Where cables pass through shafts, walls, slabs, or enter electrical panels or cabinets, openings shall be tightly sealed with firestopping materials in accordance with design requirements. Process flow: reserved openings → busway installation → distribution box positioning and installation →. The following charts give the number of 3M pillows needed to completely firestop an opening that cable tray passes through. UL Listed Systems Concrete Wall - C-AJ-4056 3 HR F-Rating, 3/4 HR T-Rating Gypsum. FIRSTO firestops are designed to seal multi-cable and cable tray penetrations of fire-rated walls and floors. In other words, the cable tray manufacturer did not go to UL or ETL and say “test this tray penetration for 2 hours, make the hole this size, and use these pillows, compressed this.