Firestopping electrical systems

When electrical engineers review the location of rated walls, they should consider the layout of their equipment to avoid the need to firestop.

By Timothy E. Kuhlman, PE, RCDD, CH2M HILL, Portland, Ore. April 1, 2011

Firestopping may not be the first thing that comes to mind for an electrical engineer designing a power system. Too often, the solution to firestopping a bank of conduits is left to the architect or contractor. This can lead to costly construction workarounds and delays in getting a building occupancy permit. Ideally, the electrical engineer will be working with the architect to understand where the rated assemblies (walls and floors) are in a building and designing their power system with those assemblies in mind. This consideration taken into account early in the design can save a lot of construction cost and angst at the end of the project.

When electrical engineers review the location of rated walls, they should consider the layout of their equipment to avoid the need to firestop. This doesn’t mean hiding penetrations. It means avoiding the penetration of the rated walls in the first place. Figures 1 and 2 showing a rated partition wall separating a transformer and panel board. Because the conduit does not penetrate the rated partition, no firestopping is required.

Although Figures 1 and 2 illustrate the simplest of electrical systems, there are some fundamental points that can be extrapolated and applied to a more complex power design. The best way to ensure a rated assembly, such as a wall, maintains its fire rating integrity is to not penetrate the wall. A firestopping system that would be required (in Figure 1) has to be specified, installed, and maintained. All, if not done properly, can compromise the wall’s integrity. The design in Figure 2 will require additional effort by the electrical designer for placement of the gear and specifying the proper conduit and wire. On the other hand, the architect didn’t have to specify a firestop penetration and the contractor never had to install it.

When avoiding the rated assembly is not practical or possible, a firestopping system is required. Selecting a firestopping system for a power system is slightly different than selecting a system for power-limited systems such as data networks, fire alarms, and controls where the cabling is commonly routed exposed (not in conduit or enclosed wireway). There are less likely to be changes in the power system distribution than in power-limited systems. The part of the power system that is most likely to see changes in the life of a facility is from the panel boards to the outlets or lighting. Even then, this amount of change is considerably less than what is typically seen for network cabling. Therefore, selecting a firestop system that allows for multiple entries is not a priority consideration for a powered system.

To properly select a firestopping system, the power system designer should discuss the rated assemblies and their construction with the architect. The rating of the assembly, the construction materials being used, and the thickness and spacing of structural supports can affect the selection of the firestopping system. The designer also must understand the codes that define the fire and smoke protection for a building. Each authority having jurisdiction (AHJ) defines the codes applicable to their jurisdiction. For this discussion we will be referencing the International Building Code—2009, Chapter 7, Fire and Smoke Protection Features.


If you can’t avoid penetrating a rated wall, then you better plan for it. When planning for a wall penetration you need to understand three things: the rating of wall, how the wall is constructed, and the materials that will be penetrating it. The architect can tell you the wall rating and its construction. Common materials for a fire-rated wall type include layers of gypsum board, concrete, or concrete blocks. Common electrical system components likely to penetrate a wall include conduit, busduct, cable tray, and cable. Concrete and masonry walls require early coordination of the exact size and location of openings where reinforcing is necessary.

Timing, flexibility, and access are also considered while planning the penetrations. Depending on the design and construction schedule, the rated wall type may be decided long before the electrical gear is selected and its distribution determined. This could be due to a fast-track project approach or long-term phasing of the project. When planning a penetration in a concrete or block wall, a certain amount of flexibility may be required depending on the size of a cable tray or manufacturer of a busduct. Making a penetration larger in this type of wall after it’s built can be a problem. Not all rated walls have access to both sides of the wall. This may require the selection of a penetration method that can be sealed with one-sided access.


Table 1 shows a list of common electrical penetrations along with a list of typical firestopping types. Let’s consider the firestopping for a single conduit penetration. This is probably the simplest and most common type of electrical penetration. For a gypsum-board constructed, 2-hour rated wall, the common firestop system will be either caulk or composite sheet. As the penetrating hole and annular space get larger, more firestop material and structure are required to hold the firestop material in place.

For example, Figures 3 and 4 show that a single 4-in. conduit making a 5-in. penetration through a rated gypsum wall can be sealed using only caulking. This type of system is good for a gap between the conduit and penetration that is 2 in. or smaller. If the penetration gap is larger than 2 in., an approved backing material is needed to hold the caulk in place. The use of a 28-gauge metal sleeve in the penetration and packing material consisting of mineral wool insulation allows the annulus gap to be up to 3 in. The firestop caulk type is the same for both penetrations, but the larger annulus space around the conduit requires additional firestopping structure. For firestopping holes with an annulus greater than 3 in., a composite sheet material can be used.

The composite sheet is secured to the wall and is cut to close the annular space around the conduit penetration. The size of the penetration in the rated wall is limited to the system requirements of the composite sheet. For a gypsum stud wall, the composite sheet requires additional horizontal bracing in the wall to secure the top and the bottom seam. The composite sheet is often used in conjunction with a firestop sealant. Per Underwriters Laboratories (UL) system requirements, the fire sealant is used at the seams and around the gap or interface between the conduit penetration and the composite sheet.

Cable tray

As shown in Table 1, the available firestop systems for cable tray differ from those for conduit in that cable tray allows for the use of firestopping pillows. There are systems that use caulk to firestop a cable tray penetration; the caulk is used in conjunction with a mineral wool insulation. In the example in Figure 5, the penetration using caulk allows for a maximum cable cross-sectional area of 38% of the cable tray cross-sectional area to be used for loading depth.

Although the use of caulk is possible, it is not very flexible. More common are firestop pillows or composite sheeting used with cable tray penetrations. Figure 5 shows the same type of cable tray being firestopped with pillows. Re-entry of the firestop system using pillows is slightly easier because there isn’t as much caulk to be removed. For this system, some caulk is still used to seal around the voids and gaps that don’t seal around the cables. This system allows for a maximum cable cross-sectional area of 30% of the cable tray cross-sectional area to be used for loading depth.

Of the three choices, the composite sheet provides for the neatest installation while maintaining the integrity of the firestop. There aren’t any pillows that can be knocked out or vibrate out of position. This assembly may require a little more labor to fasten the composite sheet in the penetration and to trim the sheet to the profile of the cable in the tray. It’s not as easy to re-enter if cables are added, as the composite sheet will have to be removed to trim it to the new cable profile. However, a neat installation provides for a clear visual inspection of the assembly to see if it has been violated.

Busway and cablebus

Busways, also referred to as bus ducts, can be firestopped using any of the three types of penetration sealants shown in Table 1. Smaller busway assemblies can be firestopped with just the use of a firestop sealant. Larger busway constructions may require the use of packing material in conjunction with the sealant to fill the annular voids. In addition, a manufacturer’s wall plate bulkhead may have to be provided to make the system work as intended by the UL system requirements.

Firestopping pillows and composite sheet can be used with busways. Either the pillows are packed around the busway or the composite sheet is cut to fit the shape of the busway. In both applications, a small amount of sealant is required to fill any voids around the busway. Unlike cable tray, the firestop system does not impose a fill percent limitation on the busway. The bus bars are completely contained on the inside of the busway, and the firestop material is applied to the outside of the busway enclosure. It is important to work with the manufacturer of the busway product to specify the correct type of busway segment that will pass through the rated wall.

Cablebus is a unique power distribution method consisting of an enclosure that is similar to a ventilated cable tray system with a cover, insulated conductors, and cable separators. The cablebus is manufactured with cable support blocks to maintain separation between the conductors to take advantage of the free air rating of the conductors. Unlike busway, where a solid enclosure is provided by the manufacturer to penetrate walls assemblies, the cablebus system maintains its open ventilation. A cablebus penetration will require firestop components inside the cablebus enclosure in addition to firestop sealant on the outside of the cablebus. In laying out the cablebus system to be manufactured, the electrical engineer needs to provide the cablebus manufacturer with the specific location and the rating of the rated assemblies the cablebus will be passing through. While the power distribution methods using conduit and cable tray allow for a selection of firestopping methods, the firestop for cablebus is integral to the cable and cablebus manufacturer.

Special areas and special assemblies

Special areas require special assemblies. The firestopping systems considered up to this point are primarily intended to stop fire and smoke. In some industrial environments, the assembly may also have to serve as a seal against liquids or vapors, or be exposed to hazardous materials. This requires a unique type of firestopping product. These specialty products consist of a frame with block inserts. The block inserts come in a variety of sizes to adapt to a range of diameters for conduit or cable penetrations. Each block is made of concentric laminations that can be removed in small increments to allow for a tight seal around a conduit or cable. When cable or conduits are fully assembled in the penetration frame, a compression block is used to lock the sealing blocks in place.

There are some trade-offs in specifying this type of assembly. The laminated block firestop typically doesn’t allow for the same number of cables to pass through the same size of wall penetration as the more traditional firestop materials previously noted. When penetrating concrete floors or walls, the engineer needs to plan the size of the opening so that the firestopping mounting frame can be either cast or bolted into place afterward. However, this type of firestop is designed for a more challenging environment. In addition, it provides flexibility to add or delete cables in the penetration after the initial installation.

For example, in the case of a microelectronic manufacturing environment, the manufacturing tools will be increased or swapped out as the factory ramps up in yield. The microelectronics environment, an H5 occupancy, may require a firestopping assembly for the power conductors connected to the manufacturing tools coming from the power distribution panels supplied from outside of the fabrication area. Microelectronic manufacturing plants use a variety of hazardous production materials to which the firestop could be exposed.

The specialty firestop block inserts can be opened to allow new cables to pass through, or blocks can be added to seal unused openings. This can be done without disturbing the other cables and doesn’t require the re-application of caulk, trimming of composite sheet, or replacement of pillows. This system is designed for ease of assembly and disassembly. A system like this that is designed for easy re-entry is less likely to be compromised and therefore more likely to maintain its firestop and sealing integrity.

Unused penetrations

For a variety of reasons, it will be necessary to seal off a penetration through a rated wall that does not have a power conductor going through it. This might be a penetration that is made for equipment that hasn’t been installed yet or for equipment that has been relocated. Regardless of the reason, if there is an unused penetration in a rated wall, then it will need to be repaired or firestopped. Smaller holes in a rated wall may be sealed with a firestop sealant or putty. UL system requirements apply to unused penetrations (holes) just as they apply to penetrating items. Depending on the firestop caulk or putty used, the UL system application description will prescribe the size of the hole that can be sealed. For caulking-type firestopping the limit is approximately 2 in. in diameter or less, depending on the manufacturer.

Larger penetrations can be sealed with pillows or composite sheet materials. As with caulking, the UL system will prescribe the maximum size penetration that can be sealed. This is usually given in a maximum square inch area and the maximum length for one side of the penetration. Unused penetrations that are larger than the maximum allowable diameter to which the firestopping products can be applied will require the reconstruction of the rated wall to re-establish the wall’s rated integrity.

Other considerations

So far we have considered how to firestop a rated wall for the power system penetrating it. The selection of the firestop product may affect cable tray fill, and might also affect the de-rating of the conductors passing through it. Depending on the firestop system selected, the length of the cable tray, conduit, or conductor path may be insulated by the firestop system. This, in turn, can cause a hot spot in the conductor path. For example, conductors in a cable tray passing through a composite sheet used for firestopping have a limited contact area with the firestopping system. A penetration using firestopping pillows may have a 6-in. length of conductors and cable tray that is insulated by the pillows. In most applications, the requirements of NEC 310.15 and Annex B will define the ampacity of the cable. IEEE Std 666-2007, IEEE Design Guide for Electric Power Service Systems for Generating Stations, offers considerations for ampacity adjustment when the firestopping method causes a thermal gradient in the cable or other thermal change to the raceway.

Another complicated issue for firestopping a conduit is where there are back-to-back rated walls that can move independently. This can occur in a rated firewall partition located between two structures separated by an expansion joint or a structural isolation break. The designer must understand the amount of building movement, evaluate the support system, and then select the firestop system to that will work. Once again, this provides an opportunity to coordinate with the firestop manufacturer and the architect to find a workable solution.

Coordinating with the architect

The firestop construction specification is typically owned by the project architect. It is necessary to work with the architect to understand the rated assemblies throughout the facility. They will be key in providing a consistent firestop solution for each rated assembly. The design process between the architect and electrical engineer may be iterative as there are limits to the total amount of penetrations that can be made or grouped in a rated assembly and there are limits to the configuration of the power system distribution.

When an existing firestop system cannot be prescribed for a penetration, the architect may look for alternative means and methods to satisfy the intent of the building code, AHJ, or insurance carrier. This may extend beyond the specification of a firestop system and include other fire protection means such as sprinklers, early warning smoke detection, and compartmentalization of the space concerned.

An architect may also work with a firestopping manufacturer to develop an engineered solution. Firestop systems are specific to the rated assembly construction, the materials that are passing through them, and firestop materials. If any of these components are outside of the parameters of the UL system description, then that UL system cannot be applied. The firestop manufacturer may do its own testing to provide data showing that the engineered solution is viable. The architect can submit this information to the approving bodies for compliance.

Commercial facilities most likely will use a wide variety of firestopping systems that have already been tested and cataloged with a UL system number. However, coordination with the architect is still necessary to avoid delays in construction and building occupancy. In industrial facilities where the power systems are much larger and more complicated, and the environment is likely to be hazardous, using off-the-shelf solutions for firestopping becomes more difficult. The development of building code alternative means and methods or working with a manufacturer for an engineered firestop solution takes time and planning.

Kuhlman has 24 years of experience in the design and construction of telecommunications infrastructure. He is a member of Consulting-Specifying Engineer’s Editorial Advisory Board.


Firestop system, appliance, or device: Material designed to prevent the spread of fire through openings in a rated wall or floor assembly.

L rating: The L rating determines the amount of air leakage, in cubic feet per minute per square foot of opening, through the firestop system at ambient and/or 400 degrees F air temperatures at an air-pressure differential of 0.30 inches of water column.

W rating: The Class 1 W rating determines the capability of a firestop system to maintain watertightness of the penetration through a floor or wall construction at the ambient air conditions under 3 feet of water pressure head for a period of 72 hours.

Underwriters Laboratories (UL) system: A firestop system assigned a number by Underwriters Laboratories.

Source: UL XHEZ Guide


IEEE Standard 666-2007 IEEE Design Guide for Electrical Power Service Systems for Generating Stations

International Building Code, 2009

Litchfield, Karen, PE, CH2M Hill: Contributor

National Electrical Code, 2008 Edition

UL XHEZ. Guide Info Through-penetration Firestop Systems

UL Firestop System Number Application Sheets: UL W-L 1049, UL W-L 1079, UL W-L 1384, UL W-L 4005, UL W-L 4008, UL W-L 4063, UL W-L 6004, UL W-L 6005, UL W-L 6020, UL W-L 0034, UL W-L 0010, UL W-L 0020, UL C-AJ-8045, C-AJ-3160 & W-J-3052