NM-cables must be securely fastened where they enter an electric panel, so that tugging on a cable from outside the box will not pull wires loose from their terminations inside. This is usually accomplished by installation of an NM-connector at the knockout that is secured to the box from both sides and clamps down on the cable. Most NM-connectors are approved for securing only one or two cables, but there are connectors listed to handle even more.
The configuration shown above, where a cluster of wires enters the top of the box through a single opening, is called a chase nipple. It is a fast and easy way to pull NM-cables into a box, but not code approved.
The photo below shows NM-cables entering the top of a box through NM-connectors (except that one is missing, with a cable poking through it, and will need repair). The photo at right is a typical NM-connector—viewed from the side that typically would be on the outside of the panel.
