my Old Workshop

How to run electrical underground

If you’ve built a handy new shed or outbuilding out back, you could probably use a bit of power out there to run lights, power tools, or heaters. Here’s how to go about it.

You basically have two options on getting the power to the outbuilding: underground or overhead cables. Underground is preferable when possible, especially if you’re concerned about the way things look out in your garden or back yard.

Before you start, check the code for details, but these fundamentals should give you an idea of what you’re going to need to do.

You’ll want to use extra-insulated cable designed for direct burial. The conductors have a thicker-than-usual PVC insulation, and the outer jacket is thick, sunlight-, mositure-, and fungus-resistant PVC.

Plan ahead; if you want to have the option of switching power on in both in the outbuilding and in your house, run 3-wire cable; otherwise 2-wire’s fine.

You’ll need to dig a trench a couple of feet below the surface, deeper under driveways, and put in a base of about 3 inches of fine sand. The cable goes on top of here, and then you add another 3 inches of sand. This protects the cable from being damaged by stones during frost heaving. Lay a 1.5″ thick pressure-treated board over top (2x dimension), as added protection, and for added protection, you could buy a roll of bright yellow tape that says “buried power line” lay this over the plank.

At either end, the cable should extend from the building into the earth through a PVC conduit, which should end about a foot above the trench floor.

Aside from the digging, it’s a pretty simple task. Next time we’ll look at overhead.