Marcia Hultman

Cabinet Secretary

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South Dakota Electrical Commission

Thermal Barrier Reference Document

The use of an approved thermal barrier to separate NM cable from the interior of a building is a basic requirement for the NEC. The job of a thermal barrier is to limit the temperature rise that the NM cable will be exposed to. An approved thermal barrier is defined as 1/2-inch gypsum wallboard or the equivalent. ASTM E 119 defines the criteria by which alternative thermal barriers are to be qualified to limit the average temperature rise of the unexposed face to 250° F for 15 minutes of fire exposure.

Before 1975, experience had shown that foam plastics covered with plaster or 1/2-inch gypsum wallboard had performed satisfactorily in building fires. For this reason, 1/2-inch gypsum wallboard was included in the code as a minimum requirement. It was recognized that specifying a single material would not be desirable in a performance code; therefore, the words "or equivalent" were added. The thermal barrier test was selected as the appropriate method to determine equivalent performance. Gypsum wallboard is still the most commonly used thermal barrier, but equivalent materials per the following are permitted and have been used when shown to equivalent.

Time Assigned to Wallboard Membranes
Description of Finish Time (minutes)

Description of Finish


3/8-inch wood structural panel bonded with exterior glue 5
15/32-inch wood structural panel bonded with exterior glue 10
19/32-inch wood structural panel bonded with exterior glue 15
3/8-inch gypsum wallboard 10
1/2-inch gypsum wallboard 15
5/8-inch gypsum wallboard 30
1/2-inch Type X gypsum wallboard 25
5/8-inch Type X gypsum wallboard 40
Double 3/8-inch gypsum wallboard 25
1/2-inch + 3/8-inch gypsum wallboard 35
Double 1/2-inch gypsum wallboard 40