Fire-Classified Floor Box Assemblies

A critical component of building safety is fire-resistive floors that are designed to prevent a fire from rapidly spreading beyond its origin. Until now, incorporating floor boxes required additional insulating material to ensure that the floor retained its fire rating. Wiremold/Legrand has developed fire-classified metal and nonmetallic floor boxes that meet UL (Underwriters Laboratories, Inc.) and independent laboratory certification for fire rating without the subsequent addition of insulating material.

Fire-Rated Floors

There are a number of variables that determine the fire rating of a floor, such as: thickness of concrete, amount of fire proofing sprayed to the underside of the floor structure, and the type of concrete (lightweight or normal weight). Of particular concern is a D900 slab. This is a slab that has been tested to last a certain number of hours in a UL testing regimen that exposes floor assemblies to fire conditions. When the temperature of the floor surface reaches 325° F above ambient temperature, the floor is considered to have failed. Fire ratings, which are generally expressed in time to failure (e.g. 2 hours), are impacted by floor thickness and insulating materials.

Floor boxes, which provide open-space access to power and data/communications, are commonly installed in concrete floors. When boxes and conduit are installed in a concrete floor slab, they introduce into the floor assembly metal, which conducts heat. They also displace concrete, leaving voids. These conditions can impact the fire classification of the floor. According to the Uniform Building Code,

When materials are incorporated into an otherwise fire-resistive [floor] assembly which may change the capacity for heat dissipation, fire test results or other substantiating data shall be made available to the building official to show that the required fire-resistive time period is not reduced.

Thus, the presence of floor boxes and associated conduit within the floor requires the addition of insulating material to maintain the fire rating. This normally involves spraying fire retardant on the underside of the slab.

Current Practices

A typical floor box installation on steel decking involves placing, securing, and leveling the boxes. Conduit is run from electrical and communications closet areas across the decking and is attached to boxes. After the concrete is poured and cured, wires and cables are pulled to each box. Fire retardant material is sprayed on the underside of the floor in thicknesses that are determined by the location of boxes and conduit.

The practice of applying insulation to maintain fire classification, while effective, also raises questions that can impact the installation - and potentially the floor's fire rating. For example, from beneath the slab it may be difficult to determine the exact location of each floor box, and of the conduit that is encased in concrete and no longer subject to visual verification.

These uncertainties may lead to spraying the entire underside of a floor to a thickness that is normally required only directly beneath each floor box. The amount of material applied is often subjective and can vary from inspector to inspector and job to job. Further, should an inspector require the application of additional insulating material, it is an expensive proposition to have a subcontractor return to the site.

Fire-Classified Floor Box

The solution is a floor box that can be classified as a fire-resistant assembly. Such a system would contain sufficient firestopping material to be classified as a fire-rated assembly and ensure that a floor's fire rating is maintained with no additional insulation required.

This concept is not new; fire-rated poke-thru devices have been used for years. Installed in core-drilled holes through the slab, poke-thru devices compensate for the through-floor penetration with integral firestop material so that they maintain the floor slab's fire rating. As discrete, fire-classified assemblies, poke-thru devices do not require the application of additional firestopping products.

Wiremold/Legrand has developed fire-classified metal and nonmetallic floor boxes that preserve a floor's fire rating without spraying on additional insulating material. Unlike standard floor boxes, which accept wiring through conduit knock-outs located in the side of the box, Wiremold® fire-classified floor boxes have conduit stubs that extend downward, penetrating the steel decking into the space below. Horizontal conduit is located in this space, rather than within the floor slab above.

The stems of steel floor boxes that penetrate the floor contain intumescent firestop material. When exposed to high temperatures, this material expands and effectively seals the penetration from flames and heat. In addition, a pad made of insulating material is secured under the metal floor box to control heat propagation through the box into the space above. Because nonmetallic (plastic) floor boxes do not conduct heat, intumescent material located within the box itself is sufficient to maintain the fire rating.

The use of integral intumescent material in fire-classified floor boxes makes them similar to poke-thru devices. Unlike poke-thru devices, which require that the units can be placed 2ft. on center and one for every 65 sq. ft. in a span, metal fire-classified floor boxes can be placed 2ft. on center and one for every 4 sq. ft. in a span. Nonmetallic boxes have the same spacing requirements as poke-thru devices: 2ft. on center and one for every 65 sq. ft. in a span.

Designers and contractors need to be aware of the allowable copper cross section of these new boxes. Unlike standard conduit and floor boxes, the number and size of the conductors allowed is determined through UL fire testing procedures, and not by calculating cable fill percentages. Therefore, refer to the manufacturer's allowable copper cross sectional area for each box, and not the size of the conduit feeding it.

Greater Certainty, More Efficiency

Fire-classified floor boxes remove uncertainty from the design process. Because the floor box is a fire-classified assembly, data is readily available to demonstrate that fire-classified floor boxes do not change the floor's fire rating. Thus there are no gray areas concerning the location and amount of sprayed-on insulation. These floor boxes also eliminate supplemental insulating, which increases installation efficiency.