Journeyman Electrician Practice Exam

The wiring method that is not allowed in a ceiling space used as a return-air plenum is rigid PVC conduit. This is due to safety regulations regarding fire and ventilation. In areas designated as plenum spaces—such as the space above ceilings used for air return—wiring methods must meet specific fire resistance and low smoke emission standards. Rigid PVC conduit is not typically rated for the plenum applications, primarily because it can release toxic fumes when burned, posing a risk during a fire scenario. On the other hand, flexible conduit, rigid metal conduit, and type NM cable can be permitted in these applications under certain conditions as they meet specific code requirements for performance and safety in plenum spaces. For instance, type NM cable is allowed because it is generally permitted in areas with limited exposure to moisture and where other stringent conditions are met. Similarly, flexible conduit and rigid metal conduit are recognized for their durability and ability to protect against physical damage while also requiring less stringent fire ratings than PVC.