Understanding Wire Insulation
Wire insulation protects conductors and determines where wire can be safely used. Choosing the right insulation type ensures code compliance and long-term reliability.
Common Insulation Types
The most common wire insulation types:
- THHN: Thermoplastic High Heat Nylon (90°C dry)
- THWN: Thermoplastic Heat and Water resistant Nylon (75°C wet)
- THWN-2: Dual rated (90°C dry and wet)
- XHHW: Cross-linked polyethylene (90°C)
Temperature Ratings
Insulation temperature rating affects ampacity:
- 60°C (140°F): TW, UF - limited applications
- 75°C (167°F): THW, THWN - most common
- 90°C (194°F): THHN, THWN-2, XHHW - best performance
Wet vs Dry Locations
Location determines which insulation types are suitable.
Location Guide
Wet locations: outdoor, underground, areas subject to water. Dry locations: indoor, protected areas. Damp locations: partially protected outdoor areas.
Insulation Letter Codes
Understanding wire designation letters:
- T = Thermoplastic
- H = Heat resistant (75°C), HH = Higher heat (90°C)
- W = Wet location rated
- N = Nylon jacket
Choosing the Right Insulation
Select insulation based on installation conditions and code requirements.
Important
Using wire with incorrect insulation rating for the location can cause insulation failure and create fire hazards. Always verify suitability for your installation.
Conclusion
Understanding wire insulation types ensures proper wire selection for any application. THWN-2 is a versatile choice for most installations due to its dual wet/dry rating.
Use our Wire Gauge Calculator to help determine proper wire type for your installation.