Running Power to an Outbuilding
The hardest part may be digging the trench for the cable.
The first step to wiring an outbuilding is to figure out how many lights and outlets you need. If your needs are modest, you may be able to tap into an existing outlet in the main house and extend the circuit from it. Calculate the total loads for the existing circuit and the extension to see if the circuit has enough capacity. Otherwise, run a new circuit from the panel to the outbuilding.
If the outbuilding isn’t more than 50 ft. from the house and has a few lights and outlets, it usually can be supplied by a 120v, 20-amp circuit and 12/2 w/grd UF cable. But check with local building authorities before you start. Get the necessary permit, code requirements, and inspection schedules.
The hardest part of the job is usually digging the trench, because local codes typically require it to be 18 in. deep. Fortunately, you can rent a gas-powered trencher to do the digging for you. After removing the dirt from the trench, pick out any rocks or debris that could damage the cable. Then lay the UF cable in the trench, flattening it as you go 1.
Use electrical-grade PVC conduit, couplings, and adapters to protect the UF cable between the bottom of the trench and the outdoor boxes in which connections will be made. At the house, UF cable is typically spliced to an interior wire in a covered single-gang, weathertight box 2. On the other end, the cable typically passes through an LB conduit and a short length of PVC pipe before terminating in a junction box inside the outbuilding 3.
Working with PVC Conduit
PVC conduit can be heated and bent, allowing you to change pipe direction with far less cutting and fewer fittings than you would need for rigid pipe. PVC conduit is intended to be used outside: It is flexible, durable, and waterproof; but its assemblies are not as protective as metal pipe or cable. Be sure to use only the gray PVC conduit rated for electrical work (also called schedule-40 PVC conduit). White PVC plumbing pipe is unsuitable as electrical conduit; moreover, heating it can release noxious fumes. Wear work gloves to avoid burns when heating gray PVC pipe.
It’s best to use a hacksaw with a metal-cutting blade to cut PVC pipe, though any fine-tooth saw will work. Make the cut as square as possible. Use a pocket knife or curved file to remove burrs from the pipe end, then wipe it with a rag. Apply an even coat of PVC cement to the outside of the pipe and the inside of the fitting. If you’re bending pipe, screw the threaded adaptor into the outdoor box or conduit first, then glue the bent pipe to the adapter.
To bend PVC pipe, heat it with a MAP-gas torch; one common brand is the Bernz-Omatic®. You can also use a hot box or a PVC heating blanket (Greenlee®). Keep the torch point moving constantly, rotate the pipe periodically, and tape the pipe ends to contain the heat and speed up the process. When the pipe droops, it’s ready to shape. Place it against an irregular wall or foundation and it will conform to that contour. Strap the pipe close to the outdoor box and the foundation. To secure straps to the foundation, first predrill with a masonry bit. Then insert expandable plastic anchors into the holes and drive strap screws into the anchors.
Excerpted from Wiring Complete, 3rd Edition (The Taunton Press, 2017) by Michael Litchfield and Michael McAlister