Which type of insulation should not be used for wiring in wet locations?

Type NM (Non-Metallic) cable is designed primarily for dry locations and contains a thermoplastic insulating material that can deteriorate when exposed to moisture. It is not moisture-resistant, which makes it unsuitable for wet locations. In environments where exposure to water or excess humidity is a concern, using NM cable can lead to safety hazards, including insulation breakdown and increased risk of electrical failures.

The other types of insulation mentioned, such as THHN and MTW, are specifically designed to handle more extreme environmental conditions, including wet or damp areas. They are constructed with materials that can withstand moisture, making them safer and more reliable for installations in wet locations. Consequently, the recommendation against using Type NM in such scenarios is based on its inadequate protective qualities against moisture.