Fireproof sealing dimensions for cable trays

The fireproof sealing width for cable trays should generally be about 6 inches larger than the outside dimensions of the tray, with firestop materials filling gaps of 20–30 mm at slab penetrations.

Recommended Sealing Width and Gap

For cable tray penetrations through fire-rated walls or slabs, the opening should extend approximately 6 inches beyond the tray's outer dimensions to allow proper placement of firestop materials such as sealant bags or pillows (e.g., 3M Fire Barrier Pillows or Grace KBS Sealbags) to achieve a UL-classified fire rating equal to or greater than the wall or slab itself. At slab penetrations, a firestopping thickness of 20–30 mm is recommended, often combined with a fire-support plate at the top of the opening. For floor-level penetrations, a waterstop of at least 50 mm in height is suggested to prevent fire and water ingress.

Firestop Materials and Installation

Common firestop materials include: Firestop packs or pillows: Placed in an orderly sequence to fill gaps around cables and trays. Firestop mastic or intumescent caulk: Used to seal cut edges and small voids, expanding under heat to maintain the seal. Fire-resistant mineral wool or boards: Provide structural support and thermal insulation for larger openings.

The gap area between firestop packs and cables should not exceed 1 cm², and packing thickness should be sufficient to ensure a tight, void-free seal. Cover plates should be square, evenly painted, and used to close the opening for a complete fire barrier.

Practical Considerations

Schedule firestopping toward the end of the project to allow cable installation first. Ensure tight and reliable sealing without visible cracks or voids. For large openings, install a fire-resistant backing plate before sealing to maintain structural integrity.

Follow manufacturer instructions for UL-listed firestop systems to maintain compliance and fire rating. By adhering to these guidelines, cable tray penetrations can maintain the required fire resistance, preventing fire and smoke from spreading through electrical shafts or walls.