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UL-910 Plenum Test

A plenum is defined as any space used as part of an air-handling system. This includes heating/air-
conditioning ducts and air returns, which frequently include the space between suspended ceilings and
the floor above in modern office buildings. The National Electric Code (NEC) requires that "exposed
cables (those not in conduit) in plenums be listed as having adequate fire-resistant and low-smoke
producing characteristics ..."

A "Standard For Test Method For Fire and Smoke Characteristics of Cables Used in Air-Handling
Spaces" was developed by Underwriters' Laboratories (UL) to classify cables for this NEC requirement.
This test is performed in a 25 foot Stein Tunnel test furnace (also specified in ASTM E-84 test for
building materials). Designed to match the rigors of the application, this test is quite demanding.

In the UL-910 test, a single layer of 24 foot lengths of cable are supported by a one foot wide cable
rack, which is filled with cables. The cables are ignited by an 88 K W (300,000 BTU/hr) methane flame.
Flame spread is aided by a 240 ft/min draft. During the 20 minute test, flame spread is observed
through small windows spaced one foot apart. Smoke is measured by a photocell installed in the
exhaust duct.

To qualify, cables must have a flame spread of less than 5 feet beyond the end of the 4 1/2 foot
ignition flame, a peak optical density of 0.5 maximum (33% light transmission) and a maximum
average optical density of 0.15 (70% light transmission).