Aluminum. Accommodates aluminum and copper conductor combinations. Tin plated aluminum inhibits corrosion and increases conductivity. UL (Underwriter's Laboratories) listed. 4/0 Str. to 2 sol. range for equal main and tap. 2 sol. minimum tap with one maximum main. Gauge: 4/0 Str. to 2 Sol. Material: Aluminum Range for Equal Main and Tap: 4/0 Str. to 2 Sol. Min Tap with one Max Main: Certification: UL Listed