Product: Plenum Plenum Underground Preinsulated Duct System Plenum* ITEM#SizePDFDWG 20-0010 20" x 24" x 36" [508 x 610 x 914 mm] 20-0015 25" x 30" x 48" [635 x 762 x 1219 mm] *Custom plenum boxes available upon request. May require up to 2 week lead time. Plenum Adapter ITEM#SizePDFDWG 08-25608" [203 mm] 10-256010" [254 mm] 12-256012" [305 mm] 14-256014" [356 mm] 16-256016" [406 mm] 18-256018" [457 mm]