What are the requirements for NM-cables entering an electric panel box?
Saturday, July 7, 2018
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).
Also, shown below is a typical NM-connector—viewed from the side that typically would be on the outside of the panel.
The National Electrical Code [NEC 312.5 (C)] allows one exception to the requirement for securely fastening NM-cables at panel entry, as long as the cables enter the top of a surface-mounted panel box through a non-flexible raceway that is between 18-inches and 10-feet long, and meet the following additional requirements:
- Each cable is fastened within 12-inches of the outer end of the raceway.
- The raceway extends directly above the panel and does not penetrate a structural ceiling.
- A fitting is installed at each end of the raceway to avoid abrasion of the cable jacket and fitting remain accessible after installation.
- The raceway is sealed at the outer end.
- The cable sheathing is continuous through the raceway and extends a minimum of 1/4” into box enclosure.
- The raceway is securely fastened at outer end.
- Where installed as conduit, the allowable cable fill should not exceed permitted level in Table 1 of Chapter 9 of NEC Code, approximately 60%.
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