Redsy: See the FPN,
Section 300-21. Spread of Fire or Products of Combustion
Electrical installations in hollow spaces, vertical shafts, and ventilation or air-handling ducts shall be made so that the possible spread of fire or products of combustion will not be substantially increased.
Openings around electrical penetrations through fire-resistant-rated walls, partitions, floors, or ceilings shall be firestopped using approved methods to maintain the fire resistance rating.
FPN: Directories of electrical construction materials published by qualified testing laboratories contain many listing installation restrictions necessary to maintain the fire-resistive rating of assemblies where penetrations or openings are made.
An example is the 24-in. (610-mm) minimum horizontal separation that usually applies between boxes on opposite sides of the wall.
Assistance in complying with Section 300-21 can be found in these directories and product listings.
Joe Tedesco, NEC Consultant