In which applications is ENT prohibited?

The prohibition of ENT (Electrical Nonmetallic Tubing) in certain applications is primarily due to safety and regulatory standards. Specifically, ENT is not allowed in all plenum applications. Plenum spaces, which are areas used for air circulation for heating, ventilation, and air conditioning (HVAC) systems, can present a risk in terms of fire safety. Materials used in these spaces must adhere to strict flame and smoke rating standards to mitigate the risk of fire spreading.

When looking at ENT, it is important to recognize that it does not offer the same level of fire resistance and low smoke production as materials specifically rated for plenum use. This makes ENT unsuitable for plenums, as any failure in these installations could lead to significant safety hazards in the event of a fire.

In contrast, the prohibition of ENT in underground installations and outdoor settings may not be as comprehensive or strict as in plenum applications. While specific codes may outline restrictions based on environmental exposure or moisture, the critical factor is the heightened risk identified in plenum spaces. Hence, when considering the options, the comprehensive restriction in all plenum applications clearly marks it as the correct answer.