UL-20:2010 pdf free download – Underwriters Laboratories Inc. Standard for Safety General-Use Snap Switches.
4.4.2 Means for Mounting
4.4.2.1 A base intended for surface mounting shall have two or more holes for mounting screws, and a base with an area of more than 0.016 m2 (25 inch2) shall have three or more holes for mounting screws. A mounting-screw hole shall be countersunk not less than 3 mm (1/8 inch) in the material of the base; and there shall be a spacing over the surface of insulating material, between the head of the screw or washer, and the nearest uninsulated live part, in accordance with 4.9.1.
4.4.2.2 A flush switch shall be provided with means for mounting in a standard flush-device box or on a standard outlet-box cover in accordance with Clause 4.11, Provisions for Grounding. Flush-type switches rated 347 V shall be provided with means for mounting in a specific-use box having tapped holes, spaced on 89.69 mm (3-17/32 inch) centers, that are intended for the mounting of the switch. The mounting screws for a switch that is intended for mounting in a standard flush-device box shall be #6 – 32 and shall be spaced 83.34 mm (3-9/32 inch) apart.
4.4.2.3 A metal yoke, strap, or mounting ears shall not be less than 1.02 mm (0.040 inch) or more than
2.3 mm (0.09 inch) thick. If a nonferrous metal is used, it shall be of sufficient thickness to provide mechanical strength and rigidity not less than that of 1.02 mm (0.040 inch) thick steel. The yoke,strap,or mounting ears may be provided with extension plaster ears, which may be scored so that they can be broken off if not needed.
4.4.2.4 If a yoke, mounting ears, or strap as described in Clause 4.4.2.3 is made of steel, the corrosion protection for a switch for use in a flush-device box or on an outlet-box cover shall be a zinc or equivalent coating not less than 0.0038 mm (0.00015 inch) thick in accordance with the method for determining protection against corrosion in the Standard for Metallic Outlet Boxes, UL 514A, or CAN/CSA C22.2 No. 0, General Requirements – Canadian Electrical Code, Part II, or other coatings determined to be acceptable for the particular application.
4.4.2.5 A nonmetallic yoke, strap, or mounting ears shall be of a material and construction determined to be acceptable for the intended use.
4.4.2.6 A screw provided for use in mounting the switch to an outlet box or other enclosure shall project not more than 22 mm (7/8 inch) beyond the strap or cover and shall have a flat or blunt end. The end of the screw shall have no burrs, fins, or sharp edges that can damage wiring. This does not preclude thread-cleaning slots or grooves in the end of a screw.
4.4.3 Sealing
4.4.3.1 Live screw heads or nuts on the underside of a base intended for surface mounting shall be covered with a waterproof, insulating sealing compound with a softening point of 65°C (1 49°F) or higher. If such parts are spaced not less than 6 mm (1/4 inch) through-air from the mounting surface and are staked, upset, or otherwise reliably prevented from loosening, sealing compound need not be provided.
4.4.3.2 Sulphur shall not be used as a sealing material.
4.4.3.3 Determination of the softening point of a sealing compound shall be made in accordance with the test method specified in ASTM, E 28-99, Standard Test Methods for Softening Point of Resins Derived from Naval Stores by Ring-and-Ball Apparatus.
4.5 Current-Carrying Parts
4.5.1 General
4.5.1.1 Current-carrying parts and wire-binding nuts and screws shall be of metal and shall have the strength, rigidity, and ampacity to comply with the requirements outlined in Clauses 4.2 — 4.5.