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How to Seal a Busway Penetration Through a Fire-Rated Wall

By DHYN Engineering Team  ·  August 12, 2026

Quick answer: A busway crossing a fire-rated wall breaks the fire compartment in two ways: through the gap around the duct, and along the inside of the duct itself. Sealing it takes a tested firestop system in the annular gap (intumescent sealant, mineral wool with ablative coating, or firestop blocks), plus an internal fire barrier inside the busway at the wall line. Both must be tested assemblies, UL 1479 or EN 1366-3, matched to the wall's fire rating. Generic expanding foam is not a firestop and will fail inspection.

This is one of those details that nobody puts in the busway quotation and everybody argues about three days before the fire inspection. The duct installer thinks it is the firestop contractor's job, the firestop contractor has never seen a bus duct, and the inspector fails it. Here is how to get it right the first time, and who needs to do what.

Fire takes two routes past a busway

The first route is obvious: the annular gap between the duct's metal housing and the edge of the wall opening. Every penetration has one, usually 25 to 100 mm all around, and smoke pours through it the moment there is pressure difference. The second route is the one people miss. A bus duct is a long metal tube full of copper, and fire travels along it and through it. Hot gases move inside the enclosure like a chimney, and the copper bars themselves conduct heat through the wall so effectively that combustible material on the far side can ignite without a single flame crossing. Sealing only the outside gap leaves the inside of the duct wide open.

What the codes actually require

The rule in every code system we have worked under comes down to the same sentence: a penetration must not reduce the fire resistance of the element it passes through. A 2-hour wall with a busway through it must still be a 2-hour wall after the penetration is sealed. The way you prove that is with a tested system, not with a material. In North America that means a firestop system tested to UL 1479 or ASTM E814, with an F rating (flame) and ideally a T rating (temperature) equal to the wall. In Europe and most of Asia it means a penetration seal tested to EN 1366-3, with the rating stated in minutes of integrity and insulation, EI 120 for a 2-hour wall. The furnace curve behind both is the ISO 834 cellulosic curve, the same one referenced for fire-rated construction generally.

Note the difference between a penetration seal and a fire-rated busway. A fire-rated busway keeps the circuit alive during a fire (circuit integrity, tested under IEC 60331-type conditions). A penetration seal keeps the fire from crossing the wall. They solve different problems and one does not replace the other.

The four sealing methods that pass inspection

Method	Best for	Rating achievable	Notes
Intumescent sealant or mastic	Small gaps, 25 to 50 mm	Up to EI 120 with correct depth	Applied over a mineral wool backing; cheap, clean, needs full-depth tooling on both faces
Mineral wool plus ablative coating	Most busway penetrations	EI 60 to EI 120	High-density wool (140 kg/m3 class) packed tight, coated both faces; the workhorse solution
Firestop blocks or bricks	Large openings, multiple services	EI 90 to EI 120	Intumescent blocks fitted around the duct; easy to re-enter when cables get added later
Pre-formed firestop device or kit	New builds, repetitive floors	Per the kit's test	Cast-in sleeves or retrofit collars sized to the duct; fastest on a riser with 40 identical floors

Whichever method you pick, it has to be the exact tested configuration: same product, same depth, same gap range as the test report or the UL system drawing. A bucket of intumescent sealant applied at half the tested depth is not a tested system, it is decoration.

The internal fire barrier, the part that gets missed

Reputable busway manufacturers offer a fire barrier section: a short length of duct with intumescent or insulating material packed between the bars and the housing, installed exactly at the wall or slab line. When heat reaches it, the material expands and chokes the inside of the duct. On vertical risers this is doubly important, because a busway shaft is a chimney by design and every floor slab penetration is a stack-effect highway. Many consultants in the Middle East and Southeast Asia now write "internal fire barriers at every slab" straight into the busway specification, and we build them at the factory where the position of each barrier is marked on the layout drawings.

Ask for the barrier's test evidence the same way you ask for the penetration seal's. The honest version is a furnace test of the barrier section as part of a full penetration assembly, wall, duct, seal and barrier together, under ISO 834 conditions. A datasheet with no test report behind it is a marketing page.

The installation sequence that passes

Get the order right and the job is half a day per penetration. First, size the opening before the wall is built or core-drilled; the busway manufacturer gives you the housing dimensions, and you add the tested gap range, typically 25 to 50 mm per side. Second, install and test the busway completely: joint torques, insulation resistance, the works. Sealing a penetration before the electrical tests are done means breaking the seal to fix a joint, and resealing. Third, pack the mineral wool to the specified density and depth, leaving the correct recess for sealant. Fourth, apply sealant or coating to both faces of the wall, not just the visible one. Fifth, fit the internal barrier section if it is not already built into the run. Sixth, label the penetration with the system reference, date and installer. The label is what turns a gray smear of mastic into a documented, inspectable firestop, and inspectors look for it.

Five mistakes that fail inspections

Polyurethane expanding foam, the one from the hardware store, appears in failed inspections more than everything else combined. It burns, it shrinks, and no test report exists for it as a busway firestop. Second mistake: sealing one face of the wall and leaving the far side open. Third: no allowance for movement. A busway expands roughly 1 mm per meter per 40 degrees of temperature rise, and a rigid seal on a long run cracks within the first summer; tested systems include the movement allowance, follow them. Fourth: filling the inside of the duct with foam or mortar at the wall instead of a proper barrier section, which wrecks the duct's heat dissipation and usually its IP rating too. Fifth: no documentation. If the penetration has no label and no system reference on the as-built drawings, the inspector treats it as unsealed, and honestly, so should you.

What to ask your busway supplier before the walls go up

Three questions, asked early, prevent the whole argument. Can you supply internal fire barrier sections, positioned to my reflected ceiling and shaft drawings, with test evidence? Do you have a standard penetration detail drawing showing opening size, gap range and the firestop systems you have tested with? And will your installation supervisor coordinate with the firestop subcontractor on site? A manufacturer who has been through a few high-rise projects, like the risers we describe in our high-rise bus duct guide, answers all three in one email. The ones who hesitate are learning on your project, and the fire inspection is an expensive place for someone to learn.