In a joint research project involving ASHRAE and the National Research Council Canada (RP 1108), the effect of fires involving communication cable installed in air handling plenums was investigated. The project included surveys in North American office buildings to determine the types and quantities of cable in return air plenums and fire scenarios that could potentially ignite the cables. It also included fire tests performed at three scales: small, medium and full. The small-scale tests were conducted using a cone calorimeter. Medium-scale tests were conducted using a modified standard room fire test facility. Tests conducted in this facility were used to determine the behavior of communications cable with exposure to air heated to 200, 325 and 450°C as well as direct flame impingement. Finally, a series of full-scale fire tests were conducted in a facility set up specifically for this project. Tests in this facility were used to determine the behavior of communication cables in plenums with fully-developed fires in a compartment below the plenum. In this report, data from the tests are used to investigate the potential effect of fires involving cables in plenum spaces on the hazard to building occupants in compartments contaminated by smoke distributed through a building heating, ventilation and air-conditioning (HVAC) system.