Occasionally when doing electrical repairs, there is a need to alter the size of the knockout openings in electrical boxes or service panels. Sometimes the standard knockout openings in a wall box aren't large enough for the cable or conduit you need to feed into the box.
How does one handle this situation? Electricians generally make use of a tool known as a knockout punch for this kind of situation. These tools incorporate a set of die and knockout pieces, along with some kind of device to pull the two pieces together to punch a hole in the metal box. There are several different styles of a knockout punch, and some electricians own several.
Screw-Type Manual Knockout Punch
One such type of tool is a screw-type, manual knockout that can be tightened with a wrench to knock out the hole. As the wrench is used to tighten the bolt head, the two pieces are drawn together on either side of the box hole opening. As you continue tightening, the knockout is forced through the box opening.
Hydraulic Knockout Punch
The other choice is a more expensive knockout set. This one is a hydraulic knockout punch that uses the same die and knockouts but incorporates a hydraulic cylinder to drive the knockout through the box. This too can be a manual, hand-driven pump, or you can purchase a more costly electric hydraulic pump that works with the push of a button.
There are other methods for making a larger hole in electrical boxes. One method is to use what is called a unibit, also known as a stop bit. It simply is a tapered cutting drill bit that gradually cuts a bigger and bigger hole. They come in various sizes, some cutting holes up to 3/4".
Another option is called a hole saw—a round bit with cutting teeth that fit into a power drill. Hole saw kits are pretty common and the ones designed for electrical work usually include bits ranging from 1/2" to 3/4" to 1" in diameter. However, hole saw options for carpentry work can get very large in size—up to 4" in diameter or even larger.
As a footnote, remember that enlarging the size of the knockout openings should be a last resort. Remember that there are different sized electrical boxes for many different uses, and when possible, choose a larger box rather than altering a smaller one. An electrical box should not be overfilled with wires due to safety concerns. Good practice says that wall boxes and junction boxes should never be filled with more than 70 percent of capacity to ensure plenty of room for wire nut connections and the body of a switch or receptacle. Opt for larger, deeper boxes when a standard box isn't sufficient for your needs.
Larger boxes may be the only choice when a standard box is not larger enough for the knockout die you want to use. The walls of a standard box may not be large enough to handle either the hole you need to cut or the connector and locknut that needs to be installed on that larger opening. Having properly fitting connectors is necessary for proper grounding of metal boxes.