What is a missing knockout or open knockout in an electric panel box? Why is it a problem?
Monday, May 20, 2019
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.
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