What is the difference between THHN and Romex?

THHN is a single conductor meant to be pulled through conduit. Romex is a cable with multiple insulated conductors inside a common sheath, designed for interior wood-frame wiring without conduit.

Can THHN wire be used outdoors?

Only if it is inside conduit and the wire carries a dual THHN/THWN-2 rating for wet locations. Plain THHN alone is rated for dry and damp conditions, not wet.

What size THHN wire do I need for a 20-amp circuit?

A 20-amp circuit typically requires 12 AWG copper THHN. The conductor size depends on the ampacity tables in the electrical code and the specific installation conditions.

Why is THHN wire color-coded?

Color coding identifies the function of each conductor in the circuit. Black and red are typically hot conductors, white is neutral, and green or bare is the equipment ground.

Do I need a permit to replace THHN wire in conduit?

Usually yes. Replacing conductors in conduit is electrical work that most jurisdictions require a permit and inspection for, especially on branch circuits or feeders.

THHN Wire — Thermoplastic Conductor for Conduit Runs

A THHN wire is a single thermoplastic-insulated conductor rated for use in dry and damp locations inside conduit or raceways.

THHN Wire diagram — labeled parts and installation context

What It Is

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. It is one of the most common building wire types used inside conduit for residential and commercial branch circuits. The conductor is solid or stranded copper (or aluminum in larger feeder sizes), surrounded by PVC insulation rated to 90 degrees Celsius in dry conditions and a thin nylon outer jacket that resists oil, gasoline, and abrasion during pulling. The 90-degree rating gives THHN a higher ampacity than older 60-degree or 75-degree insulation types, although the NEC ampacity tables in Article 310 often require derating based on the termination temperature of the connected device.

Unlike cable assemblies such as NM-B (Romex) that bundle multiple conductors under one sheath, THHN is a single conductor pulled through conduit along with other individual wires to make up a complete circuit. A typical 20-amp, 120-volt branch circuit in conduit consists of one black 12 AWG THHN (hot), one white 12 AWG THHN (neutral), and one green 12 AWG THHN (equipment ground).

Types

Solid THHN is used for smaller gauges, primarily 14 AWG and 12 AWG, where the conductor is stiff enough to terminate easily at screw terminals and push-in connectors. Stranded THHN is standard for 10 AWG and larger sizes because the individual strands make the wire flexible enough to pull through conduit bends without kinking or damaging the insulation.

Dual-rated THHN/THWN-2 wire is the most commonly stocked product at supply houses. The THWN-2 designation adds a wet-location rating at 90 degrees Celsius, meaning the same conductor can be used in both dry conduit runs inside a building and wet or outdoor conduit runs without switching wire types. Aluminum THHN is available in feeder sizes (typically 4 AWG and larger) for service entrance and subpanel feeds where cost savings over copper are significant.

Where It Is Used

THHN wire is used inside EMT (electrical metallic tubing), rigid metal conduit, IMC (intermediate metal conduit), PVC conduit, and flexible metal conduit for branch circuits, feeders, and equipment connections. It is the standard wiring method in commercial and industrial buildings where conduit is required, and it is also used in residential construction for garage circuits, exposed basement runs, and outdoor conduit runs.

In panel interiors, short lengths of THHN are used as pigtails and jumpers between bus bars and devices. On large feeders, multiple THHN conductors are pulled through oversized conduit to supply subpanels, motors, and HVAC equipment.

How to Identify One

THHN wire has continuous printed markings along its insulation at regular intervals showing the manufacturer name, wire gauge (e.g., 12 AWG), insulation type (THHN or THHN/THWN-2), voltage rating (typically 600V), and UL listing information. The insulation is available in standard NEC color codes: black, red, and blue for hot conductors, white or gray for neutral, and green for equipment ground.

The nylon jacket gives THHN a slightly glossy, slick feel compared to older THWN wire, which lacks the nylon layer. This slickness is intentional because it reduces friction during conduit pulls, lowering the pulling force needed and reducing the risk of insulation damage.

Replacement

Replacement is needed when insulation is cracked from age or overheating, when the nylon jacket is damaged from a rough pull through conduit with tight bends or burrs, or when the conductor size no longer matches the circuit load after a panel upgrade or code change. Replacing conductors in existing conduit involves pulling the old wires out and feeding new ones in, which requires wire-pulling lubricant and properly deburred conduit fittings to protect the new insulation.

A permit is required for conductor replacement in most jurisdictions because the work involves branch circuits or feeders that must be inspected for proper sizing, termination torque, and code compliance. When upgrading a circuit from 15 amps to 20 amps, for example, the existing 14 AWG THHN must be replaced with 12 AWG throughout the entire conduit run.