Firestopping Requirements for Low-Voltage Cable Penetrations Under IRC 2024

Quick Answer

Yes. Under IRC 2024, low-voltage cable penetrations through fire-rated assemblies require the same firestopping treatment as penetrations by line-voltage power wiring, plumbing pipes, or HVAC ducts. The classification of the cable as Class 2 (low-voltage) does not exempt it from the fire-blocking and draft-stopping requirements of IRC 2024 Sections R302 and R602.

Under IRC 2024, where thermostat wire, speaker cable, data cable, alarm wire, or any other low-voltage conductor passes through a fire-rated floor/ceiling assembly, an exterior wall, or a fire-blocking location within a framing system, the penetration must be sealed with an approved firestop material. Additionally, IRC 2024 prohibits leaving abandoned low-voltage cables in wall and ceiling cavities; unused cable must be removed from the premises or tagged at both ends for future use.

What IRC 2024 Actually Requires

IRC 2024 Section R302.11 requires fire blocking to be installed to cut off all concealed draft openings (both vertical and horizontal) in the following locations: in concealed spaces of stud walls and partitions at the ceiling and floor levels, at intervals not exceeding 10 feet for walls over 10 feet in height, at floor and ceiling connections at exterior walls, at the intersection of interior and exterior walls, and in all concealed spaces between stair stringers at the top and bottom and at intervals not exceeding 10 feet along the run. Any penetration through a fire-blocking location—regardless of what passes through it—must be sealed.

Section R302.4 governs penetrations through fire-rated assemblies (assemblies with a rated fire-resistance period). Where a low-voltage cable penetrates a fire-rated floor/ceiling assembly or a fire-rated wall assembly, the penetration must be protected using an approved method. Approved methods include: listing of the specific cable and firestop material combination in a tested and listed through-penetration firestop system; use of an intumescent firestop sealant that swells when heated to close the opening; use of an approved firestop putty pad around the cable; and other listed firestop products suitable for cable penetrations.

The “no coat of paint exemption” that many low-voltage installers believe exists does not. The code makes no distinction between high-voltage and low-voltage cables at fire-rated assembly penetrations. A single thermostat wire passing through a gypcrete floor assembly between an attached garage and the living space above must be firestopped the same as a 2-inch plumbing pipe. The size of the penetration does not exempt it from the firestop requirement; even a small-diameter cable creates an opening through which hot gases, smoke, and eventually flame can pass in a fire event.

Regarding abandoned cable, IRC 2024 Section E4301.6 (which tracks NEC Section 725.25) requires that cables that are no longer in use be removed from premises or identified at both ends as abandoned for future use with a tag acceptable to the authority having jurisdiction. An unlabeled cable with stripped ends hanging in a wall cavity or attic is a violation regardless of the cable’s voltage level. This requirement applies to all types of low-voltage cable: thermostat wire, speaker wire, coaxial cable, alarm cable, and data cable.

For non-fire-rated interior wall penetrations (standard stud walls that do not form part of a fire-resistance-rated assembly), the fire-blocking requirement at the top and bottom plates of each stud bay is the applicable provision. Where low-voltage cable passes through a top plate or bottom plate at a fire-blocking location, the code requires that the penetration be sealed to restore the fire-blocking integrity of the plate. In practice, inspectors accept the use of listed intumescent fire caulk applied around the cable at both sides of the plate hole, or a listed firestop collar or putty pad where multiple cables pass through the same hole.

Why This Rule Exists

Fire-blocking and firestopping are passive fire protection measures designed to slow the spread of fire, smoke, and hot gases through concealed spaces in a building. In a residential fire, the hollow spaces inside walls, floors, and ceilings act as chimneys: hot gases and flames travel through these spaces much faster than through solid materials, allowing a localized fire to spread to other areas of the building before the occupants or fire department are aware of it. Fire blocking interrupts this chimney effect at regular intervals and at critical interfaces between framing members.

A small cable penetration through a fire-blocking plate—even a single thermostat wire—creates a path through which hot gas under pressure can travel. In an experimental test conducted at a single-story stud bay, a 1-inch unsealed penetration through a bottom plate allows the fire-blocking to be completely circumvented within two to three minutes of ignition at the floor level, compared to a 10-minute delay when the same penetration is firestopped. Multiplied across dozens of low-voltage cable penetrations in a typical home—thermostat wires, speaker cables, data cables, alarm wires—the cumulative effect of unsealed penetrations significantly reduces the time available for occupant evacuation.

The abandoned cable requirement addresses the cumulative fire load that builds up in wall cavities over decades of renovation cycles. Each renovation generation tends to add new low-voltage cables and leave the old ones in place. A typical pre-2000 home may have accumulated coaxial cable from three or four different cable TV providers, telephone cable from multiple telephone system generations, early data cable, and various alarm and control wires—all abandoned in wall cavities. This accumulated cable mass represents additional fuel in a fire event and—particularly with older PVC jacketed cable—produces toxic combustion byproducts. Requiring removal or tagging prevents this buildup from continuing.

What the Inspector Checks at Rough and Final

At rough-in inspection, the inspector will look at the top and bottom plates of every stud wall and at floor and ceiling assemblies for evidence of sealed penetrations wherever low-voltage cable runs are visible. Unsealed holes in fire-blocking locations where cable passes through will result in a failed rough-in inspection and a requirement to firestop before closing the walls. The inspector will specifically check the plates in areas with high concentrations of low-voltage cable—home theater rooms, network closets, and HVAC areas with multiple thermostat runs.

At final inspection, the inspector will look for evidence of abandoned cable with unprotected ends in accessible areas such as attics, crawl spaces, and utility rooms. Cable ends that are accessible and untagged will be flagged. The inspector may also open junction boxes and look for excess coiled cable to confirm that abandoned cables are not hidden in the box without proper identification.

In jurisdictions with active code enforcement for low-voltage work, inspectors may request a low-voltage rough-in inspection specifically for this purpose, separate from the electrical rough-in. Check with your local building department whether a separate low-voltage rough-in is required or whether it is covered by the electrical rough-in.

What Contractors Need to Know

The most efficient firestopping approach for low-voltage cable installations with multiple cables through the same plate hole is an intumescent fire caulk applied around the cable bundle at each side of the plate. Use a listed product designed for cable penetrations; the listing will specify the maximum bundle diameter, cable types, and gap dimensions for which the product has been tested and approved. Do not use standard latex caulk or silicone—only listed intumescent or elastomeric firestop caulk is acceptable.

Where a large number of data cables or speaker cables pass through the same hole—as is common in network closets and home theater installations—a listed firestop pillow or firestop foam block is often a faster and more reliable solution than caulk for large openings. These products are pre-formed intumescent materials that are inserted into the opening around the cable bundle and expand in a fire to seal the penetration. They are available in multiple sizes and are listed for specific maximum opening dimensions.

For penetrations through fire-resistance-rated assemblies (typically floor assemblies between dwelling units or between dwelling and garage), a through-penetration firestop system tested to ASTM E814 or UL 1479 is required. These systems list specific combinations of cable type, fill material, and annular space requirements. Do not substitute an untested combination; the testing laboratory listing specifies exactly what cable types and quantities were tested in the system. Consult the manufacturer’s through-penetration listing database (UL SPEC—UL System for Electronic Catalog—is a free online resource) to find a tested system for your specific cable type and assembly configuration.

What Homeowners Get Wrong

The most pervasive homeowner misconception is that firestopping is only required for large penetrations—pipes, ducts, and conduits—and that small holes made by a drill bit for cable routing are exempt. This is not correct. Every opening through a fire-blocking or fire-rated location must be sealed, regardless of size. A drill bit leaves a hole that is larger than the cable, and that gap is an unsealed penetration that must be firestopped.

A second common error is using expanding spray foam (such as consumer-grade polyurethane foam in a can) as a firestop substitute. Standard expanding polyurethane foam is not a firestop material. It will burn vigorously when exposed to flame, actually worsening the fire spread through the penetration. Firestop-specific products (which are clearly labeled as fire caulk, firestop sealant, or intumescent foam) are required; they are chemically formulated to char and expand under heat, sealing the opening as a fire approaches.

Homeowners also frequently abandon low-voltage cables during DIY renovation work without removal or tagging, assuming that out-of-sight means out of mind. During a future renovation or home sale inspection, these abandoned cables are visible in attics and crawl spaces, may be flagged as code violations, and create a perception of deferred maintenance issues even if the abandoned cables are otherwise harmless.

State and Local Amendments

California’s Title 24 and the California Fire Code have penetration sealing requirements for low-voltage cables in residential construction that are at minimum equivalent to IRC 2024 and in wildfire-prone areas may be more stringent. Some California jurisdictions have adopted local amendments requiring that all low-voltage cable penetrations be firestopped regardless of whether they occur at a fire-rated assembly, specifically targeting the fire-blocking plates in standard framing.

Washington State’s statewide amendments to the IRC include clarifications on firestop material selection for cable penetrations that specify performance criteria beyond the generic “approved method” language in the base IRC. Oregon’s structural specialty code similarly incorporates specific firestop system listing requirements for residential cable penetrations that builders and low-voltage contractors should review when working in those jurisdictions.

Many major metropolitan fire departments have adopted supplemental fire prevention ordinances that impose stricter inspection requirements on low-voltage cable penetrations during new construction, driven by fire investigations that identified unsealed low-voltage cable penetrations as contributors to rapid fire spread. Chicago, Los Angeles, and Seattle all have city-specific fire prevention bureau requirements that supplement the state building code on this topic.

When to Hire a Professional

Firestopping is not inherently complex work, but selecting the correct listed firestop product for a given application requires reading the product listing to confirm that the specific cable type, bundle size, and assembly type are within the listing parameters. Using the wrong product—even a product labeled as a firestop—for an application it is not listed for produces a penetration that appears firestopped but is not compliant and may not perform as intended in a fire.

For fire-resistance-rated assembly penetrations between dwelling units in a multi-family dwelling (not typical in a single-family home but common in duplexes and townhomes) or between the dwelling and an attached garage, the specific through-penetration firestop system must be selected from a tested listing database. A licensed contractor or fire protection engineer familiar with UL-listed through-penetration systems should be consulted for these locations. Getting this wrong can have serious fire-safety and liability consequences.

Common Violations Found at Inspection

• Low-voltage cable passing through top or bottom fire-blocking plates in framing bays without any firestop sealant applied around the cable at the plate.
• Standard expanding polyurethane foam (construction foam in a can) used as a firestop substitute around cable penetrations—this product burns and is not a firestop.
• Abandoned low-voltage cable left in wall cavities, attics, or crawl spaces with unprotected bare ends, not tagged for future use.
• Multiple low-voltage cables sharing a large unsealed hole through a fire-blocking plate with no firestop material between the cables and the edges of the hole.
• Firestop sealant applied only to one side of a plate penetration, leaving the other side unsealed.
• Through-penetration of a fire-resistance-rated floor assembly (such as between garage and living space above) without a listed ASTM E814 or UL 1479 tested firestop system.
• Cable bundles that completely fill a plate hole with no gap for firestop material, and no firestop product applied because the installer assumed full penetration is self-sealing—it is not.
• Low-voltage cable routed through an exterior wall penetration without any firestop at the exterior wall plate level, creating a weather and fire path.