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If you want to waterproof a length of outdoor (or even indoor) electrical cable, silicone sealant is the best protectant to use. Silicone creates an extremely tight and durable seal while also retaining its flexibility, making it a perfect outer coating for electrical wires that may need to be moved or rearranged in the future. In addition to being water- and weatherproof, it can also withstand very high temperatures. Silicone sealant can be found in any hardware or home improvement store.
Mix the silicone sealant if required. Silicone sealant is composed of several different compounds, and some brands need to be mixed before use.
Disconnect power to all of the electrical cables you are working with. Flipping the power switch on your main circuit breaker will usually accomplish this.
Clean the housing of all the electrical wiring you wish to waterproof with a wet rag. The cable housing should be free of dirt, dust and any other debris.
Hand-tighten your cable connectors with a 1.1 cm (7/16 inch) spanner. Make sure they are as tight as possible.
Apply the silicone sealant to the electrical cables. If your sealant comes in a tube and is inserted into a caulking gun, make sure you apply it evenly and coat the wire entirely. If your sealant comes in a tub, use a small paintbrush to apply it to the cable.
Cover all lengths of wire adequately and leave it to dry once finished. Avoid sealing over the connectors at either side of the wire, as this will prevent you from being able to remove it without cutting through the threads.
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