Except as provided for in Section 18-28-307.2.4, auxiliary drain pans shall be installed under all coils on which condensation will occur and under units containing coils that are located in attic spaces, suspended ceiling spaces, furred spaces or any area where damage will occur to the building or its contents, as a result of an overflow from the equipment or appliance drain pan or from a stoppage in the condensate drain piping. Auxiliary drain pans shall have a minimum depth of 1 1/2 inches (38 mm), shall be not less than 3 inches (76 mm) larger than the unit or coil dimensions in width and length, and shall be constructed of a corrosion-resistant material. Metallic pans shall have a minimum thickness of not less than 0.0276-inch (0.7 mm) galvanized sheet steel. Nonmetallic pans shall have a minimum thickness of not less than 0.0625 inch (1.6 mm). A separate drain line shall extend from the pan to a conspicuous point and serve as an alarm which indicates that the primary drain is restricted. As an alternative to a separate drain line, an approved water level detector or float switch device shall be used to control overflow by automatically shutting down the equipment or appliance that produces the condensate.