THHN/THWN-2 Conduit Fill Guide
Thermoplastic High Heat-resistant Nylon-coated / Thermoplastic Heat and Water-resistant Nylon-coated (THHN/THWN-2) conductors are rated for 90 degrees C (194 degrees F) in dry locations. This page provides complete conduit fill data for THHN/THWN-2 conductors in all standard conduit types and sizes per the National Electrical Code. Use the tables below to find the minimum conduit size for your installation, or visit the calculator for custom fill calculations.
THHN/THWN-2 Conductor Specifications
THHN/THWN-2 Conductor Areas (NEC Table 5)
The cross-sectional areas below include the conductor insulation and are used for all conduit fill calculations. These values come from NEC Chapter 9, Table 5. The area determines how many conductors of each size can fit in a given conduit per the NEC fill percentage rules (53% for 1 conductor, 31% for 2, and 40% for 3 or more).
| Wire Size | Area (sq.in.) | Typical Use |
|---|---|---|
| 14 AWG | 0.0097 | 15A branch circuits, lighting |
| 12 AWG | 0.0133 | 20A branch circuits, receptacles |
| 10 AWG | 0.0211 | 30A circuits, dryers, water heaters |
| 8 AWG | 0.0366 | 40-50A circuits, ranges, large equipment |
| 6 AWG | 0.0507 | 55-65A circuits, subpanels, large motors |
| 4 AWG | 0.0824 | 70-85A feeders, subpanels |
| 3 AWG | 0.0973 | 85-100A feeders |
| 2 AWG | 0.1158 | 95-115A feeders, service entrance |
| 1 AWG | 0.1562 | 110-130A feeders |
| 1/0 AWG | 0.1855 | 125-150A service entrance, large feeders |
| 2/0 AWG | 0.2223 | 145-175A service entrance |
| 3/0 AWG | 0.2679 | 165-200A service entrance |
| 4/0 AWG | 0.3237 | 195-230A main service entrance |
| 250 AWG | 0.3970 | 215-255A large services |
| 300 AWG | 0.4608 | 240-285A commercial services |
| 350 AWG | 0.5242 | 260-310A commercial services |
| 400 AWG | 0.5863 | 280-335A commercial/industrial |
| 500 AWG | 0.7073 | 305-380A large industrial feeders |
Minimum Conduit Size for THHN/THWN-2 Conductors
The table below shows the minimum conduit trade size needed for various numbers of THHN/THWN-2 conductors in EMT conduit. This is the most commonly referenced conduit type. For other conduit types, use the calculator or browse the individual conduit type pages.
| Wire Size | 3 wires | 4 wires | 6 wires | 8 wires | 10 wires | 12 wires |
|---|---|---|---|---|---|---|
| 14 AWG | 1/2" | 1/2" | 1/2" | 1/2" | 1/2" | 1/2" |
| 12 AWG | 1/2" | 1/2" | 1/2" | 1/2" | 3/4" | 3/4" |
| 10 AWG | 1/2" | 1/2" | 3/4" | 3/4" | 3/4" | 1" |
| 8 AWG | 1/2" | 3/4" | 1" | 1" | 1-1/4" | 1-1/4" |
| 6 AWG | 3/4" | 3/4" | 1" | 1-1/4" | 1-1/4" | 1-1/2" |
| 4 AWG | 1" | 1" | 1-1/4" | 1-1/2" | 2" | 2" |
| 3 AWG | 1" | 1-1/4" | 1-1/4" | 1-1/2" | 2" | 2" |
| 2 AWG | 1-1/4" | 1-1/4" | 1-1/2" | 2" | 2" | 2-1/2" |
| 1 AWG | 1-1/4" | 1-1/2" | 2" | 2" | 2-1/2" | 2-1/2" |
| 1/0 AWG | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 2-1/2" | 2-1/2" |
| 2/0 AWG | 1-1/2" | 2" | 2" | 2-1/2" | 2-1/2" | 3" |
| 3/0 AWG | 1-1/2" | 2" | 2-1/2" | 2-1/2" | 3" | 3" |
| 4/0 AWG | 2" | 2" | 2-1/2" | 3" | 3" | 3-1/2" |
| 250 AWG | 2" | 2-1/2" | 3" | 3" | 3-1/2" | 4" |
| 300 AWG | 2-1/2" | 2-1/2" | 3" | 3-1/2" | 3-1/2" | 4" |
| 350 AWG | 2-1/2" | 2-1/2" | 3" | 3-1/2" | 4" | N/A |
| 400 AWG | 2-1/2" | 3" | 3" | 4" | 4" | N/A |
| 500 AWG | 2-1/2" | 3" | 3-1/2" | 4" | N/A | N/A |
About THHN/THWN-2 Conductors
THHN/THWN-2 is the most widely used building wire in the United States. It is the standard conductor for branch circuits, feeders, and general-purpose wiring in commercial, industrial, and residential construction. Because it carries a dual rating (THHN for dry and THWN-2 for wet), a single conductor can be used in virtually any indoor or outdoor conduit installation. This versatility and its compact insulation thickness make it the default choice for electricians nationwide.
Insulation Details
The insulation consists of a thermoplastic PVC inner layer covered by a thin nylon (polyamide) outer jacket. The nylon jacket provides excellent resistance to oil, gasoline, and physical abrasion during pulling. This dual-layer construction allows a thinner overall insulation compared to single-layer insulation types like XHHW, which is why THHN/THWN-2 conductors have smaller cross-sectional areas and allow more wires per conduit.
History and Development
THHN/THWN-2 evolved from earlier TW and THW insulation types as polymer science advanced. The addition of the nylon jacket in the 1970s allowed thinner insulation while maintaining abrasion resistance, and the dual THHN/THWN-2 rating became standard as manufacturers optimized the insulation system for both dry heat resistance and wet-location performance.
Advantages of THHN/THWN-2
- Smallest insulation diameter among common wire types, maximizing conduit fill
- Dual-rated for dry (90 degrees C) and wet (75 degrees C or 90 degrees C for THWN-2) locations
- Nylon jacket resists oil, gasoline, and abrasion during pulling
- Available in all standard sizes from 14 AWG through 1000 kcmil
- Lowest cost per foot among high-temperature building wires
- Most widely stocked wire type at electrical supply houses
Limitations
- Not suitable for direct burial without additional protection
- PVC insulation has limited flexibility at very low temperatures
- Maximum conductor temperature must be derated when more than 3 current-carrying conductors share a conduit
- Not rated for hazardous locations without additional listing
Other Wire Types
Compare conduit fill with other conductor insulation types. Different insulation materials have different thicknesses, which affects how many conductors fit in a given conduit.