Dear Mr. Electrician: What is the best way to remove the pre-punched concentric knockouts in an electrical circuit breaker panel without loosening the remaining outer knockout rings?
Answer: Remove them very carefully. If you look closely at the way the concentric knockouts are stamped on the electrical load center you will see that they alternate between being punched in and being punched outwards. You could start with the smaller center KO first, but I usually go with whatever is easier and most convenient for each job. I use a hammer and an old screwdriver for making the first punch. Begin the knockout removal by banging it in the direction that it was pre-punched from the factory. Depending on the manufacturer, some concentric knockouts are easier to remove than others. NOTE: Text links below go to applicable products on Amazon.com
Concentric Knockout Removal
Once you get it started by hitting with the screwdriver you need to twist it or knock it forth and back a few times to loosen it up. If you have a good pair of diagonal pliers or a pair of BX Cutters you can sometimes get in there and cut one or more of those welds or the ring itself. Just don’t try to get it real fast. Be patient and work it out. You will feel the knockouts get loose as you move them forth and back.
For subsequent knockout rings sometimes you can use your pliers to carefully pry the ring halves until they are up enough to squeeze them together and then move the pliers forth and back parallel to the anchor point until it breaks off. If you try to rock in the other direction it will tend to twist the next ring up. Just keep removing the rings in the direction that they were punched and you should be fine.
In a situation where the electrical sub-panel is recessed in a wall, you can use a long thin screwdriver and stick it between the edge of the wall and the edge of the panel to apply a little pressure on one of the rings from the outside to get it started.
Each electrical manufacturer’s concentric knockouts are different in strength and ease of removal. Sometimes they pop out too easily and you have a much bigger opening then you need. Fortunately there are reducing washers available in all trade sizes, made to transition from the larger opening down to the size that you need. If you punch out the wrong knockout, it can be sealed up again using a knockout seal. Both of these items can be purchased online or at almost any local electrical supply company.
The knock outs on a load center cover tend to be much easier to remove to allow for each circuit breaker. However there is always a possibility that the wrong knock out gets removed or a circuit breaker is relocated thereby leaving an open knock out on the load center cover. Fortunately every electrical load center manufacturer also has available blank filler plates to cover up the empty slots in the circuit breaker cover.
You might find the replacement of a four gang electric meter socket base interesting.