Knockouts are perforated circles cut into the sides, top, bottom, and back of an electric panel box that can be “knocked out” for the entry of wiring in conduit or NM-cable. A knockout that is opened but not used for wiring installation—with an NM-connector or conduit connection that closes it—becomes an opening to the outside that would allow an arcing inside the panel to escape and start a fire. So these openings are required to be resealed, and there are ready-made plugs available that snap into place in the unused holes.The Residential Edition of the Florida Building Code (FBC) and the International Residential Code (IRC) states the requirement as shown below.

    The reference number at the end of the citation refers to where it was taken from the National Electrical Code (NEC). Also, because the breakers in a panel create heat, the interior of a panel box is a warm place for wasps, lizards, frogs, and mice to nest in the winter when an open knockout gives them access. Mud dauber wasps are especially fond of outdoor service panels in Florida and will build their nests right over the bus bars and wire connections. 

    Here’s a live lizard we found recently in a panel, which is unusual, because ordinarily we find them dead and dried-up, killed by electrocution when their body made a connection between the two bus bars.

    The code makes an exception for “openings intended for mounting purposes,” so the panel shown below has multiple open knockouts in the bottom that should be closed, but the two unused mounting slots on the back are considered okay.

    The openings in the dead front (front cover plate) of a panel box that are removed so the breakers can come through are called “twistouts” and are also required to be closed if not used. See our blog post What is a "missing twistout" at an electric panel? for more on this.

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 

Here’s links to a collection of our other blog posts about ELECTRIC PANELS:

• What causes copper wires to turn green or black in an electric panel?  

• What is the maximum number of circuit breakers allowed in an electric panel?

• When should a corroded or damaged electric panel cabinet or disconnect box be replaced? 

• What is a tandem circuit breaker? 

• When did arc fault circuit interrupter (AFCI) breakers first become required?

• Can an electric panel be located in a closet? 

• Can an electric panel be located in a bathroom? 

• Can you add circuit breakers by different manufacturers to an electric panel if they fit?

• My circuit breaker won't reset. What's wrong?  

• What is a split bus electric panel?

• How do I identify a combination AFCI (CAFCI) circuit breaker? 

• What does a circuit breaker with a yellow or white test button indicate? 

• What is the maximum gap allowed between the front of a recessed electric panel box and the wall surface surrounding it? 

• What are the requirements for NM-cables entering an electric panel box?

• Why is a fuse box/panel an insurance problem for homebuyers? 

• Why is bundled wiring in an electric panel a defect?

• What is the difference between GFCI and AFCI circuit breakers? 

• Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

• What happens when you press the "TEST" button on a circuit breaker in an electric panel?

• What is a Dual Function Circuit Interrupter (DFCI)? 

• What is the difference between a Combination Arc Fault Circuit Interrupter (CAFCI) and an Arc Fault Circuit Interrupter (AFCI) circuit breaker?  

• What is the difference between "grounded" and "grounding" electrical conductors? 

• What does it mean when a wire is "overstripped" at a circuit breaker?

• Why is an old fuse panel dangerous?  

• Who is the manufacturer of those "bad" electric panels?

   Visit our ELECTRIC PANELS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.