THHN/THWN is the most commonly used type of wire in Electrical Installations

Understanding THHN and THWN Copper Wires in Electrical Installations

Copper wires are integral to electrical installations, and selecting the right type of wire is crucial for ensuring the safety, efficiency, and longevity of electrical systems. Two commonly used types of copper wire are THHN (Thermoplastic High Heat-resistant Nylon-coated) and THWN (Thermoplastic Heat and Water-resistant Nylon-coated). This article aims to provide a comprehensive overview of these wire types, focusing on their properties, applications, and the relevant standards from the Philippine Electrical Code (PEC).


What is THHN Wire?

THHN wire is a specific type of building wire with thermoplastic insulation and a nylon or PVC (Polyvinyl Chloride) jacket. The THHN designation indicates that the wire is suitable for use in high-heat environments up to 90°C. Key characteristics include:

  • Thermoplastic Insulation: Provides durability and resistance to heat.
  • High Heat Resistance: Suitable for environments up to 90°C.
  • Nylon Jacket: Adds a layer of protection against chemicals, gasoline, and oil.
  • Temperature Rating: THHN wire can handle temperatures up to 90°C in dry locations and up to 75°C in wet locations.
  • Voltage Rating: THHN wire is rated for 600 volts.
  • Conductor Material: Typically, THHN wire is made from copper due to its excellent conductivity and durability.

What is THWN Wire?

THWN wire, on the other hand, is similar to THHN but with additional properties making it suitable for wet environments. The designation stands for Thermoplastic Heat and Water-resistant Nylon-coated wire. Key characteristics include:

  • Heat and Water Resistance: THWN wire can withstand temperatures up to 75°C in both dry and wet locations.
  • Thermoplastic Insulation: Like THHN, it provides durability and heat resistance.
  • Nylon Jacket: Offers chemical resistance and additional protection against moisture.
  • Voltage Rating: THWN wire is also rated for 600 volts.
  • Conductor Material: Like THHN, THWN wire is generally made from copper.

Applications of THHN and THWN Copper Wires

THHN Wire Applications

THHN wire is commonly used in various applications where the environment is dry or occasionally damp, such as:

  • Residential Wiring: Ideal for house wiring, THHN wire is used for connecting electrical outlets, switches, and other fixtures.
  • Commercial Buildings: It is used in commercial settings for powering lighting, heating, and cooling systems.
  • Conduit and Cable Tray Systems: THHN wire is often installed in conduit or cable tray systems for additional protection.

THWN Wire Applications

THWN wire is used in environments where exposure to water or moisture is a concern. Typical applications include:

  • Outdoor Installations: Suitable for outdoor wiring, such as in gardens, pools, and other wet locations.
  • Industrial Settings: Used in industrial environments where water resistance is crucial, such as in wastewater treatment plants.
  • Underground Wiring: THWN wire is often used for underground installations where it may be exposed to moisture.

Both THHN and THWN wires are used extensively in various electrical installations. Their applications include:

  • Residential Wiring: Used for branch circuits, feeders, and service conductors.
  • Commercial Buildings: Ideal for wiring in conduits and raceways.
  • Industrial Settings: Suitable for use in control and power circuits in machinery and equipment.
  • Conduit and Raceway Systems: Both wires are suitable for installation in conduit systems, ensuring safety and compliance with electrical codes.

Philippine Electrical Code Standards

The Philippine Electrical Code (PEC) provides guidelines and standards for the use of electrical wiring to ensure safety and compliance. Table from the PEC outlines the allowable ampacities of insulated conductors rated up to and including 2000 volts, not more than three current carrying conductors in raceway, cable or earth, based on ambient temperature of 30°C and conductor size.

THHN and THWN Copper Wire Ampacities Table

THHN and THWN Copper Wire Ampacities Table

Conductor Size (mm2)Copper

For more than Three Current-Carrying Conductors Adjustment Factors shall be applied. This can be seen in the PEC 2017 Article 3.10, Table For correction factors based on 30°C Table

Key Points from the Table

According to the table provided, the allowable ampacities of THHN and THWN wires are determined by factors such as conductor size and ambient temperature. Here are some critical points:

  • Ambient Temperature: The ampacity of the wire must be adjusted based on the ambient temperature. For example, THHN and THWN wires have different ampacities at 75°C and 90°C.
  • Conductor Size: Larger conductor sizes have higher ampacity. For instance, a 2.0 mm² copper wire has an ampacity of 25 amps at 90°C, while a 200 mm² copper wire has an ampacity of 360 amps at the same temperature.
  • Installation Conditions: The installation environment (dry, wet, or hazardous locations) impacts the wire choice and ampacity.
  • Adjustment Factors: For more than Three Current-Carrying Conductors Adjustment Factors shall be applied.

Practical Tips for Installation

  1. Choose the Right Wire Type: Select THHN or THWN wire based on the installation environment. Use THHN in dry and high-heat areas, and THWN in wet or damp locations.
  2. Check Code Requirements: Always refer to the latest Philippine Electrical Code for specific ampacity ratings and installation guidelines.
  3. Consider Ambient Temperature: Adjust the ampacity based on the ambient temperature to ensure safe operation.
  4. Use Proper Conduit: Install wires in appropriate conduit systems to protect against physical damage and environmental factors.


THHN and THWN copper wires are vital components in electrical installations, offering high heat resistance and suitability for dry and wet locations, respectively. Understanding their specifications, applications, and compliance with Philippine Electrical Code standards ensures safe and efficient electrical systems.

Scroll to Top