The National Electric Code (NEC), Article 300.5 Table 300.5, covers the installation of underground wiring systems. Column 1 of Table 300.5 covers the requirement for direct burial cable installations. Depending on location and the method of burial, the required depth for direct burial cable varies from 18 inches to 24 inches. Study the requirements of Article 300.5 Table 300.5, and be sure to get the required electrical permits before undertaking this project. For this article, we will assume that the cable will feed a sub-panel installed in a garage on private property.
Before beginning the actual excavation work, call the local utilities, the telephone company, and the cable company to determine if they have any buried cables or pipes in the area you intend to trench. Lay out your trench with the trenching shovel. Mark out the length and width of the trench with the trenching shovel. Carefully remove the sod in squares, and set the blocks aside. You will replace this sod after the cable is in place.
Using the power trencher, begin excavating a trench at least 20 inches deep. According to Article 310.5 Table 310.5 of the NEC, a minimal burial depth of 18 inches on residential property and going a couple of inches deeper assures you that the minimum depth will be met. Begin in the middle and work both ways to get as close to the building as possible with the trencher. Once the main portion of the trench is completed, finish out the ends using the trenching shovel. Once the trench is completed, determine how you are going to bring the underground feeder cable into the house/main building. If you bring it in though a basement wall below grade, you will have to protect it with a sleeve made of rigid conduit.
Using the core drill and the appropriate size core bit, drill a hole through the basement wall. Measure the thickness of the wall and add 3 inches for the conduit sleeve. Cut, thread and de-bur a piece of rigid conduit cut to this length. Screw a conduit end bushing in place on one end of this sleeve and slip it through the hole in the basement wall. Hold it tightly in place while a helper screws another end bushing in place on the opposite end on the opposite side of the basement wall. The end bushings not only help keep the sleeve in place; they keep the cable from being cut on the ends of the sleeve. According to Article 310.5(D)(1)(2), the cable must be protected at any point where it may be subjected to damage.
On the other end, you will be bringing the cable in to the garage grade level. Drill a hole through the building wall and create a nipple/sleeve much as we did for entering the basement except on this sleeve we will only use an end bushing on one end. We will attach an "LB" condulet where the sleeve exits the wall and we will screw a length of conduit to the opening in the "LB" that extends to the bottom of the trench. There will be an end bushing placed on the trench end of this nipple to protect the cable from abrasion.
Once the cable has been laid and inspected, backfill the ditch. If the backfill contains sharp stones, pieces of payment, etc., you will need to protect the cable with a running board. Replace the sod and tamp in place.