THHN vs. Romex NM-B: When to Use Each Wire Type
THHN and NM-B (Romex) are the two wire types electricians use most frequently, and they serve fundamentally different installation methods. Choosing between them depends on where the wire is installed and whether conduit is part of the installation. Using the wrong type in the wrong application violates the National Electrical Code. Here is a clear comparison.
What Is THHN?
THHN stands for Thermoplastic High Heat-resistant Nylon-coated. It is a single insulated conductor (one wire with insulation around it, no outer jacket). THHN is designed to be pulled through conduit (EMT, PVC, rigid, or flexible conduit) and is never installed loose through wood framing like NM-B. THHN is rated for dry and damp locations at 90 degrees Celsius. The nylon outer coating reduces friction when pulling through conduit, making it easier to install in long runs and conduit with multiple bends. THHN is sold by the foot or in spools, and you purchase each conductor separately (you pull individual hot, neutral, and ground wires through the conduit).
What Is Romex NM-B?
NM-B (Non-Metallic sheathed cable), commonly called Romex, bundles 2 or 3 insulated conductors plus a bare ground wire inside a single thermoplastic jacket. The “/2” or “/3” after the gauge indicates the number of insulated conductors (12/2 has two insulated conductors plus ground; 12/3 has three insulated conductors plus ground). NM-B is designed for direct installation through wood-framed walls, ceilings, and floors without conduit. It is the standard residential wiring method in most U.S. jurisdictions. NM-B is only rated for dry locations at 90 degrees Celsius.
When to Use NM-B (Romex)
NM-B is the right choice for interior residential wiring through wood-framed walls, ceilings, and floors in dry locations. This covers the vast majority of household circuits: bedroom outlets, kitchen circuits, bathroom GFCI circuits, lighting, switches, and general receptacles. NM-B is faster and less expensive to install than THHN in conduit because it requires no conduit material, no conduit fittings, and no wire pulling. You simply staple the cable to framing members and run it to each device.
When to Use THHN in Conduit
THHN in conduit is required or preferred for exposed wiring in garages, basements, and utility areas where NM-B would be physically vulnerable, for exterior and outdoor installations (in weatherproof conduit), for commercial and industrial installations where code requires conduit, for runs through masonry, concrete block, or anywhere NM-B cannot be stapled to wood framing, for damp or wet locations where NM-B is not permitted, and for large feeder circuits (100A+ sub-panel feeds) where individual conductors in conduit provide better heat dissipation than bundled cable.
Cost and Labor Comparison
NM-B costs less per foot than the equivalent THHN conductors plus conduit. For a typical 20-amp circuit using 12 AWG wire, the NM-B cable costs roughly half of the combined cost of three THHN conductors plus EMT conduit, connectors, and fittings. The labor difference is even larger: running NM-B through open framing is 2 to 3 times faster than installing conduit, pulling individual wires, and making conduit connections. This is why NM-B dominates residential construction where code allows it.
Can You Use THHN Instead of NM-B in a House?
Yes, if you install it in conduit. THHN in conduit is always code-compliant anywhere NM-B is allowed (plus additional locations where NM-B is not). Some jurisdictions (notably the City of Chicago) require all residential wiring in conduit and do not permit NM-B at all. In these areas, THHN is the standard residential wire. However, in the majority of U.S. jurisdictions, NM-B is permitted and preferred for standard residential wiring due to its lower cost and faster installation.
IB Lighting stocks both THHN and NM-B from Southwire in all standard residential and commercial gauges. Browse our complete electrical wire and cable collection or call (800) 674-9019 for help selecting the right wire for your project.