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312.10, 314.5 Screws Protruding into Boxes – How Much is Allowable

Revision/New

Leviton Comment: We are covering Articles 312.10 and 314.5 together because they are very similar, and both relate to screws protruding into boxes.

Change Summary

  • The NEC has set limits on how far screws can protrude into boxes.

NEC®Text

Article 312 – Cabinets, Cutout Boxes, and Meter Socket Enclosures
312.10 Screws or Other Fasteners. Screws or other fasteners installed in the field that enter wiring spaces shall be as provided by or as specified by the manufacturer or shall comply with the following as applicable:
(1) Screws shall be machine type with blunt ends.
(2) Other fasteners shall have blunt ends.
(3) Screws or other fasteners shall extend into the enclosure no more than 6 mm (1⁄4 in.) unless the end is protected with an approved means.
Exception to (3): Screws or other fasteners shall be permitted to extend into the enclosure not more than 11 mm (7⁄16 in.) if located within 10 mm (3⁄8 in.) of an enclosure wall.

Article 314 – Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings and Handhole Enclosures
314.5 Screws or Other Fasteners. Screws or other fasteners installed in the field that enter wiring spaces shall be as provided by or specified by the manufacturer or shall comply with the following as applicable.:
(1) Screws shall be machine type with blunt ends.
(2) Other fasteners shall have blunt ends.
(3) Screws attaching a cover shall extend no more than 10 mm (3/8 in.).
(4) Screws or other fasteners, other than in (3), penetrating a cover shall extend no more than 8 mm (5/16 in.).
(5) Screws or other fasteners penetrating a wall of a box exceeding 1650 CM3 (100 in.3) shall extend no more than 6 mm (1/4 in.), or more than 11 mm (7/16 in.) if located within 10 mm (3/8 in.) of an adjacent box wall.
(6) Screws or other fasteners penetrating the wall of a box not exceeding 1650 CM3 (100 in.3) and not covered in 314.23(B)(1) shall be made flush with the box interior.
(7) Screws or other fasteners penetrating the wall of a conduit body shall be made flush with the conduit body interior.
Exception to (3) through (6): A screw shall be permitted to be longer if the end of the screw is protected with an approved means.

Material taken from the National Electric Code. is reprinted with permission from NFPA 70., 2023 edition. National Electrical Code®, Copyright 2022, National Fire Protection Association, Quincy, MA. All rights reserved.

Expert Analysis

A new section has been added to address the possibility of screws and fasteners damaging conductors within boxes. In all instances the only permitted style of screws are the type with machine threads and a blunt end. This reduces the likelihood of damage to conductors due to coarse threads and sharp points typically found on sheet metal screws. The penetrating length varies depending on the application and location.

Any screw sanctioned by the equipment manufacturer will be automatically acceptable. Other screws, such as those used to attach identifying medallions, must have machine threads and have blunt ends.

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