What is THHN Wire?


  • A Brief History of THHN Wire
  • Definition of THHN Wire
  • THHN Wire Composition
  • Properties of THHN Wire
  • Uses of THHN Wire
  • Comparison to Other Wires
  • Compliance and Certifications for THHN Wire
  • Safety Tips and Precautions for Using and Installing THHN Wire

A Brief History of THHN Wire

THHN wire development aligns with the evolution of electrical wiring. As electricity use increased in the late 19th and early 20th centuries, so did the demand for safe, efficient wiring.

Initially, copper emerged as a prime conductor but needed insulation to prevent oxidation. Early insulations like rubber or cloth had limitations, which led to the adoption of PVC, a thermoplastic, offering superior insulation and heat resistance.

The innovation of adding a nylon coating provided extra mechanical protection and facilitated wire pulling through conduits, marking the birth of modern THHN wire. Its usage has grown alongside electrical system advancements, now spanning residential to industrial applications.

Definition of THHN Wire

THHN stands for “Thermoplastic High Heat-resistant Nylon-coated.” It offers durability, heat resistance, and a smooth surface for easy installation. THHN wire consists of a solid or stranded copper conductor, PVC insulation, and a nylon coating, making it suitable for various electrical wiring applications.

THHN Wire Composition

ConductorCopper (Solid or Stranded)Provides excellent electrical conductivity.
InsulationPVC (Polyvinyl Chloride)Protects against environmental factors and physical wear.
Outer JacketNylonEnhances durability and eases installation by enabling smooth pulling.

Properties of THHN Wire

  • Heat Resistance: Handles temperatures up to 90°C in dry locations.
  • Resilience to Adverse Conditions: Resistant to moisture, gasoline, and certain chemicals.
  • Ease of Installation: The nylon jacket creates a smooth surface, easing pulling through conduits.
  • Variety of Gauge Sizes: Available in a range of sizes from 1000 MCM to 14 AWG.
  • Color-Coding for Safety: Simplifies identification and enhances safety in complex wiring systems.

Uses of THHN Wire

  • Conduit and Cable Trays: Commonly used for service, feeder, and branch circuits.
  • Lighting Fixtures: Reliable for wiring up lighting fixtures due to its conductivity and heat resistance.
  • Heating Units and Air Conditioners: Suitable for high-temperature environments.
  • Control Circuits: Ensures optimal functionality for appliances and machinery.
  • Machine Tools: Ideal for challenging environments where moisture or high temperatures are present.

Comparison to Other Wires

Wire TypeCostEfficiencyDurabilityApplicable Use Cases
THHNModerateHighHighConduits, cable trays, lighting fixtures, heating units, air conditioners, control circuits, machine tools
TFFNLowModerateModerateIndoor use, lighting fixtures, control circuits, machine tools (dry locations)
UF-BHighModerateHighDirect burial, outdoor lighting, pumps
NM-BLow to ModerateModerateLow to ModerateIndoor residential wiring (dry locations)
XHHWHighHighHighWet or dry locations, conduits, raceways for services, feeders, and branch circuit wiring

Compliance and Certifications for THHN Wire

ULUnderwriter LaboratoriesUnited StatesUL 83Standards for thermoplastic-insulated wires and cables. Indicates that the THHN wire has passed tests for safety and performance.
CSACanadian Standards AssociationCanadaCSA C22.2 No. 75Standards for thermoplastic-insulated wires and cables. Indicates that the THHN wire meets applicable safety and performance standards in Canada.

Safety Tips and Precautions for Using and Installing THHN Wire

Safety Tips and PrecautionsDescription
Correct InstallationFollow National Electrical Code (NEC) guidelines. Ensure the power is off before working with electrical wiring.
Limitations AwarenessSuitable for dry locations (up to 90°C) and wet locations (up to 75°C). Avoid exceeding these limits.
Professional InstallationUse appropriate tools and follow safety protocols.
Personal Protective Equipment (PPE)Wear insulated gloves, safety glasses, and flame-resistant clothing.
Power OffEnsure the power is off before working with electrical wiring to prevent electric shock. Use a voltage tester to verify.