A one-piece liquid-level float switch housing and drain line assembly for use with a collection tray or pan to collect condensate and shut off the source of condensate production when it exceeds a pre-established threshold level. The assembly is configured for rapid connection through a pre-formed drain line opening in the perimeter wall of a condensate collection tray/pan to place the deployable float body within its housing in a level orientation relative to the tray/pan without additional leveling adjustment. The housing has an open bottom and vertically-extending slots providing fluid inlet, debris blocking, and air venting functions for enhanced float body operation. The condensate pan/tray used with the assembly has a nesting configuration that includes a perimeter wall with varying combinations of gussets, curved ribs, angled corner configurations, and support shelves, and a central bottom surface with raised patterns, all of which strengthen the tray/pad during installation and use.