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How to Waterproof the Intersection Between a Pond Liner & an Electrical Cable

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Using a flexible pond liner is an easy way to create a custom pond in the homeowner's choice of size and shape. Pond liners made from EPDM rubber, a tough, waterproof material commonly used for flat roofs, will hold up for a long time and resist damage caused by sunlight, temperature changes and aquatic life. Installing pond lighting or filtration often requires penetrating the EPDM liner, though, and this can be a source of leaks if not properly done.

Below is how you can run an electrical cable through an EPDM liner and seal the site to maintain watertight integrity:

Tools and materials needed

  • Scrap pieces of EPDM
  • Tube of urethane adhesive
  • Caulk gun
  • Scissors
  • Cable ties
  • Utility knife
  • Wooden stake
  • Mallet

Step-by-step procedure

1. Secure the cable behind the liner—It is important to immobilize the electrical cable so it does not shift when the pond is filled or during settling of the surrounding material. Too much movement of the cable after it has been installed can cause the waterproof connection to be damaged and result in a leak.

To secure the cable, partially drive a 12-inch wooden stake into the soil beneath the liner but near the top of the pond. Next, measure out your electrical cable so you know exactly how much needs to be extended down the side of the pond under the liner. Where the cable passes the wooden stake, use a couple of cable ties to secure it to the protruding part of the stake, then drive the remainder of the stake into the ground until it is flush with the surface.

2. Make an opening through the liner—Once the cable is secured, determine exactly where you wish to pass it through the liner. At that point, make a small, straight incision with a utility knife; it is important to keep the length of the cut as minimal as possible, so remove plugs or other fixtures or fittings to decrease the diameter of the cable that needs to pass through the opening.

3.Insert the cord and build up reinforcement—After you have cut a slit in the liner, carefully push the cable through the opening so as not to enlarge the hole.

Once the cable is pushed through and is in its final position, you need to install reinforcing layers of EPDM rubber around the cable. To do so, use scissors to cut four circles from a scrap piece of EPDM that measure at least two inches larger in diameter than the length of the slit. Next, make straight cuts from the edges of the circles to the centers of the circles. Slip the circle cutouts around the cable where it passes through the pond liner, one on each side, and align the cable so it is in the center of the circles.

Secure the circles to the liner using urethane adhesive, and also be sure to put a generous amount of urethane around the cable where it penetrates the lining. Repeat this same process with the remaining two circles so that the area surrounding the cable on each side of the liner is built-up with two additional reinforcers.

4. Finish waterproofing the location—Cut two, 8-inch long pieces of EPDM rubber in 1-inch wide strips. Apply urethane adhesive to the electrical cable on both sides of the liner for a distance of 4 inches. While holding one end of a strip in place, begin wrapping it tightly around the cable as close as possible to the intersection of the cable with the liner, and continue wrapping it with slight overlaps until you reach the end.

Fasten a couple of cable ties around the EPDM strip to hold it in place as the urethane cures. Repeat this same procedure for the opposite side of the liner and add urethane to all areas of exposed EPDM reinforcing.