Knockout holes, or knockouts, in electrical boxes are made at the factory and have standard sizing to match cable clamps. But sometimes the standard knockout openings in a wall box aren't large enough for the cable clamp or conduit fitting you need to attach to the box, so you have to enlarge the hole. In other situations, electricians routinely enlarge holes or make new holes in metal service panel boxes. As a result, there are several different tools available for these tasks. But for most amateurs, a simple drill and unibit or hole saw is the tool of choice.
A unibit, also called a step bit or step drill, is a special drill bit that has a conical shape and can cut a range of hole sizes. The farther you go up the bit, the larger the hole it cuts, up to 1 inch or more. Unibits work with ordinary drill-drivers, just like standard drill bits. Self-starting, or self-tapping, unibits can start their own holes in solid material; other types need a pilot hole to begin cutting.
A hole saw is a cylindrical bit with cutting teeth that fits into a standard power drill. Hole saw kits designed for electrical work usually include bits ranging from 1/2 inch to 1 inch in diameter, but hole saw sizes can go up to 6 inches. For electrical boxes, a hole saw should be designed for cutting metal.
Electricians typically use a tool known as a knockout punch for cutting new holes or enlarging existing holes in metal boxes. These tools incorporate a set of dies and punches, along with some kind of device to pull the two pieces together to punch a hole in the metal box. There are two main types of knockout punches: screw-type and hydraulic.
A screw-type, or manual, knockout punch is tightened with a wrench to knock out the hole. A pilot hole is drilled into the box (as needed), then a draw bolt and die are inserted into the hole and a punch is threaded onto the draw bolt. The draw bolt is turned by a wrench to pull the punch through the metal, making the hole.
A hydraulic knockout punch is a more expensive tool that uses a similar die and punch set but incorporates a hydraulic cylinder to drive the knockout through the box. The hydraulic device can be a manual, hand-driven pump or an electric hydraulic pump that works with the push of a button.
A Word of Caution
As a general rule, enlarging the size of a knockout opening should be a last resort. There are different sizes of electrical boxes for many different uses; whenever possible, choose a larger box rather than altering a smaller one.
An electrical box should not be overfilled with wires because this can cause the wires to overheat, potentially leading to serious safety hazards. Good practice dictates that wall boxes and junction boxes should never be filled more than 70 percent of capacity to ensure plenty of room for wire nut connections and the body of a switch or receptacle. When a standard box isn't sufficient for your needs, opt for larger, deeper box.