What Is THHN Wire? Everything You Need to Know

If you're working on a residential, commercial, or industrial electrical project, you've probably come across the term THHN wire. But what exactly is it?

In this guide, we’ll explain what THHN wire is, its key features, where it’s used, and how it compares to other common types of electrical wire. Whether you're an electrician, contractor, or DIYer, this article will help you choose the right wire for the job.

What Is THHN Wire

🔍 What Does 'THHN' Stand For?

THHN is an acronym defined by the National Electrical Code (NEC), and it stands for:

• T – Thermoplastic insulation

• H – Heat-resistant (up to 75°C)

• H – (Again) Higher heat resistance (up to 90°C in dry locations)

• N – Nylon jacket for mechanical protection

So, THHN wire is a thermoplastic-insulated, heat-resistant wire with a durable nylon coating.

🧪 THHN Wire Construction

A standard THHN cable consists of:

• Conductor: Solid or stranded copper (or sometimes aluminum)

• Insulation: PVC (polyvinyl chloride)

• Jacket: Nylon sheath to improve abrasion and chemical resistance

This construction makes THHN wire durable, lightweight, and easy to pull through conduit.

⚙️ Key Features of THHN Wire

• 🔌 Voltage Rating: Up to 600V

• 🌡️ Temperature Rating:

  • 90°C (194°F) in dry locations

  • 75°C (167°F) in wet locations (when dual-rated as THWN)

• 💧 Moisture Resistance: Good (especially when dual-rated THHN/THWN)

• 🔥 Heat Resistance: Excellent for dry conditions

• 💪 Mechanical Strength: Strong nylon jacket resists oil, gas, and abrasion

• 🧱 Installation Method: Suitable for conduit and raceway only (not for direct burial without protection)

🏗️ Common Uses of THHN Wire

THHN wire is one of the most commonly used building wires in the U.S. It is widely used in:

• Commercial and residential building wiring

• Power distribution panels and control circuits

• Conduit installations

• HVAC equipment

• Lighting and outlet wiring

Note: THHN is not rated for direct burial unless installed in a conduit. For underground wiring, consider UF-B or USE-2 wire types.

🆚 THHN vs Other Wire Types

THHN is ideal for indoor conduit work, while THWN-2 and XHHW provide better moisture protection for outdoor or wet locations.

💡 Frequently Asked Questions

What is THHN wire used for?
→ THHN is used in conduit for electrical wiring in buildings, especially for power distribution and branch circuits.

Is THHN wire rated for outdoor use?
→ Yes, but only when installed in conduit. It’s not UV-resistant or waterproof on its own.

Can I bury THHN wire directly in the ground?
→ No. THHN wire must be installed inside conduit if buried underground.

Is THHN copper or aluminum?
→ Most THHN wire is copper, but aluminum options are available for larger gauge power wiring.

📦 Available THHN Wire Sizes

At TOT Wire & Cable, we supply THHN wire in various sizes and colors, including:

• Gauge sizes: 14 AWG to 600 MCM

• Conductors: Solid or stranded copper

• Color options: Black, white, red, green, blue, yellow, and more

• Packaging: Available in coils, reels, or custom-cut lengths

📞 Need Help Choosing THHN Wire?

Still wondering if THHN cable is right for your project? Contact our expert team at TOT Wire & Cable for technical support, custom quotes, or large volume discounts.