THHN Wire: Meaning, Uses, Ratings and THWN Differences

THHN wire is a common type of single conductor building wire used in US electrical systems, especially inside conduit and raceways. The name describes the conductor's insulation and protective covering while its AWG size identifies the conductor's physical size. This guide explains what THHN means, where it is used, how its ratings work and how THHN compares with THWN and THWN 2.

Key Takeaways

THHN is a thermoplastic, high heat resistant, nylon jacketed building wire commonly used in conduit and raceway installations. The important points to remember are:

THHN describes the wire type

THHN identifies insulation and construction characteristics, it does not tell you the conductor's AWG size.

AWG describes conductor size

A 12 AWG THHN conductor and a 6 AWG THHN conductor are the same general wire type but have different conductor sizes and electrical characteristics.

THHN is rated for 90°C in dry and damp locations

Under the current NEC conductor classification THHN has a 90°C maximum operating temperature rating for dry and damp locations.

Many products are dual rated

Modern building wire is commonly marked THHN/THW 2 giving the conductor a wet location rating as well as its THHN rating.

What Is THHN Wire?

THHN stands for Thermoplastic High Heat resistant Nylon coated. It is a single conductor building wire with thermoplastic insulation and a nylon outer covering. The THHN designation describes the construction and performance characteristics of the conductor rather than its size.

THHN is commonly installed in conduit and raceway systems for residential, commercial and industrial electrical wiring. The conductor itself may be copper or another permitted conductor material depending on the product. THHN can also be manufactured in different conductor constructions, including solid and stranded versions.

What Does THHN Stand For?

Each part of the THHN designation provides information about the conductor:

The two H letters are important because THHN is classified for a 90°C maximum operating temperature in dry and damp locations. The current NEC conductor table lists THHN as flame retardant, heat resistant thermoplastic insulation with a nylon jacket or equivalent covering.

What Is THHN Wire Used For?

THHN is commonly used as individual conductors in conduit and raceway systems. Typical applications include:

The exact application depends on the conductor's complete marking, size, installation method, environmental conditions and applicable electrical requirements. THHN is therefore not simply a universal replacement for every type of electrical cable.

• Branch circuit wiring
• Feeder conductors
• Commercial building wiring
• Industrial electrical systems
• Motor and equipment wiring
• Lighting circuits
• Control wiring
• Conduit runs between electrical panels and equipment

THHN Wire Ratings

A THHN conductor is commonly associated with a 600 volt rating and a 90°C temperature rating for dry and damp locations, although the complete product marking and manufacturer specification should always be checked. Southwire's building wire products for example provide product specific ratings and NEC based ampacity information.

Temperature rating and ampacity are related but they are not the same thing. The temperature rating describes the maximum conductor operating temperature under the applicable listing. Ampacity describes how much current the conductor can carry under specified conditions.

For example the NEC ampacity tables provide different values for 60°C, 75°C and 90°C conductor temperature ratings. THHN appears in the 90°C column but the final allowable ampacity for an actual installation can be affected by termination temperature limitations, ambient temperature and the number of current carrying conductors.

THHN Wire and AWG: What Is the Difference?

One common misunderstanding is treating THHN and AWG as two competing wire size systems. They are not.

THHN identifies a type of conductor insulation and covering.

AWG identifies the conductor's size using the American Wire Gauge system.

For example:

As the AWG number decreases the conductor becomes larger. Therefore 6 AWG is larger than 12 AWG. The appropriate conductor size cannot be selected from the THHN designation alone.

Load current, ampacity, conductor material, installation conditions, overcurrent protection, voltage drop and applicable NEC requirements may all affect the final selection. For broader wire size information see our wire size chart.

• 12 AWG THHN = 12 gauge THHN conductor
• 6 AWG THHN = 6 gauge THHN conductor

Common THHN Wire Sizes

THHN is available in many conductor sizes from smaller AWG conductors to large conductors measured in kcmil. The exact sizes available depend on the product and manufacturer.

The NEC conductor tables cover THHN across a broad range of AWG and kcmil sizes. Common searches include:

The size should not be selected simply because a particular AWG number appears frequently online. For a real circuit start with the load and installation requirements then check ampacity and voltage drop where applicable.

Our wire size calculator explains that conductor selection can depend on current, conductor material, temperature, installation conditions and voltage drop.

• 12 AWG THHN wire
• 10 AWG THHN wire
• 6 AWG THHN wire
• 2 AWG THHN wire
• 2/0 THHN wire
• THHN copper wire

Is THHN Wire Solid or Stranded?

THHN can be manufactured with either solid or stranded conductors depending on the size and product. Smaller conductors may be available in solid construction while larger conductors are commonly stranded to make installation and pulling through conduit more practical.

The construction does not change the basic meaning of THHN. The conductor must still meet the applicable product and listing requirements.

When selecting a specific conductor check the manufacturer's product specification rather than assuming that every THHN size is available in the same construction.

What Is the Difference Between THHN and THWN?

The main difference is the wet location rating. THHN is classified for dry and damp locations at a 90°C maximum operating temperature. THWN includes a wet location rating with the standard THWN classification associated with a 75°C wet location temperature rating. THWN 2 is rated for 90°C in wet locations.

Always read the actual printing on the conductor. A wire marked THHN/THWN 2 has both designations and can be used according to the requirements associated with those listings.

THHN vs THWN 2: Why the Difference Matters

If a conductor is being installed where moisture can be present the wet location listing matters. For example underground raceways can be exposed to moisture so simply seeing THHN in a product description is not enough.

The conductor's actual marking should be checked to determine whether it also carries a wet location listing. This is one reason dual rated THHN/THWN 2 conductors are common in modern building wire products.

THHN vs Romex: Are They the Same?

No. THHN is generally an individual conductor used in conduit or raceway systems. Romex is a common trade name associated with nonmetallic sheathed cable such as NM B. An NM B cable contains multiple insulated conductors within an overall nonmetallic sheath.

The two therefore differ in both construction and typical installation methods. A THHN conductor may be used as an individual wire inside conduit, whereas NM B is a complete cable assembly designed for applications permitted by its listing and the applicable electrical requirements.

Does THHN Wire Affect Voltage Drop?

Yes but the important factor is not simply the word THHN. Voltage drop depends on factors such as:

A larger conductor generally has lower resistance which can reduce voltage drop for a given current and conductor length. For a deeper explanation see our Voltage Drop Formula & Calculation Table.

If you are selecting a THHN conductor for a long circuit, check voltage drop separately from the ampacity calculation. Our wire voltage drop calculator can help evaluate conductor size and voltage loss.

• Conductor resistance
• Conductor size
• Circuit current
• Circuit length
• Conductor material
• AC or DC system characteristics
• Circuit configuration

Is THHN the Same as Copper Wire?

No. THHN describes the insulation and covering designation; it does not by itself mean that the conductor is copper.Copper THHN is extremely common and manufacturer specifications identify conductor material separately.

Southwire, for example, publishes THHN/THWN building wire products with copper conductors and product specific electrical characteristics. Therefore THHN and copper describe different properties:

THHN = conductor insulation/listing type

Copper = conductor material

You should check both when evaluating a particular wire.

How Do You Choose the Right THHN Wire Size?

Do not choose a THHN size from the wire type alone. A basic sizing process is:

NEC ampacity tables are based on specified conditions so a table value should not automatically be treated as the final allowable ampacity for every installation. For a broader explanation see our wire size by amperage guide.

• Determine the circuit load and expected current.
• Identify the conductor material.
• Check the applicable ampacity requirements.
• Consider conductor temperature, ambient temperature and current carrying conductors.
• Check overcurrent protection requirements.
• Evaluate voltage drop when circuit length or application makes it important.
• Verify the final installation against the NEC edition adopted by the local authority and applicable equipment requirements.

Conclusion

THHN wire is a common US building wire type identified by its thermoplastic insulation, high heat rating and nylon covering. The designation tells you about the conductor's construction and rating, it does not tell you the conductor's AWG size or automatically determine the correct ampacity for an installation.

When choosing THHN check the complete conductor marking, conductor material, AWG size, installation environment and applicable electrical requirements. If the circuit is exposed to moisture, verify whether the conductor is also listed as THWN or THWN 2.

For actual wire selection remember that ampacity and voltage drop are separate considerations. Use the appropriate wire sizing and voltage drop calculations rather than choosing a THHN size from the label alone.

FAQs

What does THHN mean in wire?

THHN means Thermoplastic High Heat resistant Nylon coated. It identifies a type of thermoplastic insulated building wire with a nylon covering and a 90°C classification for dry and damp locations under the NEC conductor designation.

What is the difference between AWG and THHN wire?

AWG identifies conductor size while THHN identifies the conductor's insulation and covering type. For example 12 AWG THHN describes both the conductor size and the THHN construction.

What's the difference between THHN and THWN?

THHN is classified for dry and damp locations while THWN adds a wet location rating. THWN 2 is rated for 90°C in wet locations making it different from standard THWN.

Is THHN wire stranded or solid?

THHN can be manufactured with solid or stranded conductors depending on the conductor size and product. Always check the manufacturer's specification for the exact wire.

What is THHN wire used for?

THHN is commonly used as an individual building wire conductor in conduit and raceway systems for branch circuits, feeders, equipment, motors, lighting and other electrical applications where its listing is appropriate.

Is THHN the same as Romex?

No. THHN is typically an individual conductor installed in conduit or raceway while NMB, commonly called Romex which is a multi conductor nonmetallic sheathed cable assembly.