When designing or installing electrical systems in commercial or industrial buildings, one of the most common choices for wiring is THHN wire. But what exactly is THHN, and why is it used so widely? This article provides a comprehensive explanation of its structure, performance, rating, installation requirements, and common issues.
1. What is THHN Wire?
THHN stands for Thermoplastic High Heat-resistant Nylon-coated. It is a type of insulated building wire commonly used for power distribution in conduit systems.
A standard THHN wire consists of:
A copper or aluminum conductor (solid or stranded).
PVC insulation (thermoplastic).
An outer nylon jacket, which adds abrasion and chemical resistance.
This combination makes THHN suitable for indoor, dry environments, especially where wires must be pulled through conduits.
2. Key Features of THHN Wire
Some reasons for its popularity include:
Heat resistance: Rated up to 90℃ in dry locations.
Chemical and oil resistance: The PVC + nylon layer withstands exposure to oils, solvents, and industrial chemicals.
Flexibility: Stranded THHN wires bend easily, making conduit installation smoother.
Durability: The nylon jacket protects against abrasion when pulled through metal conduit.
3. Voltage Rating of THHN Wire
Most THHN wires are rated for 600V, making them suitable for:
Branch circuits.
Feeders.
Power distribution inside buildings.
This rating ensures it covers the majority of commercial and industrial needs.
4. Common Applications of THHN Wire
Because of its thermal and mechanical properties, THHN is widely used in:
Commercial buildings: Wiring for outlets, lighting, and equipment.
Industrial plants: Motor connections, control systems, and distribution panels.
Switchboards & electrical panels: Internal wiring.
Conduit installations: EMT, rigid metal conduit, or PVC conduit.
⚠ Important: THHN is not suitable for direct burial or outdoor wet locations unless it is also rated THWN-2.
5. Difference Between THHN and THWN-2
This is a common point of confusion.
THHN:
Rated for dry locations only.
Temperature rating: 90℃.
THWN-2:
Rated for wet and dry locations.
Temperature rating: 90℃ wet and dry.
Suitable for underground conduit or damp environments.
