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Measuring and Protecting the Trailer Wiring Harness

The author using a metal tape measure to measure his utility trailer to know how long to cut the sheathing for the wires

Once I had a plan in mind, the next step was to measure the trailer to determine how long the braided sleeving needed to be for the section of the harness from the front connector through the lateral branches.

The reason I covered the trailer wiring harness with braided sleeving was to protect the trailer wiring. It was one of several options I considered. The others were split loom, spiral loom, and PVC electrical conduit. You can read more about my decision-making process here.

My decision having been made, the next steps were to measure the trailer, cut the transverse wires that ran to the marker lamps to size, and cover those sections of wire with expandable braided sleeving.

I'd actually planned to do all of these steps outside using eyeball measurements, which actually works out pretty well most of the time when fitting wiring to an existing thing like a trailer. But alas, an imminent torrential rainstorm caused me to change my plans. Using electrical tools in the rain is dangerous and dumb.

Not one to be deterred that easily, however, I decided to go outside and measure the trailer, keeping all of my measurements on the long side (it's always easier to make wire and sleeving shorter than it is to make it longer), and do the cutting and sheathing steps inside.

Once I took the measurements, I went back inside and encased all five wires (the left pair, the right pair, and the ground) in a single one-quarter-inch diameter length of braided expandable sleeving that would run from the front connector to the beam under the trailer where the wires split left and right. I sealed the sleeving to the trailer connector at the front of the harness with heat-shrink tubing.

I also encased the left and right wire pairs in one-eighth-inch sleeving, and secured them to the center section by tucking an inch or so of the eighth-inch sleeving on each side under the quarter-inch sleeving coming back from the connector. The next day, after a quick trip to the hardware store, I sealed that part of the harness in heat-shrink tubing, as well.

The purpose of the sleeving is to protect the wires from abrasion, UV light, road debris, and so forth. The purpose of the heat-shrink tubing is to hold the sleeving in place and prevent it from unraveling.

The only tools I needed for this part of the job were a tape measure, a scissor, alligator clips, and a heat gun. A hot knife would have helped reduce or eliminate the fraying; but I didn't have one, and I don't use braided sleeving often enough to make it worth buying one.

Here's a video that hopefully will make this step of the process easier to understand.

Video: Measuring and Sheathing the Trailer Wiring Harness

The next step of my DIY trailer-wiring project was to install the main trailer wiring harness.