THHN vs THWN vs THHN-2 Wire Types - Complete Comparison Guide

When selecting electrical conductors for building wire applications, understanding the differences between THHN, THWN, THHN-2, and THWN-2 insulation types is essential for ensuring code compliance, optimal performance, and cost-effectiveness. These thermoplastic insulation designations represent the most commonly used wire types in modern electrical installations, yet many electricians and contractors don't fully understand their characteristics, proper applications, and the advantages of dual-rated conductors. This comprehensive guide explains everything you need to know about these insulation types to make informed decisions for your electrical projects.

Understanding Wire Insulation Designations

The letters in wire insulation designations are abbreviations that describe specific characteristics of the insulation material and its performance ratings. Understanding what each letter means helps you decode any wire type designation you encounter.

Breaking Down the Abbreviations

• T = Thermoplastic insulation material (as opposed to thermoset rubber insulation)
• H = Heat resistant (90°C or 194°F rating)
• HH = High heat resistant (90°C in dry locations, some applications allow higher)
• W = Wet location rated (suitable for use in damp or wet locations)
• N = Nylon jacket over the insulation (provides mechanical and chemical protection)
• -2 = Second generation rating with enhanced moisture and heat resistance (90°C wet)

All THHN, THWN, and their "-2" variants use thermoplastic insulation materials, typically PVC (polyvinyl chloride) for the base insulation and nylon for the outer jacket. The specific formulations and thicknesses create the differences in performance ratings.

THHN Wire Characteristics and Applications

THHN (Thermoplastic High Heat-resistant Nylon-coated) is one of the most popular wire types for general building wire applications in dry and damp locations.

Temperature Ratings

• Dry locations: 90°C (194°F) maximum operating temperature
• Wet locations: 75°C (167°F) maximum - effectively limiting use in wet locations
• Ampacity: Can use 90°C ampacity tables from NEC Article 310.16 in dry locations

Physical Characteristics

THHN features relatively thin insulation compared to older wire types like THW. The nylon jacket provides excellent abrasion resistance and protection against chemicals, oils, and gasoline. This thin profile means THHN conductors have smaller overall diameters, allowing more conductors in a given conduit size compared to thicker insulation types.

Approved Applications

• Interior branch circuit wiring in residential and commercial buildings
• Feeder and service entrance conductors in dry locations
• Control circuits and industrial wiring in dry environments
• General purpose building wire where moisture exposure is minimal
• Conduit and cable tray installations in dry or damp locations

Limitations

THHN should not be used in wet locations unless specifically marked as dual-rated THHN/THWN or THHN/THWN-2. Direct burial applications require wet location rated wire. The 75°C wet rating effectively precludes THHN-only wire from many outdoor and underground installations.

THWN Wire Characteristics and Applications

THWN (Thermoplastic Heat and Water-resistant Nylon-coated) adds moisture resistance to heat resistance, expanding the range of acceptable applications.

Temperature Ratings

• Dry locations: 75°C (167°F) maximum operating temperature
• Wet locations: 75°C (167°F) maximum operating temperature
• Ampacity: Must use 75°C ampacity tables from NEC Article 310.16

Physical Characteristics

THWN features slightly thicker insulation than THHN to provide enhanced moisture resistance. The insulation formulation resists water penetration, making it suitable for wet and damp locations. The nylon jacket provides mechanical protection similar to THHN.

Approved Applications

• Wet and damp location wiring (basements, outdoor installations in conduit)
• Underground installations in conduit (not direct burial unless specifically rated)
• Areas with high humidity or condensation concerns
• Commercial and industrial buildings where moisture may be present
• Service entrance conductors exposed to weather in conduit

Limitations

The 75°C temperature rating means THWN has lower ampacity than THHN when operating in dry locations. This often requires using larger wire sizes to achieve the same current-carrying capacity, increasing material costs. THWN should not be used for direct burial unless specifically marked for that application.

THHN-2 and THWN-2 - Enhanced Second Generation Wire

The "-2" designation represents second-generation insulation materials with improved performance characteristics, particularly enhanced moisture and heat resistance.

THHN-2 Specifications

• Dry locations: 90°C maximum temperature
• Wet locations: 90°C maximum temperature (major improvement over THHN)
• Ampacity: Can use 90°C tables in both dry and wet locations

THWN-2 Specifications

• Dry locations: 90°C maximum temperature
• Wet locations: 90°C maximum temperature
• Ampacity: Can use 90°C tables in both dry and wet locations

Dual-Rated Wire - The Modern Standard

Most building wire manufactured today carries dual or even triple ratings printed on the jacket, such as "THHN/THWN-2" or "THWN-2/THHN". This dual rating means the wire meets the requirements of both specifications, providing maximum flexibility in applications.

Advantages of Dual-Rated Conductors

• Universal application: Single wire type works in both dry and wet locations at 90°C
• Inventory simplification: Contractors and suppliers can stock one wire type instead of multiple varieties
• Maximum ampacity: Can use 90°C ampacity tables in all applications, allowing smaller wire sizes
• Cost effectiveness: Despite slightly higher per-foot cost, smaller wire sizes often result in overall savings
• Future flexibility: Installations can be modified or extended without wire compatibility concerns

Reading Wire Jacket Markings

Wire jackets contain printed information that repeats every few feet. Typical markings include: manufacturer name, wire size (AWG or kcmil), voltage rating (typically 600V), insulation type (THHN/THWN-2), and applicable standards (UL 83, etc.). Always verify the jacket markings match your specifications before installation.

Comprehensive Comparison Table

Characteristic	THHN	THWN	THHN-2 / THWN-2
Dry Temp Rating	90°C (194°F)	75°C (167°F)	90°C (194°F)
Wet Temp Rating	75°C (167°F)	75°C (167°F)	90°C (194°F)
Wet Locations	Limited use	Yes	Yes
Damp Locations	Yes	Yes	Yes
Ampacity Tables	90°C dry, 75°C wet	75°C all locations	90°C all locations
Insulation Thickness	Thin	Medium	Medium
Conduit Fill	More per conduit	Fewer per conduit	Fewer per conduit
Relative Cost	Lower	Medium	Medium to Higher
Direct Burial	No	No (unless marked)	No (unless marked)
Modern Availability	Usually dual-rated	Usually dual-rated	Standard

Temperature Ratings and Ampacity Considerations

Understanding how temperature ratings affect ampacity is crucial for proper conductor sizing and code compliance.

Using the Correct Ampacity Tables

NEC Article 310.16 (now 310.15(B)(16) in recent code editions) provides ampacity tables at different temperature ratings: 60°C, 75°C, and 90°C. The wire's temperature rating determines which column you use:

• THHN in dry locations: Use 90°C column (highest ampacity)
• THHN in wet locations: Use 75°C column (reduced ampacity)
• THWN in all locations: Use 75°C column
• THHN-2/THWN-2 in all locations: Use 90°C column (highest ampacity)

Practical Ampacity Example

This ampacity difference means that for a given current requirement, you might be able to use smaller wire with THHN-2/THWN-2 compared to THWN, potentially offsetting the higher material cost.

Terminal Temperature Limitations

Conduit Fill Considerations

The different insulation thicknesses between wire types affect how many conductors fit in a given conduit size.

Cross-Sectional Area Comparison

Using 12 AWG as an example from NEC Chapter 9, Table 5:

• 12 AWG THHN/THWN: 0.0133 square inches
• 12 AWG THHN-2/THWN-2: 0.0133 square inches (same as THHN)
• 12 AWG THW: 0.0181 square inches (significantly larger)

THHN and THWN have identical dimensions in NEC tables because modern manufacturing produces both types with the same insulation thickness. The difference lies in the insulation material formulation rather than thickness.

Practical Impact on Installations

The thin insulation of THHN/THWN means you can fit more conductors in a given conduit compared to older insulation types like THW. This often allows using smaller conduit sizes, reducing installation costs. When comparing wire types, always reference NEC Chapter 9, Table 5 for exact cross-sectional areas rather than making assumptions based on wire type alone.

Cost Comparison and Economic Considerations

Material cost represents a significant factor in project budgets, but lowest cost per foot doesn't always mean lowest total cost.

Typical Price Relationships

• THHN (single-rated): Baseline cost (when available separately)
• THWN (single-rated): 5-10% premium over THHN (when available separately)
• THHN-2/THWN-2 (dual-rated): 5-15% premium over basic THHN, but this is now the standard

In practice, most wire sold today is dual-rated, so price comparisons between single-rated and dual-rated wire are largely academic. The real economic comparison is whether you can use smaller wire sizes due to higher ampacity ratings.

Total Cost Analysis

Consider this scenario: A circuit requires 50 amperes continuous load in a wet location. Using 75°C ampacity, you need 8 AWG. Using 90°C ampacity (with THHN-2/THWN-2 and proper derating), you might be able to use 10 AWG in some configurations. The smaller wire costs less per foot, fits in smaller conduit, and requires less labor to install - potentially offsetting the higher per-foot cost of the premium insulation.

Selecting the Right Wire Type for Your Application

Decision Framework

Use this decision tree to select appropriate wire insulation:

1. Will the wire be in a wet location?
   • Yes → Use THWN, THWN-2, or dual-rated THHN/THWN-2
   • No → Continue to step 2
2. Do you need maximum ampacity?
   • Yes → Use THHN-2 or dual-rated THHN/THWN-2 to access 90°C tables
   • No → THHN may be adequate if available
3. Is future flexibility important?
   • Yes → Use dual-rated THHN/THWN-2 for maximum application range
   • No → Match insulation type to specific application

Special Considerations and Applications

Direct Burial Applications

Neither THHN, THWN, THHN-2, nor THWN-2 is inherently rated for direct burial. For underground installations without conduit, you need conductors specifically marked for direct burial, typically:

• USE-2: Underground Service Entrance cable with 90°C wet rating
• UF: Underground Feeder cable for branch circuit applications
• Direct burial rated conductors: Specifically marked on the jacket

THHN/THWN-2 can be used in underground conduit installations, but the conduit itself must be suitable for underground use (PVC Schedule 40, PVC Schedule 80, or RMC are common choices).

High Temperature Applications

Some specialty applications involve ambient temperatures exceeding standard conditions. For these situations, consider:

• XHHW-2: Cross-linked polyethylene insulation rated for 90°C wet, 150°C short-term dry
• RHW-2: Thermoset rubber insulation for enhanced heat resistance
• Ambient temperature derating: NEC 310.15(B)(2)(a) requires derating ampacity when ambient exceeds 30°C (86°F)

Corrosive Environments

The nylon jacket on THHN/THWN provides good resistance to many chemicals, oils, and gasoline. However, extreme corrosive environments may require:

• Specialty wire with enhanced chemical resistance jackets
• Additional conduit protection (PVC-coated rigid conduit, for example)
• Consulting manufacturer specifications for specific chemical exposure

Code Compliance and Installation Requirements

NEC Requirements

Key NEC articles governing wire insulation selection:

• Article 310: Conductors for general wiring, including insulation type tables
• Article 310.10(C): Conductor application and insulation requirements
• Table 310.104(A): Lists application provisions for various insulation types
• Chapter 9, Table 5: Provides cross-sectional areas for conduit fill calculations

Wet Location Definition

Understanding what constitutes a "wet location" is essential for proper wire selection. NEC defines wet locations as:

• Locations subject to saturation with water or other liquids
• Underground installations in direct contact with earth
• Locations subject to weather and not protected from moisture
• Interior locations subject to moderate degrees of moisture (basements, some storage areas)

Frequently Asked Questions

Can I use THHN wire outdoors?

THHN-only rated wire (without THWN rating) should not be used in wet locations. However, dual-rated THHN/THWN-2 wire can be used outdoors when installed in appropriate conduit. The conduit must protect the wire from direct sunlight exposure and physical damage.

Are THHN and THWN the same thing?

No, they have different ratings. THHN is rated 90°C dry but only 75°C wet. THWN is rated 75°C in both dry and wet locations. However, most modern wire is dual-rated as THHN/THWN-2, providing 90°C operation in both conditions.

Why is most wire sold today dual-rated?

Manufacturers found they could produce wire meeting both THHN and THWN-2 specifications with essentially the same construction and cost. This dual rating provides contractors with maximum flexibility and simplifies inventory management. It's become the industry standard because the benefits far outweigh any marginal cost increase.

Do I always get to use 90°C ampacity with THHN-2/THWN-2?

Not exactly. While the wire itself is rated for 90°C, NEC 110.14(C) limits ampacity based on terminal temperature ratings. Most equipment is rated for 75°C terminals, so you must size conductors using 75°C ampacity tables. However, the 90°C rating is valuable when applying derating factors for conductor bundling or elevated ambient temperature.

Can I mix THHN and THWN in the same conduit?

Yes, you can mix different insulation types in the same conduit. However, each conductor must be rated appropriately for its specific circuit requirements and the environment where the conduit is located. All conductors count toward conduit fill calculations using their specific cross-sectional areas from NEC Table 5.

Conclusion

Understanding the differences between THHN, THWN, THHN-2, and THWN-2 wire insulation types enables you to make informed decisions that ensure code compliance, optimize performance, and manage project costs effectively. Modern dual-rated THHN/THWN-2 conductors provide the most versatile solution for general building wire applications, offering 90°C temperature ratings in both dry and wet locations, simplified material management, and maximum flexibility for current and future installation needs.

When selecting wire for your projects, consider not just the immediate application requirements but also potential future uses, the installation environment, ampacity optimization opportunities, and total installation costs beyond just material price per foot. By specifying appropriate insulation types matched to your specific needs, you ensure safe, code-compliant installations that perform reliably throughout their service life.