What Are THHN and THWN Wires?

THHN (Thermoplastic High Heat-resistant Nylon-coated) and THWN (Thermoplastic Heat and Water-resistant Nylon-coated) wires are widely used electrical conductors for residential, commercial, and industrial wiring. Both are thermoplastic-insulated wires, typically made with copper or aluminum conductors, and are designed for installation in conduits or raceways. They comply with standards such as the National Electrical Code (NEC), UL 83, and UL 1063, ensuring safety and reliability. THHN is suited for dry or damp locations with high heat resistance, while THWN offers additional moisture resistance for wet environments. Their versatility and durability align with cables like NM, UF, or 6 AWG wires discussed previously, but they are optimized for conduit-based installations.

Construction and Specifications

THHN and THWN wires share similar construction but differ in environmental ratings:

• Conductor: Solid or stranded copper (59.6 MS/m conductivity) or aluminum (61% of copper’s conductivity), available in sizes from 14 AWG to 1000 kcmil, similar to 6 AWG or household wires. Stranded conductors enhance flexibility.
• Insulation: Polyvinyl chloride (PVC) provides electrical isolation and heat resistance, with THHN rated for 90°C in dry conditions and THWN for 75°C in wet conditions (THWN-2 extends to 90°C wet).
• Jacket: A nylon outer layer reduces friction for easy pulling in conduits and protects against abrasion, similar to RHW or XHHW insulation.
• Grounding: Often paired with a separate grounding conductor in conduit systems, per NEC requirements, as discussed in grounding wire applications.

Specifications include a voltage rating of 600V, compliance with UL 83 and NEC, and ampacity ratings per NEC tables (e.g., 65A for 6 AWG copper at 75°C). The nylon jacket ensures durability, with outer diameters varying by size (e.g., 0.249 inches for 10 AWG THHN).

Key Differences Between THHN and THWN

While THHN and THWN are similar, their differences lie in environmental suitability and temperature ratings:

• Moisture Resistance: THHN is rated for dry or damp locations, while THWN is suitable for wet locations (e.g., underground conduits, outdoor installations). THWN-2, an advanced version, matches THHN’s 90°C rating in wet conditions.
• Temperature Rating: THHN supports 90°C in dry environments, while standard THWN is limited to 75°C in wet conditions. THWN-2 extends to 90°C in both wet and dry settings.
• Insulation Properties: THWN’s PVC insulation is formulated for enhanced water resistance, making it more robust in humid or submerged environments, similar to UF or marine cables.
• Applications: THHN is preferred for indoor or protected installations, while THWN is used in outdoor or wet environments, aligning with RHW or H07RN-F cables for specific conditions.
• Cost: THWN may be slightly more expensive due to its water-resistant insulation, though THHN/THWN dual-rated wires are common, reducing cost differences.

Many wires are dual-rated as THHN/THWN, offering flexibility for both dry and wet applications, similar to versatile cables like XHHW or 6 AWG wires.

Applications of THHN and THWN

THHN and THWN wires are used in various electrical installations, often within conduits:

• Residential: THHN powers indoor branch circuits, lighting, and outlets in dry areas, while THWN is used for outdoor circuits, basements, or garages, similar to NM or UF cables.
• Commercial: Both wires support lighting, HVAC, and equipment in offices, retail, or warehouses, with THWN used in wet areas like parking structures, akin to 6 AWG or multi-conductor cables.
• Industrial: THHN/THWN wires connect motors, control panels, and machinery in factories, with THWN preferred for wet environments like processing plants, similar to H07RN-F or trailing cables.
• Outdoor: THWN is ideal for exterior lighting, pumps, or EV chargers, with moisture resistance comparable to UF or marine cables.
• Solar: Both wires are used in PV systems for DC or AC connections, leveraging low resistance, similar to 6 AWG or CCS applications.

Their conduit-based installation ensures protection, aligning with the durability of household or 6 AWG wires.

Safety Considerations

Safe use of THHN and THWN wires requires adherence to electrical codes and best practices:

• Correct Sizing: Match wire size to load (e.g., 30A for 10 AWG copper at 75°C) per NEC ampacity tables to prevent overheating, as with 6 AWG or RHW wires.
• Environmental Suitability: Use THHN in dry/damp locations and THWN in wet areas to avoid insulation degradation, similar to UF or marine cables.
• Grounding: Include a grounding conductor in conduit systems, connected to grounding rods or busbars, as discussed in grounding wire applications.
• Terminations: Use 60°C/75°C-rated connectors for standard THHN/THWN, or 90°C for THWN-2, per NEC guidelines, to avoid overheating.
• Conduit Protection: Install wires in conduits or raceways to protect against physical damage, per NEC requirements, unlike NM or UF cables.
• Professional Installation: Engage licensed electricians to ensure NEC compliance, proper conduit sizing, and secure connections, reducing risks like arcing or faults.

These measures ensure safety, aligning with the focus on LSOH, grounding, or H07RN-F cables.

THHN vs. THWN Comparison Table

THHN and THWN differ in environmental ratings and applications. The table below summarizes their key attributes:

Note: Ampacity and specifications based on NEC 2020. Verify with NEC or electrician for specific conditions.

Tips for Choosing Between THHN and THWN

Selecting the appropriate wire ensures performance and safety:

• Assess Environment: Use THHN for dry indoor conduits and THWN for wet or outdoor areas; opt for THWN-2 for high-heat wet conditions, similar to UF or H07RN-F cables.
• Check Ampacity: Select wire size per NEC tables (e.g., 40A for 8 AWG copper at 75°C) to match load and breaker, as with 6 AWG or household cables.
• Consider Dual Ratings: Choose THHN/THWN dual-rated wires for versatility in mixed environments, reducing inventory needs.
• Ensure Grounding: Include a grounding conductor in conduit systems, as discussed in grounding applications.
• Verify Standards: Confirm compliance with NEC, UL 83, or AS/NZS 5000 for safety and legal requirements.
• Calculate Voltage Drop: For runs over 50 ft, use voltage drop calculators to ensure efficiency, as with 6 AWG or MV cables.
• Consult Professionals: Engage electricians for load calculations, conduit sizing, and NEC-compliant installations, as with household or H07RN-F cables.

These steps ensure reliable wiring, akin to selecting THHN, LSOH, or 6 AWG cables.

Conclusion

THHN and THWN wires are essential for conduit-based electrical installations, offering high heat resistance, durability, and compliance with NEC and UL standards. THHN excels in dry or damp indoor environments with a 90°C rating, while THWN provides moisture resistance for wet locations, with THWN-2 extending to 90°C in all conditions. Their copper or aluminum conductors, PVC insulation, and nylon jackets support diverse applications, from residential lighting to industrial machinery. Proper sizing, grounding, and professional installation are critical to prevent hazards like overheating or faults. By choosing between THHN, THWN, or dual-rated wires based on environmental needs and load requirements, users can ensure safe and efficient power delivery, building on the reliability of cables like 6 AWG, household, or H07RN-F discussed previously.