Cable Tray Firestop Calculator

Cable trays pass through fire-rated walls as a continuous bundle rather than a single round penetrant, so this calculator sizes a different kind of firestop: intumescent pillows stacked to plug the void space around the cables, plus wrap strips around the cables themselves and sealant along the tray's cut edges. The 40% cutoff on Cable fill matters more than any single number the calculator prints — it flags Exceeds 40% fill the moment cable fill crosses that line, because NFPA guidance for ladder-type cable trays generally caps fill around 40% of the tray's cross-section, both for ampacity/heat-dissipation reasons and because packing a tray denser than that leaves too little void space for the firestop pillows to seal properly. Tray width and height set the total opening area the void formula works against, so a wider or taller tray raises pillow count even at the same fill percentage.

Fire rating (hrs) is the one input on this page that changes cost without changing quantities: it drives the labor-cost term directly ($45 per hour of required rating) while pillow count, wrap strips, and sealant stay identical whether the wall is rated for 1 hour or 4. Wall thickness (in), by contrast, sits idle: the field is on the form and echoed in the step-by-step breakdown, but no line of the engine touches it — changing it from 8 inches to 24 inches leaves every output on this page unchanged, so treat it as a record for your own drawings rather than a driver of the pillow or cost estimate.