THHN vs. Romex (NM-B): Choosing the Right Wire Type
THHN and Romex (a brand name for NM-B cable) are two of the most common wire types in electrical construction. They serve different purposes and are not interchangeable in every application. Understanding the distinction helps you order the correct product for your project.
THHN / THWN Wire
THHN (Thermoplastic High Heat-resistant Nylon-coated) is a single-conductor wire with a thermoplastic insulation jacket and a nylon outer coating. It is rated for use in dry locations at up to 90 degrees Celsius. THWN adds wet-location rating. Most wire labeled as THHN is actually dual-rated THHN/THWN-2, making it suitable for both wet and dry environments.
THHN must be installed inside conduit (EMT, PVC, or flexible metal conduit) because it has no outer protective jacket beyond its own insulation. It is the standard wire type for commercial construction, industrial wiring, and any residential application that requires conduit (such as exposed runs in garages, basements, or exterior locations).
Romex (NM-B) Cable
Romex is the popular trade name for Type NM-B (Non-Metallic Sheathed) cable. It bundles two or three insulated conductors plus a bare ground wire inside a plastic outer jacket. The jacket provides physical protection, eliminating the need for conduit in approved locations. NM-B is the standard wire type for interior residential branch circuit wiring, running through wall cavities, attics, and between floor joists.
When to Use Each Type
Use NM-B (Romex) for standard interior residential wiring concealed within walls, ceilings, and floors. Use THHN in conduit for any exposed runs, commercial installations, outdoor wiring, damp or wet locations, and where local code prohibits NM-B cable. Some jurisdictions, notably the City of Chicago, require all residential wiring to be in conduit, making THHN the standard even for homes.
IB Lighting stocks both THHN/THWN individual conductors and NM-B cable in all common gauges. Call (800) 674-9019 for bulk pricing and availability.