NEMA publishes OS1 and OS2 box standards
The National Electrical Manufacturers Association (NEMA) has published OS 1 “Sheet-Steel Outlet Boxes, Device Boxes, Covers, and Box Supports” and OS 2 “Non-metallic Outlet Boxes, Device Boxes, Covers, and Box Supports”.
The two documents describe standard configurations for outlet boxes 100
ci or less, as well as manufacturer specifications for materials,
corrosion protection, dimensions of openings, supports, cover screws,
markings and provisions for grounding. The standards specify dimensions
necessary for compatibility with NEMA standard wiring devices and help
manufacturers ensure that box constructions comply with UL listing
The standards were last revised in 2003. Two of the changes made to OS
1 include additional marking requirements for boxes for
ceiling-suspended fan support and clarification to marking requirements
for clamps furnished as part of a box. OS 2 includes updated
definitions, revised requirements for fixture boxes, and a modified
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