Trailer Wiring Harness Design

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Trailer Wiring Harness Design

Postby Andrew Herrick » Fri Jul 07, 2017 9:52 am

There's a lot of public information about power distribution in the teardrop trailer, but, seemingly, not so much information on designing a high-quality underbody wiring harness to run your marker, clearance, license, and stop/turn/tail lights. Not that I have all the answers! I'm still learning.

I've played around with different methods.

- I've run surface-mounted wiring inside the camper using Legrand's Cordmate II raceway system. Looks pretty. Works well. Easy to spend several HUNDRED dollars on a full system.
- I've done the Harbor Freight thing and just run bare jacketed wire along the inside of the trailer frame, attached it with wire clips, nutted all the connections, and hoped to God it survived!
- I've run wiring through metal conduit welded/bolted to the trailer frame. Very clean, very safe, but not very accessible or versatile. I prefer to standardize my designs; I don't want to be installing new conduit every time I shift the axle location or add electric brakes.

So I've moved towards an automotive-style wiring harness where all the wiring (16 gauge) is encased in polyethylene split loom and no connection is left exposed to the elements. The wiring harness was held to the inside of the trailer frame with adhesive-backed nylon loom clamps. Butt connections were made with heat-shrink crimp terminals. Other connections were made with quick splices or T-taps (I use LED lights, so I don't need much metal-to-metal contact) and then encased the terminal in a Tee connector. The Tee connectors were really nifty, but I could only get one terminal to fit within one connector. The trailer connector was a 7-way trailer connector completely encased in molded plastic - very rugged! The connector went to a 7-way trailer screw terminal junction box mounted on the trailer tongue, which then delivered power to all the components. Oh, and I ran a separate ground wire rather than using the trailer frame as the ground.

That system worked fairly well, but the Tee connectors were a pain. And I started to worry about the adhesive-backed clamps on the paint - even though they did have quite a grip! So my latest addition is to use plastic junction boxes at every trailer light. And to use standard vinyl-coated loom clamps to hold the wiring harness, connected to the trailer frame using either Christmas Tree panel fasteners or nylon snap rivets. No screws to rust or vibrate loose.

So, that's where I'm at. I feel like I'm getting close to a rugged, weather-proof system that is versatile enough to be used for any camper, big or small, with reliable heat-shrunk connections and easy access in the future. Still chipping away at the details.

What other systems do you use? Anything to add/subtract from mine?
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Re: Trailer Wiring Harness Design

Postby John61CT » Fri Jul 07, 2017 10:06 am

Great stuff, thanks!
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Re: Trailer Wiring Harness Design

Postby QueticoBill » Fri Jul 07, 2017 10:53 am

What you describe is about the direction I was thinking but use non-slip tubing - 1/2" ENT wireway. Maybe the slit is just as protective and easier.

What type of wire? Any reason not to use plain old THHN?
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Re: Trailer Wiring Harness Design

Postby H.A. » Fri Jul 07, 2017 1:40 pm

Seems like sound electrical/mechanical practice.
Fwiw, I dont care for stickybacks if exposed to weather.
I agree they grip tight when new. But over time, even from the best manufacturers seen them fail adhesion. Agree with use of clamps of some kind & affixed via screws, christmas trees, etc.
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Re: Trailer Wiring Harness Design

Postby GuitarPhotog » Fri Jul 07, 2017 1:46 pm

My trailer is all-aluminum, painted brown (not my doing). Stickybacks don't stay stuck to hot metal. I ended up screwing cable clamps to the frame to hold my wiring harness.

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Re: Trailer Wiring Harness Design

Postby M C Toyer » Fri Jul 07, 2017 4:00 pm

On my latest trailer, a 4 x 8 with a removable shell to transport my motorcycle, all the wiring for the tail, stop and clearance lights runs through 1/2" pvc electrical conduit or clear vinyl tubing The conduit runs through the drilled frame crossmembers and tongue.

All connections are inside plastic electrical boxes which are mounted on the backside of the bracket or frame member for the light fixture. The short length of wiring to the connector is encased in clear vinyl tubing sealed to the conduit with heat shrink tubing.

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Last edited by M C Toyer on Fri Jul 07, 2017 7:33 pm, edited 1 time in total.
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Re: Trailer Wiring Harness Design

Postby Andrew Herrick » Fri Jul 07, 2017 4:16 pm

Thanks for the helpful replies (and forthcoming pictures), all.

There's a part of me that wants to just use conduit - whether PVC or EMT - but once I start thinking about all the drilling, hanging, clamping, cutting, my eyes start to glaze over :p It seams unbeatable for ruggedness, though.

And good to hear from those with experience with the "stickyback" clamps. I'll have to find another use for the other 30 in my bag.

As far as type of wire goes ... I guess it's a question of how much overkill you want in the system. I think that automotive primary wire has a more abrasion- and heat-resistant coating than regular stranded THHN? Then again, we're not building hot rods.

You can also buy bonded trailer cable in 7, 6, 5, 4 and 3-conductor varieties. Some come with a molded PVC jacket around them. They look like you could beat 'em with a hammer. The downside is price, especially if you don't buy in bulk (500+ feet). And so far I haven't founded any conventional 4-wire "Y' harnesses with the PVC jacket. Just straight cable.
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Re: Trailer Wiring Harness Design

Postby John61CT » Fri Jul 07, 2017 5:51 pm

Marine-rated wiring is good stuff, also proper gas-tight crimping.

genuinedealz.com will do it all for you at what I consider bargain pricing.
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Re: Trailer Wiring Harness Design

Postby M C Toyer » Fri Jul 07, 2017 8:17 pm

Andrew Herrick wrote:Thanks for the helpful replies (and forthcoming pictures), all.

There's a part of me that wants to just use conduit - whether PVC or EMT - but once I start thinking about all the drilling, hanging, clamping, cutting, my eyes start to glaze over :p It seams unbeatable for ruggedness, though.



In my case there weren't many holes to drill and the floor had been removed so I could work from topside. My main concern was to protect the wiring from the axle, springs and shackles. There is a fused terminal block in the main box in the center and a universal ground wire to all the lights' mounting plates.

On my "Log Cabin" I ran all the wires through the clear vinyl tubing so only needed 3/8" holes. There is a large watertight box on the underside of the tongue just behind the hitch. I have a large terminal block fused for each separate circuit and couple of switches: One isolates the cabin battery from being charged by my tow vehicle. Another energizes the electric jack from either the tow vehicle or the cabin battery. There is also a quick disconnect for the 12V cooling fan for my generator when it is mounted on the tongue and running in the sound deadening box.
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