How to Wire a 7-Way Trailer Plug (It's Easier Than You Think)

7-ways are some of the most common harnesses found on trailers. 7-ways provide the required running lights, turn signals, brake lights, and ground for the trailer. In addition, they provide three additional pins for a 12V hot lead, electric brakes, and reverse lights.Trailer wiring can be one of the most intimidating components of your towing setup, but it doesn't have to be. Most of us aren't electricians, but that doesn't mean wiring a trailer is beyond us. We'll walk you through the trailer-wiring process for 7-way plugs below, so you can get your trailer up and running in no time.
Watch video installation of 7-way connector (vehicle end)
Watch video installation of 7-way connector (trailer end)
7-Way Round Connector

What is the Color Code for 7-Way Trailer Wiring?

Before we get into the step-by-step walkthrough, we'll first go over the standard color code for 7-way wiring harnesses. This will make it easier when it comes time to make your wiring connections.The color code for wiring harnesses varies between industries. In particular, SAE (Society of Automotive Engineers) standard wiring differs from the RV industry's standard wiring. The standard for both is as follows:SAE Standard
  • Green: Right turn/brake light
  • Yellow: Left turn/brake light
  • Brown: Tail/running lights
  • White: Ground wire
  • Blue: Brake controller output
  • Black: Battery hot lead
  • Purple: Reverse lights
RV Standard
  • Green: Tail/running lights
  • Yellow: Reverse Lights
  • Brown: Right turn/brake light
  • White: Ground wire
  • Blue: Brake controller output
  • Black: Battery hot lead
  • Red: Left turn/brake light
What Size Wire Gauge is Used for a 7-Way Wiring Harness?The minimum suggested wire size for a 7-way trailer plug is 16 gauge for the turn signals, brake lights, reverse lights, and running light wires. The suggested minimum for the ground, brake power, and battery hot lead wires is 12 gauge.
7-Way Flat Connector Diagram
7-Way Plug RV Standard Diagram
7-Way Plug Heavy Duty Diagram
7-Way Trailer Wiring Color Code Traditional SAE
7-Way Trailer Harness Diagram - Traditional SAE
7-Way Trailer Wiring Color Code RV Standard
7-Way Trailer Harness Diagram - RV Standard

Wiring a Trailer with a 7-Way: Step by Step

Now that we have an idea what our wires do, let's get into the step-by-step process of wiring your trailer with a 7-way connector.

Trailer Side

Step 1: Prepare for Trailer Wiring Installation

Start by making sure you have everything you need to wire your trailer, such as:
If you need to replace one or more of your lights, you can purchase trailer lights or use a complete trailer light kit, which will come with the necessary wiring included. Remove old lights before beginning the new installation.If you don't need to replace your lights, you can simply use a 7-way harness.
Trailer Junction Box
Pictured: junction box

Step 2: Locate or Install Junction Box

If you're replacing existing wiring and your trailer already has a junction box, locate it (it's typically near the front) and remove the cover. Remove the nuts from each terminal.If you're installing a new junction box, find a suitable location for installation, such as on the inside edge of your trailer tongue. The box should be close enough for your wires to reach, but away from any components it could interfere with.If you don't have a junction box and don't want to install one, you can simply tie the new wiring harness in with your trailer's existing wiring.
Junction Box on Trailer
Locate the junction box and remove the nuts from each terminal.

Step 3: Make Trailer Connections

NOTE: Check the manufacturer's instructions for any wiring harness you use. The instructions will identify the function of each wire. You won't always be able to rely on color alone to match up wire functionality.
WITH JUNCTION BOXIf you have a junction box and are simply adding a new 7-way connector, remove the nuts on each junction box stud. If you're replacing existing wiring, go ahead and remove the old wiring now by removing each terminal from the junction box.Cut any excess wiring from your new cable and strip the wires of insulation using a crimper. Next, crimp ring terminals onto the new wiring. Place the terminals onto their corresponding studs in the junction box (ground wire to ground wire, brake wire to brake wire, etc). When finished, reinstall the nuts.Secure any excess wire with wire clips.
Trailer Junction Box - Remove Wiring
Trailer Junction Box - Strip Wire Insulation
Trailer Junction Box
Trailer Junction Box Wired
WITHOUT JUNCTION BOXIf you're not using a junction box and are simply connecting the new 7-way to your existing wiring, you can use butt connectors and a heat gun to make your wiring connections.Secure any excess wire with wire clips.
Butt Connector Trailer Wires

Vehicle Side

Now that our trailer is hooked up, it's time to wire our vehicle. Let's get started:
Step 1: Prepare for Vehicle Wiring InstallationIF YOU HAVE A 7-WAY CONNECTORIf your vehicle already has a 7-way connector, then great! Simply plug the trailer-end connector into the vehicle-end connector, and you're ready to roll.IF YOU HAVE A 4-WAY CONNECTORIf your vehicle has a 4-way connector, the easiest way to add a 7-way is with an adapter kit like the ETBC7, which includes everything you need to convert your 4-way into a 7-way. Confirm the function of your vehicle wires with a circuit tester prior to connecting. Activate each function (turn signal, brake lights, etc.) in turn and check for a corresponding signal from the circuit tester to make sure you have the right wire.IF YOU HAVE NO CONNECTORIf your vehicle lacks any kind of connector, the easiest way to install a 7-way plug is to install a 4-way and use an adapter. For help installing a 4-way connector, view our how-to guide here.Also take the time to confirm the function of your vehicle wires if you are splicing or clamping wires together. Check your vehicle's owner manual or use a circuit tester to confirm each wire's function prior to connecting. You can usually tap into your vehicle's wiring system with a circuit tester at the rear of the vehicle, behind the taillights. Activate each function (turn signal, brake lights, etc.) in turn and check for a corresponding signal from the circuit tester to make sure you have the right wire.
4-way to 7-way adapter
If you have a 4-way plug, add a 7-way with a 4-to-7-way adapter
Circuit Tester in Use on Trailer
Use a circuit tester to confirm wire function
Step 2: Make Vehicle ConnectionsYour wiring harness will either plug into, clamp onto, or splice into your vehicle's existing lighting. The type of connection depends on what is available for your vehicle model. We'll go over each process in more detail below.Some general notes and tips:
  • If the vehicle connector is under the vehicle, use a mounting bracket to attach it to the vehicle. This will help prevent damage that may occur if the connector is left dangling.
  • Use a small amount of grease on all electrical connections—the plugs on your automobile and the connector itself—to help prevent corrosion.
4-WAY TO 7-WAY ADAPTERSAn adapter will plug into your existing 4-way plug and provide two additional pins for your trailer brakes and battery lead. We've put together adapter kit # ETBC7, which includes everything you need to install a 7-way adapter, including the connectors, wiring, and circuit breakers.Once you plug the adapter into your existing 4-way, you will have an additional three wires that must be connected.One of these wires (typically blue but sometimes black) will run to your electric brake control power output. Another wire (typically black or red) will run to the positive terminal of the battery via additional wire and butt connectors.Use a circuit tester to test for wire function before connecting.
How to Wire Electric Trailer Brakes Diagram SAE
How to Wire Electric Trailer Brakes Diagram RV Standard
Watch video installation of 4-way to 7-way adapter installation
PLUG-IN STYLESome vehicle manufacturers essentially "pre-wire" your vehicle so that your wires are easily accessible for connection. T-connectors such as this one simply plug into your vehicle's existing wiring, no cutting or splicing required.The plug-in location is typically near the taillights, underneath the vehicle, or behind the paneling in the back cargo area. You may have to remove your trim access cover, scuff panels, taillights, etc. in order to access the plug.
Watch video installation of T-connector
SPLICE-IN STYLEHardwire kits aren't quite as convenient as the other styles, but splicing into your vehicle wiring is actually less difficult (and scary) than it sounds.After confirming your wire functions (using your owner's manual or circuit tester), connect your wires using one of three methods.Soldering: This is the best way to connect wires. Simpy solder your wires together for the strongest, most reliable connection using a soldering gun. Use heat shrink tubing to protect the soldered connection.Butt Connectors: If you aren't comfortable soldering wires, heat shrink butt connectors and a heat gun are the next best thing.Quick Splices: The quickest, easiest way to connect wires is by using a quick splice. Quick splices force a metal piece into two separate wires, thus connecting the circuit. NOTE: Quick splices are the easiest—but least reliable—wire connection method.
Trailer T-Connector
When FinishedAfter wiring up your trailer and vehicle, it's a good idea to plug the two ends together and test the function of each wire. Make adjustments as needed. If everything lights up when it's supposed to, you're good to go!
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Questions and Comments about this Article

Keith

I have an enclosed trailer with great 12 volt DC interior LED lighting. It sits next to the garage and I do a lot of work in it. What’s the best way to Connect 120 volt AC power to run the 12 volt lighting? Currently I just hook up the truck when I need to use it but this is getting old. Is there a way to run a converter to the 7 way plug on my trailer? 98095

Reply from Jon G.

In order to accomplish this you're going to need a converter like part # 34266167 and a battery. The battery will send the power to the lights while the converter actually changes the power from AC to DC so it works with your lights. Then you can simply run an extension cord to make sure the battery is charged. If you only need the lights for a minute you won't have to hook anything up. 73169

Reply from Keith

@JonG Thank you for the response. I will give this a try. 73270

Ross T.

I have a 2002 Chevy Silverado 3500 at my company. Its got a trailer attached to it, but doesn't work. I've been trying to fix it and I notice that its wired wrong(7 wire harness). Also, there's an auxiliary battery near the trailer neck which is connected to the junction box ( meaning two wires from the truck harness itself aren't connected to the junction box). All the wires from the trailer are connected to the junction box. Is this auxiliary battery necessary? 97491

Reply from Jon G.

If it's a smaller battery and the trailer has electric brakes then that would be your breakaway battery box. It is necessary because it applies the brakes on your trailer in the event that the trailer becomes uncoupled. 72836

Dord

Hi there, For people that already have a 4-way connector (e.g. by using Curt 55370) how would they go converting that to use 7-way connector and in particular something like Hopkins HM11141144? BTW HM11141144 is the best connector I saw with OEM like quality. I got that the +12V and ground for HM11141144 will come from the battery. Also the backup trailer lights will come from the car backup lights. I see how all the other trailer lights will come from 55370. What is not clear is what should be done with the trailer ground wire on the 4-way connector out of 55370. It cannot be used for trailer ground because it cannot carry enough Amps (55370 it's protected by a relatively small fuse). For trailer ground I already got the 10 awg wire from battery. Should I just ground that wire? Thanks 85896

Reply from Chris R.

Since you already have a 4-Way installed, and have your eyes on a 7-Way connector, all that you need is the Wiring Kit # 5506 . This includes the wiring needed to activate the new circuits on the 7-Way (brake output and 12V). The existing ground for the 4-Way will remain where it is. 67899

Kevin

I have a 2013 Chevy Pickup 4-wheel drive. I hooked my trailer brake controller. It was working fine. Then I tapped into the power wire going to the controller, to power an emergency light. Just recently I reworked the wire connection and shorted out the power wire. Now I do not have power to the controller and no power at the two terminals at the front of the fuse box under the hood. The fuses look good at the front of the fuse box. Would anyone have an idea what the issue is. 84764

Reply from Chris R.

I have to imagine you popped the 12V brake controller fuse. I know you mentioned checking this, but it's all I can think of that would cause this. Check your Silverado's owner's manual to ensure you're looking at the right fuse location for that circuit. I also don't recommend tapping into that wire for anything else. 67898

Reply from Kevin

@ChrisR Chris. I apologize for my error. I have check the fuse a couple of times. But not with a meter. I finally checked the circuit with multi- meter and replaced the fuses and the system works. I could not see where the fuse was bad. Thanks for your time and comments. Kevin 68040

Reply from Chris R.

@Kevin Awesome! I'm glad you were able to get it fixed. 68330

Doru

I saw both 20A and 30A specified for the +12V Red wire with the mention that 20A will only be good for 2 and 4 brake magnets. My car towing weight capacity is max 3000lb and 200lb tongue weight - small, so the campers I'm looking at are small. So 2-4 magnets will be enough braking for me. Is there a way to find out how many magnets a trailer has? Is there a rule of thumb like one wheel one magnet? 77873

Reply from Chris R.

Each brake assembly uses a magnet - so the amount of magnets is equal to the amount of wheels on the trailer (assuming all the axles are equipped with brakes). 63395

Doru

I would like a clarification on the wiring. I have a car that has a hitch harness in the trunk (power directly from battery, turn RH, turn LH, brake light, tail lights, ground directly to car body). I do not want to install any brake controller in the car. I plan to use a wireless one like Curt C51180. So in this case do I need to hook anything up on the Blue wire (electrical brake controller power output)? 77871

Reply from Chris R.

The blue (brake output) wire isn't needed at all for the Curt Echo # C51180 . It sounds like you've already got the 12V circuit active, so you don't need to change anything with your existing setup. Just plug the Echo in, pair it to your smart phone, and you're ready to go. 63393

Anonymous

Very nice article. I would suggest adding a note that for the section at the end Soldering: This is the best way to connect wires. Simpy solder your wires together for the strongest, most reliable connection using a soldering gun. Use heat shrink tubing to protect the soldered connection. that its always important to have the right length enough to give a good overlap of the insulation after soldering and size just a little larger than the insulation, for a tight fit of heat shrink tubing already on the wire prior to soldering them together!!! Its easy to solder first and then go Hmmm... now how do I get this tube over my wire? 64759

Roy B.

I would like to know how to best replace 2012 jayco 266RKS PULL behind 2 axel brake wires at wheel hub that tire blowouts made into spaghetti. 64752

Reply from Jacob H.

You can use 12 or 16 gauge wire to make this repair. We sell each of them by the foot I will provide a link for both. 54986



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