Keeping the Fire Rating
Robert Grupe / December 2016
Q: How do you maintain the integrity of a fire-rated wall when you have to penetrate the gypsum panels for electrical outlet boxes?
A: The term “fire-rated” often refers to a wall assembly that has a specific “fire-resistance rating.” This term is defined in the 2015 International Building Code as follows: “The period of time a building element, component or assembly maintains the ability to confine a fire, continues to perform a given structural function, or both, as determine by tests, or the methods based on tests, prescribed in Section 703.”
Relating to fire-resistive construction, the code establishes “Fire Barriers,” “Fire Partitions” and “Fire Walls” definitions. However, delving into those definitions is a topic for another time.
The test method that is prescribed in Section 703 is ASTM E119, Standard Test Methods for Fire Test of Building Construction and Materials. These are test methods that yield the hourly fire resistive ratings for walls, floor/ceiling, roof/ceiling, beams and columns.
Any breach in the continuity of the gypsum panels in an assembly, wall or ceiling will have a negative impact on the integrity of the fire resistance of the overall assembly. Therefore, it would seem that installing an electrical outlet in a wall would have the same effect.
Guidance, in this case, comes from Underwriters Laboratories Online Certifications Directory. This is the directory that provides information on specific Underwriters Laboratories Designs. UL Design U411, a generic steel stud, gypsum panel assembly, is an example of what is in the directory. At the beginning of that listing, as well as all others, is a note that states, “See general information for Fire-resistance Ratings.” The information contained there is very important, and it’s worth the effort to study. Stepping back to the UL Design U411 listing, it provides an illustration of what was tested. Along with that assembly drawing and the hourly rating that was obtained is a list of products that are acceptable for incorporation. The general information may be thought of as an extension of the original design because it provides the contractor with acceptable variances. For example, UL U411 calls for a minimum 2 1/2 inch, 18 mil stud spaced 24 inches on center. The general information describes how a 6-inch, 54-mil stud at 16 inches on center can be substituted without negating the fire-resistive performance of the assembly.
The answer to the above question can be found under section four, “Walls and Partitions.” Item six, “Metallic Electrical Outlet Boxes,” is specific to our concern. It states the following: “Certified single- and double-gang metallic outlet and switch boxes with metallic or nonmetallic cover plates may be used in bearing and nonbearing wood stud and steel stud walls with ratings not exceeding 2 h.”
To meet that acceptance there are a few stipulations. They are as follows:
1. Walls should have gypsum wallboard facings similar to specific UL designs.
These designs are standard one and two-hour wood or steel framed non-load-bearing or bearing assemblies. Essentially using 5/8-inch type X gypsum board on both sides of a framing member.
2. Boxes need to be secured to the framing.
3. The gap between the box and the gypsum board should not exceed 1/8 inch.
4. The surface area of a singular box should not exceed 16 square inches.
5. The aggregate surface area of a multitude of boxes shall not exceed 100 square inches in 100 square feet of assembly.
6. Boxes on opposite sides of the wall should be offset by 24 inches.
The implication here is that nothing more needs to be done to the assembly if the above requirements are met. This may be acceptable for fire performance, but it becomes a huge issue for acoustical performance. In a prior article, there was discussion on acoustical flanking paths and how their presence has an enormous negative impact on acoustical performance. The allowable 1/8-inch gap may be permissible for fire, but not for sound. Just the presence of the box alone in the wall will influence the sound performance, add any gap, and effectively the acoustical rating is gone. It’s for this reason that there are many details available for sealing outlet boxes. Contractors can use acoustical sealant or specially made products for that application.
So, the answer to the question on the impact of outlet boxes in fire-resistant partitions is minimal if certain requirements are met. The bigger issue may be the impact on sound attenuation. This is another example where a solution for one performance condition can have a deleterious effect on another. The question that may arise is what to do if the conditions force the inability to meet the above conditions. There are solutions available that require special detailing, special materials or both. It is suggested to discuss these with a favored building product manufacturer.
Robert Grupe is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.