Type AC cable installed in a duct for environmental air is:

Type AC cable, also known as armored cable, is not suitable for installation where it is exposed to environmental air in ducts. This is primarily due to safety regulations that govern electrical installations to prevent fire hazards and ensure the integrity of the cable in various environmental conditions. In ducts, which can be more susceptible to environmental conditions, the use of Type AC cable could lead to vulnerabilities that the installation standards aim to mitigate.

The National Electrical Code (NEC) specifies certain requirements for cable types used in specific environments. Specifically, in spaces where environmental air is circulated, cables must be rated to handle the exposure to moisture, temperature variations, and potential contaminants. Type AC cable does not carry the necessary ratings to be used safely in these types of installations.

Therefore, the classification as a violation reflects adherence to safety standards that prohibit the use of cables like Type AC in scenarios where they might not perform adequately, thus preventing potential dangers associated with electrical fires or damage.