THHN/THWN conductors are typically used for what insulation type?

• A. Cross-linked polyethylene insulation
• B. Thermoplastic insulation
• C. Rubber insulation
• D. Polyvinyl chloride insulation

Answer: (B)

THHN/THWN conductors are defined by having thermoplastic insulation. This means the insulating material is a thermoplastic plastic that softens with heat and hardens when cooled, a property typical of plastics like PVC used on these conductors. Cross-linked polyethylene isn’t thermoplastic (it’s cross-linked and won’t melt), and rubber insulation is elastomeric rather than thermoplastic, so they aren’t the typical insulation for THHN/THWN. The broader, correct category is thermoplastic insulation.