Cabinets, Cutout Boxes & Meter Socket Enclosures

Article 312 governs the metal or nonmetallic box that holds a panelboard, disconnect or meter. It sets how much bending space a conductor needs where it enters a terminal, how deep the enclosure must be, how tightly it can sit against a combustible wall, and the requirement to fill unused knockouts so the enclosure stays intact.

Much of this is about the bend. A large conductor forced into too little space cracks its insulation at the terminal, exactly where heat and current concentrate. The bending-space tables translate conductor size directly into minimum enclosure dimensions.

When it applies

This covers the enclosures that house panelboards and disconnects — cabinets, cutout boxes and meter socket enclosures. It sets how the box mounts relative to a finished wall, the bending and gutter space conductors need inside, and the rule that every unused opening gets closed.

Common mistakes

• Setting a flush panel too far back so the front sits below a combustible wall surface it should be flush with or ahead of.
• Cramming large conductors into a shallow enclosure and violating the required deflection space at the terminals.
• Leaving knockouts open, which lets in fingers, rodents and debris and breaks the enclosure rating.

In the field

I check my wire-bending space before I ever mount the can. Feeders that fight the box do not seat cleanly on the lugs, and a loose lug in a tight enclosure is the kind of connection that comes back as a burned panel.