THWN Conduit Fill Guide
Thermoplastic Heat and Water-resistant Nylon-coated (THWN) conductors are rated for 75 degrees C (167 degrees F) in both dry and wet locations. This page provides complete conduit fill data for THWN 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.
THWN Conductor Specifications
THWN 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 THWN Conductors
The table below shows the minimum conduit trade size needed for various numbers of THWN 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 THWN Conductors
THWN is used in both dry and wet locations at a 75 degrees C temperature rating. In modern practice, most wire sold as THWN also carries the THHN rating (making it THHN/THWN-2), but standalone THWN-rated wire still exists in the market. THWN is suitable for conduit wiring in commercial and residential applications, feeders, branch circuits, and any installation where the conductor temperature will not exceed 75 degrees C. It shares the same physical dimensions as THHN/THWN-2.
Insulation Details
THWN uses the same dual-layer PVC and nylon insulation system as THHN. The nylon jacket provides mechanical protection and oil resistance. The insulation thickness is identical to THHN, resulting in the same conductor cross-sectional areas. The primary difference is the temperature rating: THWN is rated at 75 degrees C in all locations, while THHN adds a 90 degrees C dry-location rating.
History and Development
THWN was developed as an improvement over the earlier TW (Thermoplastic Water-resistant) insulation by adding a nylon jacket for improved abrasion resistance. It predates the modern dual-rated THHN/THWN-2 conductors that now dominate the market. While less common as a standalone rating today, THWN remains a recognized NEC insulation type.
Advantages of THWN
- Same compact dimensions as THHN/THWN-2 for maximum conduit fill
- Rated for wet locations at 75 degrees C
- Nylon jacket provides oil and abrasion resistance
- Compatible with all standard conduit types
- Well-established NEC listing with decades of field history
Limitations
- Lower dry-location temperature rating (75 degrees C) than THHN (90 degrees C)
- Largely superseded by dual-rated THHN/THWN-2 in the market
- Not suitable for direct burial without additional protection
- Requires derating when bundled with many conductors
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.