
The Yellow wire handles the Left side – left as you look at the back of the trailer. (Or, in a different way of thinking, left as you are sitting in the driver’s seat of the tow vehicle.) The Green wire handles the Right side. See the above trailer wiring diagram for a visual.
Size: See the above discussion about Wire Size. We recommend 16 gage and larger for lights, and we recommend LED lights.
Blue Wire Notes:
Typically, the blue wire is for electric trailer brakes, however, this wire is not as standard as lights and ground.
For electric brakes, on the vehicle side the blue wire goes to the brake controller. Many styles of brake controllers are available, so find one that works for your vehicle. On the trailer side, the blue wire goes to the brakes and breakaway circuit. (See the trailer wiring & breakaway diagrams above.)
Here are some other ways the blue wire is sometimes used.
- Some trailers with surge hydraulic brakes use this 5th pin to disable the brakes when the vehicle is reversing. This is not in the trailer wiring diagram above. If you do this, then connect the blue wire to the reverse lights on the tow vehicle side, and be sure to note what you’ve done. Better yet, use a purple wire instead, and label it.
- Some places label the 5th pin for “Reverse Lights”. If you do that, then make sure to note it on the trailer, because it can cause a lot of trouble if connected wrong.
Just be careful when using a 5-pin connector. Make sure the car wires match functions of the trailer.
Compatibility 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 with a light load it does not require brakes. I just tell the borrower the load capacity is 3000# (even though true 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 connects to the tow vehicle positive, DC power. Typically, auxiliary power is for charging the Breakaway battery, RV batteries, interior lights, power for accessories, etc.
The extent of routing for the Red wire is not on the above Trailer Wiring Diagram because it is optional, and different for every trailer. In the Breakaway wiring section, the schematic there shows how the Breakaway battery box connects to the Red. That maintains the battery charge.
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 (use a fuse or circuit breaker). Also, it’s good to protect the system from completely draining the tow vehicle battery.
Size: Use a wire size appropriate for the power demands. If it’s just for charging the breakaway battery, then 16 gage is fine. If you’re powering up more batteries or lights in the trailer, then use 14 or even 12 gage. Don’t overload this wire. If you do need large amounts of auxiliary power, use a generator or install special wiring from the vehicle alternator.
Wire Routing Notes:
The trailer wiring diagram above gives one flavor for routing direction – starting at the tongue connector, then wrapping around the trailer. Other people suggest splitting the wires near the tongue, then routing down both sides — Right and Left specific. Either approach is fine.
While wire routing is a personal preference, 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 so they are easier to protect. The amount of wire is almost identical for both the split and wrap around approach.
Protecting The Wires:
Hollow frame members are often the route for wires. For instance, if the frame is constructed of rectangular steel tube, then putting wires down the tube seems like an easy way to protect them. While this is true, it also means that you can’t cap (seal) the ends of the tube to keep moisture out. That’s a trade-off to consider, but it does not have to be all or nothing. On my last trailer, I routed the wires through the tongue tube, then outside the main frame members (tubes) so they can seal. Wire and light connections are outside of the frame tubes under the trailer bed.
When running wires consider the possibility of changes down the road. If you’re sure changes will never happen, button things up super tight. If you think changes might happen later, then leave access to the wires. For example, for a little camper, you might think Aux Power is on the “I’m never going to use that” list. But in a few years, you might find the solar doesn’t always cut it, and Aux Power is suddenly desirable. By leaving access to the wire routing, running the additional wire is not so difficult. Food for thought.
Another good source with a trailer wiring diagram comparing different styles of connectors is at etrailer.com Secondly, you can get most trailer electrical items at any trailer parts store, or online. etrailer and JohnsonTrailerParts.com are both good sources for electrical parts.