What Is THHN Wire?
THHN is a single-conductor building wire with thermoplastic insulation and a tough nylon jacket, designed to be pulled through conduit. Most THHN sold today is dual-rated THHN and THWN-2, which means it is rated for both dry and wet locations and carries a 90 degrees Celsius temperature rating. Unlike Romex, which bundles conductors inside one sheath for in-wall residential use, THHN is run as individual color-coded conductors inside raceway.
Where THHN Is Used
THHN is the workhorse of conduit wiring. It is used for runs inside EMT, PVC, and metal conduit, outdoor circuits where the wire is protected in conduit, service-entrance and feeder conductors, motor and equipment connections, and most commercial and industrial branch circuits. Because it is sold as individual conductors, you pull only the number and colors you need.
THHN Ampacity
THHN conductors are rated to 90 degrees Celsius, but the equipment terminations they land on are usually rated 75 degrees Celsius, so the 75 degrees Celsius column of NEC Table 310.16 typically governs the circuit. The 90 degrees Celsius rating is used as the starting point for derating when conductors are bundled or run in high ambient temperatures. As a quick reference, 12 AWG copper is protected at 20 amps and 10 AWG at 30 amps for branch circuits under NEC 240.4(D), even though the table values are higher.
Color Coding
THHN comes in multiple colors so you can identify phases and functions. Black, red, and blue are common for hot conductors, white or gray for the grounded neutral, and green for equipment ground. Buying the right colors up front keeps your installation clean and code-compliant.
THHN vs Romex
Choose Romex NM-B for concealed residential wiring inside dry framed walls and ceilings, where the cable is run without conduit. Choose THHN when the circuit runs through conduit, goes outdoors in raceway, feeds a service or subpanel, or is part of a commercial job. THHN gives you flexibility in conductor count and color, while Romex gives you a fast all-in-one cable for houses.
Code and Safety Note
Conduit fill, derating, and termination temperature limits all apply to THHN installations. Follow the National Electrical Code and local amendments, confirm with your authority having jurisdiction, and use a licensed electrician for service, feeder, and panel work.
IB Lighting carries Southwire THHN and THWN-2 building wire in multiple gauges and colors inside our electrical wire and cable collection, with free freight plus free freight insurance on every order. Call (800) 674-9019 for gauge, color, and quantity help.