Q. What’s the best way to locate an electrical box that was covered over when the drywall was installed?

A.Sean Kenny responds: First, try to narrow down the approximate location of the hidden box. The National Electric Code ( NEC) requires that outlets be placed no more than 12 feet apart, and that there be an outlet no more than 6 feet from a door opening. Most outlets are located 18 to 24 inches off the floor, and wall switches are generally located about 4 feet above the floor.

Carefully scan the wall surface in these areas for a hump created by the hidden box pushing against the drywall. If you can’t see a hump, place a long straightedge on the wall, and move it around until it rocks over the high spot.

I carry two types of electronic locators that help me pinpoint the location of the box (see photos). If the hidden box is "hot," I use my sniffer (Greenlee Textron, 4455 Boeing Dr., Rockford, IL 61109; 815/397-7070) to pinpoint the location. If there is no power at the hidden box, then I’ll tap into the wires of a downstream box and use a tone generator (Leviton, 59-25 Little Neck Pkwy., Little Neck, NY 11362; 800/833-3532) to locate the hidden box.

After making my best guess at where the center of the box is, I drill a 3/8-inch hole, being careful not to damage the wires by drilling too deep. I shine a flashlight in the hole and verify that I’ve pinpointed the box. Using short strokes with a keyhole saw, I carefully cut outward from the hole until I reach the walls of the box. Then I cut around the outside of the box, allowing the drywall to draw up tight to the studs. Nearby fasteners will stand proud of the surface and need to be driven home.

Sometimes when the wall has already been finished, I may cut out an undersized box opening, and let the bulge remain in the wall. This isn’t the prettiest approach, but it eliminates the need to patch the wall, and may work in a utility room or other secondary space. The NEC requires that the face of the box must be flush with the wall surface, so I always install a box extension to satisfy code.

Sean Kenny is a master electrician living and working in Amesbury, Mass.