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.