The National Electrical Manufacturers Association (NEMA) published NEMA LE 4-2012 Recessed Luminaires, Ceiling Compatibility.

NEMA LE 4-2012 contains definitions, dimensions, and tolerances for recessed luminaires designed to use fluorescent, high-intensity discharge, and incandescent light sources. Drawing that show examples of fluorescent lamps also apply to high-intensity discharge, compact fluorescent, incandescent, and other lamp sources. This standard does not apply to hard metric dimensioned products; soft metric dimensions or direct metric conversions, however; may be applied throughout this standard as one inch equals 25.4 millimeters.

NEMA LE 4-2012 may be purchased in a hard or electronic copy for $69 by visiting NEMA’s website or by contacting IHS by phone at 800-854-7179 (within the U.S.) or
303-397-7956 (international), or by visiting www.global.ihs.com.
 
NEMA is the association of electrical equipment and medical imaging manufacturers, founded in 1926 and headquartered in Arlington, Virginia. Its member companies manufacture a diverse set of products including power transmission and distribution equipment, lighting systems, factory automation and control systems, and medical diagnostic imaging systems. Worldwide annual sales of NEMA-scope products exceed $120 billion.​