THHN Wire: The Complete Guide to Individual Conduit Conductors
THHN is the workhorse wire of commercial electrical construction and any residential project that uses conduit wiring methods. Unlike NM-B (Romex), which bundles multiple conductors in a single jacket for direct installation through walls, THHN consists of individual conductors pulled through conduit. Understanding THHN is essential for any electrician, contractor, or informed homeowner working on commercial projects, service upgrades, panel work, or conduit-based residential wiring.
What THHN Stands For
The designation breaks down as follows: T stands for Thermoplastic insulation. H indicates the insulation is rated for high heat (75 degrees Celsius dry, or 90 degrees Celsius in newer dual-rated versions). HN indicates the second H (high heat) plus N (Nylon jacket). The nylon outer jacket over the thermoplastic insulation gives THHN its characteristic smooth, slippery surface that pulls through conduit with significantly less friction than non-nylon-jacketed conductors.
THHN/THWN-2: The Current Standard
Nearly all THHN wire sold today carries a dual THHN/THWN-2 listing. THWN adds a W (wet location) rating, and the “-2” suffix indicates a 90-degree Celsius temperature rating in both dry and wet conditions. This dual listing makes the wire suitable for dry and wet conduit installations, making it the universal choice for conduit work in all environments. When you see “THHN” at a supply house or online, it almost certainly carries the THWN-2 dual listing on its UL label.
Available Gauges and Their Applications
THHN is available in a wide range of AWG gauges. The most common residential and light commercial sizes are 14 AWG (15-amp circuits), 12 AWG (20-amp circuits), 10 AWG (30-amp circuits), 8 AWG (40-amp circuits, typically for EV chargers and large appliances), 6 AWG (55-amp circuits, service conductors and subpanel feeds), 4 AWG (70-amp), 3 AWG (85-amp), 2 AWG (95-amp, common for 100-amp subpanel feeds), 1 AWG (110-amp), 1/0 AWG (125-amp), 2/0 AWG (145-amp), 3/0 AWG (165-amp), and 4/0 AWG (195-amp, common for 200-amp service entrance conductors).
Standard Color Coding
THHN wire uses color-coded insulation to identify the conductor function. Black and red are used for hot conductors on 120-volt and 240-volt circuits. White or gray designates the neutral conductor. Green or bare copper designates the equipment ground conductor. Blue is used for three-phase systems and some multi-circuit applications. Other colors (orange, yellow, brown, purple) are used in three-phase commercial wiring for specific phase identification per NEC requirements. For residential work, the four colors you will use most are black (hot), red (second hot in 240V circuits), white (neutral), and green (ground).
THHN vs NM-B: Which to Use
The choice between THHN in conduit and NM-B cable is primarily determined by the installation environment and local code requirements. NM-B is faster to install in concealed residential locations (inside walls, ceilings, floors) because it runs as a complete cable without requiring conduit. THHN in conduit is required for exposed locations (basements, garages, commercial buildings), wet locations, installations where future circuit changes are anticipated (adding conductors to conduit is simple; fishing new NM-B through finished walls is difficult), and jurisdictions that prohibit NM-B entirely (such as Chicago).
Pulling THHN Through Conduit
THHN pulls through conduit more easily than any other conductor type due to the nylon jacket. For short conduit runs, THHN can be pushed and pulled by hand. For longer runs, a fish tape or wire-pulling lubricant (specifically formulated for electrical conductors) reduces friction significantly. Pull all conductors for a circuit simultaneously rather than one at a time: the conductors twist around each other during pulling, which actually reduces friction and prevents the wires from binding against the conduit interior.
Buying THHN Wire
THHN is sold by the spool in lengths from 500 feet to 2,500 feet at the full spool level, and by the foot or in smaller rolls for smaller projects. IB Lighting carries THHN/THWN-2 from Southwire in multiple gauges and colors. Browse our wire and cable selection for current stock and pricing, or call (800) 674-9019 for contractor volume pricing and color-matched sets. Free freight insurance on every order. Visit the Southwire brand page for the complete THHN product lineup.