UL 1996-2009 pdf download – Electric Duct Heaters.
4 Enclosures 4.1 Enclosures for duct heaters shall be so formed and assembled that, they have the strength and rigidity to resist abuse during shipment, installation, or use without increasing their fire and accident hazards due to a total or partial collapse that could result in a reduction of spacings, a loosening or displacement of parts, or any other defect. 4.2 Enclosures for individual electrical components and wiring, outer enclosures, and combinations of the two, shall be considered in determining compliance with the requirements of 4.1 . 4.3 Among the factors to be used in determining the acceptability of an enclosure are: a) Physical strength, b) Resistance to impact, c) Moisture-absorptive properties, d) Flammability, e) Resistance to corrosion, and f) Resistance to distortion or melting caused by the temperatures that is possible under conditions of anticipated use or by electrical disturbances within. g) Intended installation within ducts and plenums. 4.4 Nonmetallic enclosures or parts thereof shall be considered to comply with 4.3 (d) and (f), when: a) They comply with the flammability test specified in the Standard for Polymeric Materials – Use in Electrical Equipment Evaluation, UL 746C; and b) They have temperature ratings not less than the maximum temperatures to which exposure during normal operation is possible. 4.5 A sheet metal enclosure shall be evaluated with respect to its size, shape, metal thickness, and use in a particular application. Sheet steel having a thickness of less than 0.66 mm (0.026 inch) when uncoated or 0.74 mm (0.029 inch) when galvanized, or nonferrous sheet metal having a thickness of less than 0.91 mm (0.036 inch) is not acceptable, except for relatively small areas or for surfaces that are curved or corrugated, or otherwise reinforced such as by angles, channels, flanges, or ribs.
4.6 The enclosure of a unit shall be provided with means for mounting in the intended manner. Any special fittings necessary for intended mounting shall be shipped with the unit. 4.7 An electrical part within the outer cabinet need not be individually enclosed when the assembly complies with all of the following: a) The construction and location of the part do not permit the emission of flame or molten metal through openings in the outer cabinet, or it can be shown that malfunction of the component does not result in a risk of fire. b) There are no openings in the bottom of the compartment in which the part is located that permit molten metal or the equivalent to drop onto flammable material. See 1 6.8 for units for outdoor installation. c) The part is not near flammable material other than electrical insulation. d) Sheet metal thickness of the outer cabinet is in compliance with Section 5. e) The part is not located in an air-handling compartment. f) The part is not subject to unintended contact by persons, as specified in Section 6. g) There are no openings in the top surface of the outer cabinet that permit objects to fall on or near uninsulated live parts.