THHN wire (Thermoplastic High Heat-resistant Nylon-coated) is a PVC-insulated, nylon-jacketed single-conductor building wire rated for 90°C in dry locations and 75°C in wet locations. It is the most widely used general-purpose wire for branch circuits, feeder wiring, and motor connections in residential, commercial, and industrial buildings installed inside conduit. It is not approved for direct burial without conduit.

This guide covers everything you need to know about THHN wire: what it is, how it is constructed, where it is used, its full ampacity ratings, how it compares to alternatives, and answers to the most common questions electricians and contractors ask.

What Is THHN Wire?

THHN wire is a single-conductor electrical wire designed for installation inside conduit systems in buildings and structures. It is governed by UL Standard 83 and referenced throughout the National Electrical Code (NEC). Each letter in the name describes a specific construction or performance property:

  • T — Thermoplastic: The insulation is made from polyvinyl chloride (PVC), a thermoplastic material.
  • H — High heat-resistant: The wire is rated for a minimum of 75°C operating temperature.
  • H — Higher heat: Rated up to 90°C in dry locations.
  • N — Nylon: A nylon (polyamide) outer jacket is applied over the PVC insulation. This is the defining feature that separates THHN from older THW wire types.

THHN wire is also widely called building wire. Most THHN wire sold in the United States today is dual-rated THHN/THWN-2, meaning it meets both the dry-location THHN standard and the wet-location THWN-2 standard (90°C wet). Always check the printed markings on the wire jacket to confirm the applicable ratings before specifying or installing.

How Is THHN Wire Constructed?

THHN wire has a three-layer construction. Each layer performs a specific function.

1. Conductor — Copper or Aluminium

The conductor is the current-carrying core of the wire. THHN wire is available with two conductor materials:

  • Copper — the most common choice. Copper offers superior electrical conductivity (~58 MS/m), higher tensile strength, and greater corrosion resistance. It is standard for residential and commercial wiring.
  • Aluminium — lighter and lower cost per foot. Aluminium has approximately 61% the conductivity of copper, so a larger gauge is required to carry the same current. Commonly used for large feeder circuits and service entrance conductors.

Conductors are available as solid (single strand, typically 14–10 AWG) or stranded (multiple twisted strands, more flexible, common in 8 AWG and larger).

2. Insulation — PVC (Polyvinyl Chloride)

The conductor is wrapped in a layer of thermoplastic PVC insulation. PVC provides electrical isolation, flame resistance, and protection against moisture and many chemicals. Insulation thickness varies by wire gauge per UL 83 and NEC requirements. The insulation is colour-coded by convention: black or red for hot conductors, white or grey for neutrals, green or bare for equipment grounding conductors.

3. Outer Jacket — Nylon (Polyamide)

The outermost layer is a thin but tough nylon outer jacket. This is the feature that defines THHN and differentiates it from THW wire. The nylon jacket provides resistance to abrasion during conduit pulling, resistance to oils, gasoline, and many solvents, a smooth low-friction surface that makes conduit installation easier, and additional mechanical protection against cuts and nicks during handling.

Key Features and Ratings of THHN Wire

1. Temperature Rating

90°C (194°F) in dry locations. 75°C (167°F) in wet or damp locations (when the wire carries a THWN rating). Wire dual-rated THWN-2 is rated 90°C in wet locations as well. This makes THHN suitable for use near motors, heating equipment, and industrial machinery.

2. Voltage Rating

THHN wire is rated for a maximum of 600 volts. This covers the vast majority of branch circuit, feeder, and service wiring applications in residential, commercial, and light industrial buildings.

3. Chemical and Oil Resistance

The nylon jacket resists oils, gasoline, and many solvents. The PVC insulation layer adds resistance to acids and alkalis. Together they make THHN suitable for industrial environments where chemical exposure is a concern.

4. UV and Sunlight Resistance

Many THHN wires carry a sunlight-resistant (SR) rating printed on the jacket. SR-rated wire is acceptable for exposed outdoor conduit installations. Always confirm the SR marking before specifying wire for outdoor above-ground conduit runs.

5. Flame Resistance

THHN wire is flame-retardant per UL 83 and NEC requirements. Under standard test conditions it will not propagate a flame along its length, reducing fire spread risk in buildings.

6. Available Size Range

THHN wire is available from 14 AWG through 1,000 kcmil, covering everything from residential 15-amp branch circuits to large commercial and industrial feeder conductors.

THHN Wire Ampacity Chart — NEC Table 310.16

The table below shows the maximum allowable ampacity for THHN wire based on conductor material, size (AWG or kcmil), and temperature rating. Values are from NEC Table 310.16 for conductors in conduit with no more than three current-carrying conductors, at an ambient temperature of 30°C (86°F). Always apply NEC Article 310 correction and adjustment factors for your specific installation conditions.

Wire Gauge (AWG / kcmil) Copper – 60°C (140°F) NM-B, UF-B Copper – 75°C (167°F) THW / THWN Copper – 90°C (194°F) THHN / THWN-2 Aluminum – 75°C (167°F) THW / THWN Aluminum – 90°C (194°F) THHN / THWN-2
14 AWG 15 A 20 A 25 A — —
12 AWG 20 A 25 A 30 A 20 A 25 A
10 AWG 30 A 35 A 40 A 30 A 35 A
8 AWG 40 A 50 A 55 A 40 A 45 A
6 AWG 55 A 65 A 75 A 50 A 55 A
4 AWG 70 A 85 A 95 A 65 A 75 A
3 AWG 85 A 100 A 115 A 75 A 85 A
2 AWG 95 A 115 A 130 A 90 A 100 A
1 AWG — 130 A 145 A 100 A 115 A
1/0 AWG — 150 A 170 A 120 A 135 A
2/0 AWG — 175 A 195 A 135 A 150 A
3/0 AWG — 200 A 225 A 155 A 175 A
4/0 AWG — 230 A 260 A 180 A 205 A
250 kcmil — 255 A 290 A 205 A 230 A
300 kcmil — 285 A 320 A 230 A 260 A
350 kcmil — 310 A 350 A 250 A 280 A
500 kcmil — 380 A 430 A 310 A 350 A
600 kcmil — 420 A 475 A 340 A 385 A
750 kcmil — 475 A 535 A 385 A 435 A
1000 kcmil — 545 A 615 A 445 A 500 A

Where Is THHN Wire Used? Applications and Installations

THHN wire must be installed inside an approved raceway — conduit, cable tray, or wireway — in most applications. Its broad temperature and voltage ratings make it appropriate across all major electrical installation categories.

1. Residential Wiring

In residential construction, THHN wire is used for branch circuit wiring run in conduit, panel feeder connections, sub-panel feeds, garage and workshop circuits, and outdoor circuits protected in conduit. It is not the same as NM-B (Romex) cable — THHN is a single-conductor wire that must be installed in conduit, whereas NM-B is a multi-conductor cable assembly.

2. Commercial Buildings

Commercial electrical systems use THHN wire extensively for power distribution panels, lighting circuits, HVAC control wiring, emergency lighting circuits, and elevator and escalator control systems. Its 600V rating and broad size range cover virtually all commercial load requirements.

3. Industrial Facilities

Industrial applications demand wiring that tolerates heat, mechanical stress, and chemical exposure. THHN wire is well-suited for motor branch circuits, motor control centre (MCC) wiring, machine tool wiring, process control panels, conveyor systems, and automation control wiring.

4. Outdoor and Exposed Conduit Installations

When installed in outdoor conduit — rigid metal conduit (RMC), intermediate metal conduit (IMC), or schedule 40/80 PVC conduit — THHN/THWN-2 dual-rated wire is suitable for exposed overhead runs, service entrance conduit, outdoor distribution panels, site lighting, and utility junction boxes. The sunlight-resistant (SR) marking must appear on the wire jacket for any exposed outdoor run.

Advantages of THHN Wire

  • High temperature tolerance — 90°C dry rating makes it suitable for motor and industrial environments where heat build-up is a concern.
  • Chemical and oil resistance — the nylon jacket protects against solvents, oils, and fuels commonly found in industrial settings.
  • Easy conduit pulling — the smooth nylon surface reduces friction, making long conduit runs easier to complete.
  • Wide availability — stocked at virtually every electrical distributor in North America in all standard sizes.
  • Code compliant — listed under UL 83 and accepted by the NEC for most building wiring applications.
  • Dual-rated options — most THHN wire sold today also carries a THWN-2 rating, covering both dry and wet location installations.
  • Broad size range — available from 14 AWG through 1,000 kcmil to cover small branch circuits and large feeder conductors alike.

Limitations of THHN Wire

  • Conduit required — THHN is a single-conductor wire that requires installation inside an approved conduit or raceway for most applications. It is not a standalone cable assembly.
  • Not approved for direct burial — THHN wire alone cannot be buried directly in the ground. Underground direct burial requires USE-2 or UF-B rated cable. THHN/THWN-2 dual-rated wire is acceptable inside underground conduit.
  • Temperature limits must be respected — exceeding the rated temperature through overloading or high ambient conditions will degrade the insulation over time, creating a safety risk.
  • Higher installed cost than NM-B for simple residential circuits — running THHN in conduit has a higher combined material and labour cost than NM-B cable for straightforward residential branch circuits.

THHN Wire vs. Alternatives — Comparison

Wire Type Temperature Rating Wet Location Rated? Direct Burial? Best Use Case
THHN 90°C dry / 75°C wet (if THWN dual-rated) Only if THWN-2 dual-rated No — conduit required General building wiring in conduit
THWN / THWN-2 75°C (THWN) / 90°C (THWN-2) wet Yes No — conduit required Wet or damp locations; underground conduit runs
XHHW-2 90°C dry and wet Yes No — conduit required Industrial and commercial; higher moisture resistance
NM-B (Romex) 60°C assembly / 90°C conductor No No Residential branch circuits in dry concealed locations
USE-2 90°C dry and wet Yes Yes Service entrance; direct burial; solar PV wiring
UF-B 60°C Yes Yes Residential direct burial for outdoor branch circuits
MTW 60°C (UL) / 90°C (CSA) Yes No Machine tool and industrial control panel wiring
TFFN 90°C dry No No Fixture wiring, signs, and luminaires; 300V maximum

THHN Wire — Key Facts Summary

  • Full name: Thermoplastic High Heat-resistant Nylon-coated wire
  • Temperature rating: 90°C dry / 75°C wet (90°C wet if THWN-2 dual-rated)
  • Voltage rating: 600V maximum
  • Insulation material: PVC (polyvinyl chloride) — not TPE
  • Outer jacket: Nylon — provides abrasion, oil, and chemical resistance
  • Conductor materials: Copper or aluminium
  • Installation requirement: Must be installed in conduit for most applications
  • Direct burial: Not approved unless inside conduit and THWN-2 dual-rated
  • Available sizes: 14 AWG through 1,000 kcmil
  • Governing standards: UL 83 / NEC Table 310.16
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Frequently Asked Questions About THHN Wire

What does THHN wire stand for?

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. T = thermoplastic (PVC) insulation, H = high heat-resistant, H = rated to 90°C in dry locations, N = nylon outer protective jacket. The nylon jacket is the defining feature that separates THHN from older THW wire types, which have no outer nylon jacket.

What is THHN wire suitable for?

THHN wire is suitable for general-purpose building wiring installed inside conduit. Common applications include residential branch circuits (in conduit), commercial power distribution, industrial motor and control wiring, lighting circuits, feeder conductors, and sub-panel feeds. It is suitable for dry locations at 90°C and wet locations at 75°C (or 90°C if dual-rated THWN-2).

What is the difference between THHN and THWN wire?

THHN is rated for dry locations at 90°C. THWN (Thermoplastic High heat-resistant Water-resistant Nylon-coated) adds water resistance and is rated for wet locations at 75°C. THWN-2 is rated 90°C in both dry and wet conditions. Most THHN wire sold today in the United States is dual-marked THHN/THWN-2 on the jacket. The practical difference is that THWN-2 dual-rated wire can be used in underground conduit runs, while THHN-only wire cannot.

What is THHN copper wire?

THHN copper wire is a THHN-rated conductor with a solid or stranded copper core, PVC insulation, and a nylon outer jacket. Copper THHN is the most common type used in residential and commercial wiring because copper provides superior conductivity, easier termination at standard devices, and compatibility with most breakers, receptacles, and lugs without special connectors or anti-oxidant compound.

What is the difference between AWG and THHN wire?

AWG (American Wire Gauge) and THHN describe different properties of the same wire. AWG is the measurement system for wire conductor diameter — a lower AWG number means a larger, heavier gauge (4 AWG is thicker than 12 AWG). THHN describes the insulation and jacket type. A complete wire specification combines both: for example, “12 AWG THHN copper” means a 12-gauge copper conductor with THHN-rated PVC insulation and a nylon jacket.

Can THHN wire be used underground?

THHN-only rated wire is not approved for direct burial underground. Wire marked THHN/THWN-2 is acceptable for installation inside underground conduit. For direct burial without conduit, USE-2 or UF-B rated cable is required. Always check the jacket markings and your local NEC-based code requirements to confirm what is permitted for your specific underground installation.

Is THHN wire the same as building wire?

Yes — THHN wire is one of the most common types referred to informally as building wire. The term building wire covers several wire types including THW, XHHW, and USE, but THHN/THWN-2 is the most widely specified and stocked product in this category for North American construction and industrial projects.