THHN and THWN are the two most common building wire designations in North America, and most electricians use the terms interchangeably. In practice, nearly all modern building wire is dual-rated THHN/THWN-2, meaning a single conductor carries both ratings. But the designations themselves describe different performance characteristics — and understanding the difference matters when you're reading spec sheets, pulling permits, or selecting wire for wet or high-temperature environments.
This guide breaks down what each letter in the designation means, compares temperature and moisture ratings, covers NEC ampacity requirements, and explains when the distinction between THHN and THWN actually affects your wire selection.
| Property | THHN | THWN | THWN-2 | THHN/THWN-2 (Dual-Rated) |
|---|---|---|---|---|
| Voltage Rating | 600V | 600V | 600V | 600V |
| Dry Location Temp | 90°C | 75°C | 90°C | 90°C |
| Wet Location Temp | Not rated for wet | 75°C | 90°C | 90°C |
| Insulation | PVC | PVC | PVC | PVC |
| Nylon Jacket | Yes | Yes | Yes | Yes |
| Wet Locations | No | Yes | Yes | Yes |
| UL Standard | UL 83 | UL 83 | UL 83 | UL 83 |
| NEC Reference | Article 310 / Table 310.4(1) | Article 310 / Table 310.4(1) | Article 310 / Table 310.4(1) | Article 310 / Table 310.4(1) |
What Do the Letters in THHN and THWN Mean?
Each letter in a building wire designation tells you something specific about its construction and rated environment. Decoding the name is the fastest way to understand what the wire can and cannot do.
| Letter | Meaning |
|---|---|
| T | Thermoplastic insulation (PVC) |
| H | Heat resistant (75°C rated) |
| HH | High heat resistant (90°C rated) |
| W | Suitable for wet locations |
| N | Nylon jacket over the insulation |
| -2 | 90°C rated in wet locations (instead of 75°C) |
So THHN = Thermoplastic, High Heat (90°C dry), Nylon jacket. THWN = Thermoplastic, Heat (75°C), Wet-rated, Nylon jacket. And THWN-2 = Thermoplastic, Heat, Wet-rated at 90°C, Nylon jacket. When you see THHN/THWN-2 printed on a wire, it means that single conductor meets the requirements of both designations.
What Is THHN Wire?
THHN is a 600V thermoplastic-insulated building wire rated for 90°C in dry locations. The "HH" indicates high heat resistance, and the "N" indicates a nylon jacket over the PVC insulation. THHN is designed for use in conduit, raceways, and other dry environments where its 90°C rating provides maximum ampacity.
The nylon jacket is a thin but important layer — it provides abrasion resistance during pulling, chemical resistance against oils and solvents, and gasoline resistance. This makes THHN easier to pull through conduit and more durable in industrial environments where the wire contacts lubricants or chemicals.
THHN Specifications
- Voltage: 600V
- Temperature: 90°C dry locations only
- Insulation: PVC (polyvinyl chloride)
- Jacket: Nylon (polyamide)
- Conductor: Copper (solid or stranded)
- Sizes: 14 AWG through 1000 kcmil
- Standard: UL 83
- NEC Article: 310
By itself, THHN is not rated for wet locations. If you need wire for outdoor conduit, underground raceways, or any location exposed to moisture, you need THWN or the dual-rated THHN/THWN-2.
What Is THWN Wire?
THWN is a 600V thermoplastic-insulated building wire rated for both dry and wet locations. The "W" in the designation is the key — it indicates the insulation has been tested and approved for use in wet environments, including outdoor conduit, underground raceways, and damp locations.
Standard THWN (without the "-2" suffix) is rated 75°C in both wet and dry locations. This lower temperature rating compared to THHN's 90°C means THWN has a lower ampacity when used in dry environments. The trade-off is moisture resistance.
THWN vs THWN-2
THWN-2 is the upgraded version of THWN. The "-2" suffix indicates the wire is rated 90°C in wet locations (compared to 75°C for standard THWN). This is a significant improvement because it means THWN-2 matches THHN's 90°C rating in dry locations while also being fully rated for wet environments at the same temperature. In practice, THWN-2 has almost entirely replaced standard THWN in the market.
Why Is Most Building Wire Dual-Rated THHN/THWN-2?
Modern manufacturing processes produce insulation compounds that meet the requirements of both THHN and THWN-2 simultaneously. Rather than producing separate products, manufacturers make a single wire that passes both sets of UL 83 tests and print both designations on the jacket.
This dual-rated wire gives you the best of both worlds: 90°C performance in dry locations (from the THHN rating) and 90°C performance in wet locations (from the THWN-2 rating), plus the nylon jacket for pulling and chemical resistance. When you buy "THHN" from any major distributor today, you are almost certainly getting dual-rated THHN/THWN-2.
The practical implication: for most projects, you don't need to choose between THHN and THWN. The dual-rated product covers both environments at the highest temperature rating. The distinction matters only if you encounter older stock rated solely as THHN (no wet rating) or solely as THWN (75°C only).
How Do Temperature Ratings Affect Ampacity?
Temperature ratings directly determine how much current a conductor can carry. A higher temperature rating means the insulation can tolerate more heat from current flow, which translates to higher ampacity for the same conductor size.
| AWG Size | 60°C (TW) | 75°C (THWN) | 90°C (THHN / THWN-2) |
|---|---|---|---|
| 14 | 15A | 15A* | 15A* |
| 12 | 20A | 20A* | 20A* |
| 10 | 30A | 35A | 40A |
| 8 | 40A | 50A | 55A |
| 6 | 55A | 65A | 75A |
| 4 | 70A | 85A | 95A |
| 3 | 85A | 100A | 115A |
| 2 | 95A | 115A | 130A |
| 1 | 110A | 130A | 145A |
| 1/0 | 125A | 150A | 170A |
| 2/0 | 145A | 175A | 195A |
| 3/0 | 165A | 200A | 225A |
| 4/0 | 195A | 230A | 260A |
| 250 | 215A | 255A | 290A |
| 350 | 260A | 310A | 350A |
| 500 | 320A | 380A | 430A |
*14 and 12 AWG are limited to 15A and 20A respectively by NEC® 240.4(D) overcurrent protection rules, regardless of insulation temperature rating. The 90°C column ampacity may only be used for derating purposes.
The 90°C ampacity column is particularly useful when you need to apply derating factors. For example, if you have more than three current-carrying conductors in a conduit, NEC® Table 310.15(C)(1) requires derating. Starting from the higher 90°C ampacity gives you more headroom before the derated value drops below your circuit's actual load. See our AWG Wire Gauge Guide for full ampacity tables with derating factors.
Terminal Temperature Limitation
Even when using 90°C-rated wire, the ampacity you can actually use is often limited by the terminal temperature rating on the equipment. Most circuit breakers, panelboards, and devices are rated for 75°C terminals. Per NEC® 110.14(C), you must use the ampacity corresponding to the lowest temperature rating in the circuit — which means the 75°C column applies unless all termination points are rated for 90°C. The exception, again, is when applying derating or ambient temperature correction factors, where you may start from the 90°C ampacity.
Wet Locations vs. Dry Locations: When Does It Matter?
The NEC defines three location types that determine which wire ratings are acceptable:
- Dry locations — Indoor areas not normally subject to dampness or moisture. Most interior conduit runs, panel interiors, and junction boxes in climate-controlled spaces.
- Damp locations — Partially protected areas subject to moderate moisture. Covered porches, some basements, cold-storage warehouses, and canopied outdoor areas.
- Wet locations — Installations underground, in concrete slabs in contact with earth, in outdoor conduit, and anywhere subject to saturation with water. Underground conduit is always considered a wet location even if it appears dry at the time of installation.
THHN alone is rated only for dry locations. THWN and THWN-2 are rated for dry, damp, and wet locations. This is the core practical difference: if your conduit run passes through any wet or damp section, the wire must carry a "W" rating. Since dual-rated THHN/THWN-2 carries the "W" rating, it works everywhere.
Common Wet Location Scenarios
- Outdoor conduit runs (even above ground — condensation is possible)
- Underground conduit and raceways
- Conduit in concrete slabs on or below grade
- Wet basements and mechanical rooms with high humidity
- Exterior junction boxes and enclosures
- Rooftop equipment connections
Where Is THHN/THWN Building Wire Used?
THHN/THWN-2 is the workhorse of electrical construction in the United States. It's used in virtually every commercial and residential project that involves conduit or raceway wiring.
Primary Applications
- Branch circuits — 15A, 20A, and 30A branch circuits for lighting, receptacles, and dedicated equipment in conduit
- Feeders — Larger gauge conductors feeding subpanels and distribution equipment
- Service entrance conductors — In conduit from the meter to the main panel
- Motor circuits — Individual conductors in conduit for motor branch circuits and feeders
- Commercial construction — The default conductor type for conduit-based wiring in commercial buildings
- Industrial plants — Power distribution in conduit and raceway systems
Installation Methods
THHN/THWN is designed for installation in conduit and raceways. It is not suitable for direct burial (use UF-B or direct burial-rated cable), open-air installation without a raceway, or as a substitute for NM-B (Romex) inside walls without conduit. The wire must always be enclosed in a recognized wiring method such as EMT, RMC, IMC, PVC conduit, or wireways.
For cable tray installations requiring THHN-type conductors, VNTC tray cable provides a multi-conductor assembly with an overall PVC jacket rated for cable tray use.
Copper vs. Aluminum THHN/THWN
THHN/THWN-2 is manufactured in both copper and aluminum. Copper is the standard for branch circuits and most feeders through 4/0 AWG. Aluminum building wire becomes cost-effective for larger feeders and service entrance conductors, where the material savings offset the need for larger conductors and AL-rated terminations.
| Factor | Copper THHN/THWN-2 | Aluminum THHN/THWN-2 |
|---|---|---|
| Typical sizes | 14 AWG – 1000 kcmil | 8 AWG – 1000 kcmil |
| Ampacity (same size) | Higher | ~78% of copper |
| Weight | Heavier | ~30% of copper weight |
| Cost per foot | Higher | Significantly lower |
| Terminations | Any CU-rated lug | Must use AL/CU-rated lugs with anti-oxidant |
| Best for | Branch circuits, small feeders | Large feeders (4/0+), service entrances |
For a deeper comparison of conductor materials, see our How to Choose the Right Cable guide.
Solid vs. Stranded Conductors
THHN/THWN-2 is available in both solid and stranded conductor configurations. The choice depends on wire size and installation requirements.
- Solid conductors (14 AWG – 10 AWG) — A single copper wire. Easier to terminate on screw terminals and push-in connectors. Standard for residential branch circuits and smaller commercial runs. More rigid, which makes it harder to pull in long conduit runs.
- Stranded conductors (typically 10 AWG and larger, though available in smaller sizes) — Multiple smaller wires twisted together. More flexible, easier to bend and route in junction boxes, and significantly easier to pull through conduit. Standard for commercial and industrial installations. Required for larger sizes where solid copper would be too stiff to work with.
Most THHN/THWN-2 in sizes 8 AWG and larger is stranded by default. For 10 AWG and 12 AWG, both solid and stranded options are available — stranded is preferred for conduit pulls longer than 100 feet or with multiple bends.
THHN/THWN Color Coding
Building wire color coding follows NEC® requirements and industry conventions. Using the correct colors speeds up installation, simplifies troubleshooting, and is required for certain conductors.
| Color | NEC Application |
|---|---|
| Green or Green/Yellow | Equipment grounding conductor (required by NEC®) |
| White or Gray | Grounded (neutral) conductor (required by NEC®) |
| Black | Ungrounded (hot) conductor — Phase A (120/208V) or general single-phase hot |
| Red | Ungrounded (hot) conductor — Phase B (120/208V), travelers, secondary hot |
| Blue | Ungrounded (hot) conductor — Phase C (120/208V three-phase) |
| Orange | Ungrounded (hot) conductor — 277V circuits (277/480V systems), or delta high leg |
| Yellow | Ungrounded (hot) conductor — Phase B (277/480V) or switch legs |
| Brown | Ungrounded (hot) conductor — Phase A (277/480V three-phase) |
Only green (grounding) and white/gray (neutral) are mandated by the NEC®. Hot conductor colors are industry convention, not code requirement — but following the standard color scheme is considered best practice and is required by many specifications and local jurisdictions.
How Does THHN/THWN Compare to Other Building Wire Types?
THHN/THWN-2 is one of several conductor types listed in NEC® Article 310, Table 310.4(1) (formerly Table 310.16). Here's how it compares to other common building wire designations.
| Wire Type | Insulation | Dry Temp | Wet Temp | Key Difference from THHN/THWN-2 |
|---|---|---|---|---|
| THHN/THWN-2 | PVC + nylon | 90°C | 90°C | — (baseline) |
| XHHW-2 | Cross-linked PE | 90°C | 90°C | Better chemical and moisture resistance; preferred for industrial and wet environments |
| RHH/RHW-2 | Thermoset rubber | 90°C | 90°C | Rubber insulation; more flexible, better cold-weather performance |
| USE-2 | XLPE or EPR | 90°C | 90°C | Underground service entrance; rated for direct burial |
| TW | PVC (no nylon) | 60°C | 60°C | Lower temperature rating; less common in modern installations |
| MTW | PVC or nylon | 90°C | 60°C | Machine tool wire; designed for equipment internal wiring |
For most conduit-based building wiring, THHN/THWN-2 is the default choice because it offers the best combination of availability, cost, and performance. XHHW-2 is the primary alternative when superior chemical resistance or moisture performance is required.
What to Look for When Buying THHN/THWN Building Wire
Not all building wire is created equal. Here's what to verify when specifying or purchasing THHN/THWN-2.
- Dual rating — Confirm the wire is marked THHN/THWN-2 (not just THHN). This ensures full wet-location rating at 90°C.
- UL Listed — Look for the UL mark. Wire without UL listing will fail electrical inspection.
- Conductor marking — The jacket should be printed with: wire type (THHN/THWN-2), gauge, voltage rating (600V), UL file number, and manufacturer name.
- Copper quality — Specify soft-drawn annealed copper per ASTM B3 (solid) or ASTM B8 (stranded).
- Gauge accuracy — Reputable manufacturers maintain tight tolerances on conductor diameter. Undersized conductors reduce ampacity and may not meet code.
- Put-up length — Standard put-ups are 500 ft, 1,000 ft, and 2,500 ft spools. Larger projects may order 5,000 ft reels. Match your order to your actual run lengths plus 10–15% waste.
Browse our copper building wire for current availability and sizing options.
Frequently Asked Questions
Is THHN the same as THWN?
No, they are different designations. THHN is rated 90°C for dry locations only. THWN is rated for wet and dry locations at 75°C (or 90°C for THWN-2). However, most modern building wire is manufactured as dual-rated THHN/THWN-2, which carries both designations and is rated 90°C in wet and dry locations.
Can I use THHN in a wet location?
Only if the wire is also rated THWN or THWN-2. Wire marked only as THHN (without a wet rating) is not code-compliant for wet or damp locations. In practice, nearly all THHN sold today is dual-rated THHN/THWN-2 and includes the wet rating — but you should always check the jacket markings before installing in a wet location.
Can I use THHN/THWN for direct burial?
No. THHN/THWN-2 is designed for conduit and raceway installation, not direct burial. For underground runs without conduit, use UF-B cable or direct burial-rated conductors. You can run THHN/THWN-2 inside underground conduit (it is wet-rated), but the wire itself must be enclosed in the conduit — it cannot be placed directly in the earth.
What is the difference between THWN and THWN-2?
The temperature rating in wet locations. THWN is rated 75°C in wet locations. THWN-2 is rated 90°C in wet locations. This higher wet-location rating gives THWN-2 better ampacity in wet environments and makes it more versatile overall. Standard THWN has been largely replaced by THWN-2 in the market.
Why does THHN have a nylon jacket?
The nylon (polyamide) jacket over the PVC insulation provides abrasion resistance during conduit pulls, resistance to oils, gasoline, and common solvents, and improved mechanical protection. It makes the wire significantly easier to pull through conduit without damaging the insulation, which is why THHN is the preferred conductor type for conduit wiring.
Can I use THHN/THWN in cable tray?
Individual THHN/THWN-2 conductors are typically not permitted in cable tray unless specifically listed for tray use. For cable tray installations, VNTC tray cable (which uses THHN-type conductors inside an overall PVC jacket rated for tray use) or Type MC cable listed for cable tray are the standard options. Check NEC® Article 392 for cable tray wiring methods.
What gauge THHN do I need for a 200-amp service?
For a 200A residential service, you typically need 2/0 AWG copper THHN/THWN-2 (rated 175A at 75°C per NEC® Table 310.16, which meets the 200A service requirement per NEC® 310.12). Alternatively, 4/0 AWG aluminum is commonly used for 200A services. Always verify with your local AHJ and consider voltage drop for longer service runs. See our AWG Wire Gauge Guide for complete sizing tables.
Is THHN/THWN wire the same as Romex?
No. THHN/THWN-2 is an individual conductor designed for installation in conduit or raceways. Romex (NM-B) is a multi-conductor cable with an outer jacket designed for installation inside walls and ceilings without conduit. They serve different purposes and are governed by different NEC® articles. You cannot substitute one for the other.
Related Resources
- AWG Wire Gauge Guide: Sizes, Ampacity & Selection
- How to Choose the Right Cable for Your Project
- Plenum vs. Riser Cable: CMP, CMR, CL2P & CL3P Ratings Explained
- Fire Alarm Cable: Types, Ratings & NEC Requirements
- High Temperature & LSZH Cable Guide
- Direct Burial Cable: Types, Depth Requirements & Selection
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Disclaimer: This guide is provided for informational purposes only and is not installation advice. It does not constitute professional electrical, engineering, or code-compliance advice. Installing wire & cable can be dangerous and pose a risk of possible electric shock or other hazards. Building codes, NEC editions, and local amendments change periodically. Always consult a licensed electrician and your local authority having jurisdiction (AHJ) before specifying or installing cable. Images are for illustration purposes and may not reflect actual installed products.
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