THHN vs. THWN Wire: Key Differences, Ratings, and Uses
THHN and THWN are two common types of insulated building wire used in residential, commercial, and industrial electrical installations. Although their names are similar, they have different performance characteristics, particularly regarding temperature ratings and moisture resistance.
The main difference between THHN and THWN wire is their rated suitability for different environments. THHN stands for thermoplastic high heat-resistant nylon-coated wire, while THWN stands for thermoplastic heat- and water-resistant nylon-coated wire. Traditional THHN is commonly rated for 90°C in dry locations, while traditional THWN is commonly rated for 75°C in wet and dry locations. The exact ratings depend on the product's listing and markings.
Many modern building wires carry a dual marking: THHN/THWN-2. These conductors meet the applicable requirements for both designations and are commonly rated for 90°C in wet and dry locations, subject to their specific listing.
For electricians, contractors, distributors, and project buyers, choosing between THHN and THWN requires more than comparing insulation names. The installation environment, conductor size, allowable ampacity, terminal ratings, and applicable electrical code all affect the final decision.
THHN vs. THWN Wire: Quick Comparison
Feature | THHN Wire | THWN Wire | THHN/THWN-2 Wire |
Full designation | Thermoplastic High Heat-Resistant Nylon-Coated | Thermoplastic Heat- and Water-Resistant Nylon-Coated | Meets applicable THHN and THWN-2 requirements |
Moisture resistance | Do not assume wet-location suitability from THHN alone | Designed for wet-location use within its listing | Commonly suitable for wet and dry locations when properly marked |
Typical temperature rating | 90°C in dry locations | Traditional THWN commonly 75°C in wet and dry locations | Commonly 90°C in wet and dry locations |
Insulation construction | Thermoplastic insulation with nylon coating | Thermoplastic insulation with nylon coating | Product-specific insulation system with applicable dual ratings |
Common installation | Approved raceways and building wiring | Approved raceways and wet-location wiring | General building wiring in approved raceways |
Key consideration | Confirm whether the product has additional wet-location markings | Check the exact temperature rating and listing | Verify the complete marking and installation requirements |
Important: The table describes common ratings, not universal specifications for every product. Always verify the complete conductor marking, manufacturer's datasheet, applicable product listing, and locally adopted electrical code before installation.
What Does THHN Wire Mean?
THHN stands for Thermoplastic High Heat-Resistant Nylon-Coated wire. It is a type of building wire commonly used for electrical distribution and control applications.
THHN conductors typically have thermoplastic insulation and a nylon outer covering. This construction supports installation in suitable raceways and provides insulation performance for approved electrical applications.
THHN Wire Temperature Rating
THHN wire is commonly rated for a maximum conductor temperature of 90°C in dry locations. However, the actual rating must be confirmed from the wire's markings and product documentation.
The temperature rating describes the insulation system's permitted operating temperature under specified conditions. It does not automatically establish the maximum current that the conductor can safely carry in every installation.
Allowable ampacity also depends on:
Conductor material and cross-sectional area or AWG size
Ambient temperature
Number of current-carrying conductors in a raceway
Applicable correction and adjustment factors
Equipment terminal temperature ratings
Overcurrent protection and other code requirements
For example, a 90°C-rated conductor may be used for certain ampacity correction and adjustment calculations where permitted. The final allowable ampacity must still comply with the applicable code limitations, including terminal temperature restrictions.
Common Applications of THHN Wire
THHN wire is commonly used in approved building-wire installations, including:
Branch circuits in residential and commercial buildings
Feeders and electrical distribution systems
Industrial control panels and equipment wiring
Lighting and power circuits
Conductors installed in approved electrical raceways
The specific application must be permitted by the conductor's listing and applicable electrical code. THHN marking alone should not be treated as approval for direct burial, every outdoor exposure, or every wet-location installation.
What Does THWN Wire Mean?
THWN stands for Thermoplastic Heat- and Water-Resistant Nylon-Coated wire. Like THHN, it is a building-wire designation associated with thermoplastic insulation and a nylon covering.
The important distinction is its water-resistance designation. THWN is intended for use in wet locations when the conductor is properly listed and installed according to the applicable requirements.
Wet locations can occur in more places than many people expect. For example, raceways installed outdoors or underground may accumulate moisture internally, even when the raceway is designed to protect the conductors.
THWN Wire Temperature Rating
Traditional THWN wire is commonly rated for 75°C in wet and dry locations, but the actual product marking takes precedence.
THWN-2 is a different designation. The “-2” indicates a 90°C temperature rating in both wet and dry locations under the applicable product standard and listing.
This distinction matters when selecting wire for installations that may experience both moisture and elevated operating temperatures.
Before specifying THWN wire, verify:
Whether the conductor is marked THWN or THWN-2
Its wet- and dry-location temperature ratings
Its conductor material and size
Its approved installation methods
Its compliance with the applicable electrical code
Common Applications of THWN Wire
Properly listed THWN conductors may be used in approved applications where moisture resistance is required, such as:
Electrical conductors installed in wet-location raceways
Certain outdoor electrical installations
Commercial and industrial wiring systems
Raceway installations that may be exposed to moisture
Wet-location suitability does not automatically mean that a conductor is approved for direct burial, sunlight exposure, or every outdoor use. Those applications require the appropriate markings and listing.
What Is THHN/THWN-2 Wire?
THHN/THWN-2 is a dual-rated building wire that meets the applicable requirements for both THHN and THWN-2.
This marking is common on modern building wire because it combines high-temperature performance in dry locations with wet-location suitability under the applicable listing.
Why Is THHN/THWN-2 So Common?
A dual-rated conductor can be practical for projects where the wiring route includes different environmental conditions. Instead of specifying separate conductor types for certain dry and wet raceway applications, a properly listed THHN/THWN-2 product may be suitable for both.
Its common advantages include:
A 90°C temperature rating in wet and dry locations, subject to the product listing
Suitability for a broad range of approved raceway installations
Convenient specification and inventory management
A familiar designation for electrical contractors and distributors
However, the dual marking does not mean the wire can be used in every environment or installation method. Direct burial, sunlight resistance, exposed use, and other special conditions must be verified separately.
THHN vs. THWN: Five Key Differences
1. Moisture Resistance
Moisture resistance is one of the most important differences between the two designations.
THHN identifies a high heat-resistant thermoplastic building wire, but the THHN marking alone does not establish that the product is suitable for wet locations.
THWN specifically identifies a water-resistant designation. When the conductor is properly listed for the application, it can be suitable for approved wet-location installations.
For outdoor or underground raceways, verify the complete wire marking rather than assuming that a raceway will always remain dry.
2. Temperature Ratings
Temperature ratings can affect conductor selection and ampacity calculations.
Traditional THHN is commonly rated for 90°C in dry locations. Traditional THWN is commonly rated for 75°C in wet and dry locations. THWN-2 is commonly rated for 90°C in both wet and dry locations.
These ratings are not interchangeable without considering the actual product marking and listing.
A higher insulation temperature rating also does not automatically permit a higher circuit ampacity. Terminal ratings and code requirements can limit the allowable current.
3. Installation Environment
THHN is commonly used in approved dry-location raceway installations. THWN is designed for wet-location use within its listed ratings. THHN/THWN-2 combines both designations and is commonly used in approved raceway installations that may encounter either condition.
The wiring method must still comply with the applicable code. The conductor designation alone does not determine whether the complete installation is acceptable.
4. Availability and Purchasing
THHN/THWN-2 is widely available in many modern building-wire product ranges. It can simplify purchasing where a project requires conductors suitable for both wet and dry locations.
However, purchasing decisions should not be based on availability alone. Contractors and distributors should confirm conductor size, material, voltage rating, temperature rating, certification, packaging, and intended application.
5. Application Limitations
Neither THHN nor THWN should be treated as a universal wire type.
For example, a conductor listed for use in a raceway is not automatically approved for direct burial. Similarly, wet-location suitability does not necessarily establish resistance to sunlight or approval for use outside the product's specified wiring methods.
Always confirm that the conductor and the entire wiring system are approved for the intended installation.
THHN vs. THWN: Advantages and Limitations
Advantages and Limitations of THHN
Advantages
Widely used in commercial, residential, and industrial building wiring.
Commonly available in multiple conductor sizes.
Nylon-coated construction supports installation in suitable raceways.
Many products carry a 90°C dry-location rating.
Limitations
THHN-only marking should not be assumed to establish wet-location suitability.
Actual ampacity depends on conductor size, installation conditions, and code restrictions.
Not automatically approved for direct burial or all outdoor applications.
Advantages and Limitations of THWN and THWN-2
Advantages
Designed for water resistance within the applicable product listing.
Suitable for approved wet-location installations.
THWN-2 commonly provides a 90°C rating in both wet and dry locations.
Dual-rated THHN/THWN-2 products can simplify conductor selection for many raceway projects.
Limitations
Traditional THWN and THWN-2 have different temperature ratings.
Wet-location suitability does not automatically establish direct-burial approval.
A 90°C rating does not override terminal limitations or other code requirements.
How to Choose Between THHN and THWN Wire
Selecting the right wire begins with understanding the installation environment and confirming the product's listed capabilities.
Step 1: Identify the Installation Location
Determine whether the conductor will be installed indoors, outdoors, underground, or in a location where moisture may be present.
Remember that raceways can be considered wet locations under applicable electrical codes. Do not assume that conductors inside an outdoor or underground raceway will remain dry.
Step 2: Check the Complete Wire Marking
Read the printing on the conductor jacket and review the manufacturer's datasheet.
Look for designations such as:
THHN
THWN
THWN-2
THHN/THWN-2
Also confirm the conductor size, material, voltage rating, temperature rating, and applicable product certification.
Step 3: Confirm the Required Ampacity
Determine the required circuit current and conductor size using the applicable electrical code.
Account for conductor material, ambient temperature, the number of current-carrying conductors, correction and adjustment factors, and equipment terminal ratings.
Do not select a wire solely because its insulation is marked 90°C. The permitted ampacity must satisfy all applicable restrictions.
Step 4: Verify the Approved Wiring Method
Confirm whether the conductor is approved for the intended raceway, wet location, sunlight exposure, direct burial, or other installation condition.
If the application requires a particular listing or marking, verify that the product documentation explicitly supports it.
Step 5: Follow Local Electrical Requirements
The National Electrical Code (NEC) is used in many U.S. jurisdictions, but local adoption and amendments can vary. Other countries may follow different electrical standards.
Consult the applicable code, product instructions, and qualified electrical professionals when the installation requirements are unclear.
Common Mistakes When Comparing THHN and THWN
Choosing wire based on an abbreviated name can lead to specification errors. Avoid these common mistakes.
Assuming THHN Is Always Suitable for Wet Locations
THHN-only marking does not establish wet-location suitability. Check for additional markings and the product's listed uses.
Confusing THWN With THWN-2
Traditional THWN is commonly rated for 75°C, while THWN-2 is commonly rated for 90°C in wet and dry locations. Always check the actual marking instead of assuming the ratings are identical.
Using the Insulation Rating as the Final Ampacity
The insulation temperature rating is only one factor in ampacity calculations. Terminal ratings, conductor size, ambient conditions, and code-required adjustments must also be considered.
Assuming All Outdoor Wiring Can Be Directly Buried
A conductor suitable for wet locations is not automatically suitable for direct burial. Verify the product's approved installation methods.
Ignoring Product Certifications and Markings
Two products with similar names may have different ratings or approved uses. Review the complete conductor marking and manufacturer's documentation before ordering or installation.
THHN vs. THWN for Solar and Industrial Projects
THHN and THWN are building-wire designations commonly encountered in electrical distribution and industrial installations. However, they should not automatically be substituted for specialized wire products used in photovoltaic systems.
Solar PV applications may require conductors specifically listed and marked for photovoltaic use, with appropriate voltage, temperature, environmental, and performance ratings.
For example, PV wire listed to UL 4703 and photovoltaic cable designed to applicable standards such as EN 50618 or IEC 62930 are specified according to their own requirements. These products should be selected based on the project jurisdiction, system design, environmental conditions, and applicable standards.
When sourcing conductors for solar, industrial, or export projects, buyers should confirm:
The required wire or cable standard
Rated voltage and temperature
Conductor material and cross-sectional area
Insulation and sheath materials
Wet-location, sunlight, and other environmental requirements
Required testing, certifications, and documentation
Compatibility with the intended electrical equipment and installation method
For manufacturers, distributors, and project procurement teams, a clear technical specification helps reduce product mismatches and supports more reliable installation planning.
Frequently Asked Questions
Is THHN or THWN wire better?
Neither is universally better. THHN is commonly rated for 90°C in dry locations, while traditional THWN is commonly rated for 75°C in wet and dry locations. THWN-2 is commonly rated for 90°C in both wet and dry locations. The right choice depends on the product listing and installation requirements.
Can THHN wire be used in wet locations?
Do not assume that THHN-only wire is suitable for wet locations. Verify the complete conductor marking and manufacturer's documentation. A conductor marked THHN/THWN-2 may be suitable for wet locations when properly listed and installed according to the applicable code.
What is the difference between THWN and THWN-2?
The main distinction is the temperature rating. Traditional THWN is commonly rated for 75°C in wet and dry locations, while THWN-2 is commonly rated for 90°C in both wet and dry locations. Always verify the actual product marking.
Is THHN/THWN-2 wire suitable for outdoor use?
It may be suitable for approved outdoor raceway installations when the product is listed for the application. However, outdoor use can involve wet locations, sunlight exposure, direct burial, or other conditions requiring additional ratings or approvals. Check the product documentation and applicable code.
Can THHN wire be used underground?
THHN-only marking does not establish direct-burial suitability. Underground raceways may be considered wet locations, so the conductor must be suitable for the conditions and wiring method. Verify the complete marking and local code requirements.
Does a 90°C THHN/THWN-2 rating mean the wire can carry more current?
Not automatically. The 90°C rating describes the insulation's temperature capability under specified conditions. Allowable ampacity depends on conductor size, material, installation conditions, correction and adjustment factors, and terminal temperature limitations.
Are THHN and THWN copper wires?
They can be made with copper conductors, and products using other conductor materials may also be available depending on the manufacturer's range and applicable standards. Always check the product specification for conductor material and size.
Can THHN/THWN-2 replace PV wire in a solar installation?
Not automatically. PV systems may require conductors specifically listed and marked for photovoltaic applications. Select the cable according to the applicable photovoltaic standards, system design, environmental conditions, and local electrical code.
Conclusion: Choose Wire Based on Ratings and Installation Conditions
The main difference between THHN and THWN wire is their designated performance, particularly regarding heat and moisture. THHN is commonly rated for 90°C in dry locations, traditional THWN is commonly rated for 75°C in wet and dry locations, and THWN-2 is commonly rated for 90°C in both wet and dry locations.
For many modern building-wire applications, THHN/THWN-2 offers a convenient dual-rated option. Nevertheless, the complete product marking, manufacturer's listing, allowable ampacity, installation method, and applicable electrical code should determine the final selection.
For contractors, distributors, and procurement teams, reviewing these details before ordering helps ensure that the selected wire meets the project's technical and environmental requirements.
Need help specifying cable for your next project? Contact the FRCABLE team to discuss your application, required cable standards, conductor specifications, and sourcing requirements for suitable cable solutions.
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