THHN wire is one of the most widely specified single-conductor building wires for branch circuits, feeders, and general power distribution in conduit and raceway systems. Our THHN/THWN-2 copper wire is manufactured for 600V applications with PVC insulation and a nylon jacket to support smooth pulling and long-term resistance to heat, abrasion, oil, and gasoline. For buyers comparing suppliers, the key specification is not only the THHN marking itself, but also whether the product is clearly documented with the correct dual rating, standards, available sizes, and project support information.
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What is THHN Wire?
THHN meaning in electical stands for Thermoplastic High Heat-Resistant Nylon-coated wire. In practical use, it refers to a single-conductor building wire with PVC insulation and a nylon jacket, commonly used in conduit and raceway systems for residential, commercial, and industrial wiring.
Many buyers today are actually purchasing dual-rated THHN/THWN-2 wire. THHN indicates 90°C performance in dry locations, while THWN-2 indicates 90°C performance in wet locations. Because product selection errors often happen at this point, buyers should always verify the jacket print, datasheet, and quotation sheet before confirming use in wet, damp, or outdoor conduit systems.
THHN wire is typically chosen when a project needs:
- a 600V copper building wire,
- reliable pulling performance in conduit,
- available solid or stranded constructions depending on size,
- and broad use across branch circuits, feeders, control wiring, and equipment connections.
Technical Note: Most of today’s supply is also dual-marked THHN/THWN-2. Unlike older “THWN” (rated 75°C Wet), the “-2” suffix indicates the insulation system is engineered to retain 90°C ampacity even in wet locations.
Construction Specifications (ASTM & UL Standards)
We manufacture THHN/THWN-2 wire with a focus on conductor stability, installation efficiency, and code-oriented application support.
Conductor
Soft-annealed bare copper is used for dependable conductivity, improved ductility, and secure terminations. Depending on gauge and application, the wire may be supplied in solid or stranded constructions. In general, smaller sizes are often selected in solid constructions, while larger sizes are commonly preferred in stranded constructions for easier handling and pulling.
Insulation
The conductor is insulated with a heat- and moisture-resistant PVC compound formulated for building wire service.
Jacket
A nylon jacket helps improve abrasion resistance during pulling and adds resistance to oil and gasoline. It also helps the wire move more smoothly through conduit, especially in longer runs and installations with multiple bends.
Supply note
Available conductor constructions, strand classes, and gauge options should always be confirmed on the quotation sheet and datasheet for the exact item ordered. For project inquiries, please specify: gauge, solid or stranded, color, estimated length, installation environment, and destination market
THHN Wire Specification
| Parameter | Specification |
| Voltage Rating | 600 Volts |
| Reference Standards | UL 83 (File No. [Insert if avail]), CSA C22.2 No. 75, UL 1063 (MTW) |
| Temperature Rating | 90°C Dry / 90°C Wet |
| Conductor Purity | Soft Annealed Bare Copper (ASTM B3) |
| Insulation Resistance | Excellent resistance to heat, moisture, oil, and gasoline |
| Conduit Compatibility | EMT, IMC, Rigid Steel, PVC Schedule 40/80 |
| Environmental Compliance | RoHS / REACH |
Ampacity & Temperature Constraints
To ensure code compliance, it is critical to distinguish between the Wire Rating and the Termination Rating. Although our conductor THHN is engineered to handle 90°C, most circuit breakers and lugs are limited to 75°C.
- For Sizing: Always select your wire gauge based on the 75°C column (NEC Table 310.16) to match standard equipment terminals.
- For Derating: Use the higher 90°C rating as your starting baseline when calculating adjustments for hot environments (like rooftops) or when bundling more than three wires in a conduit.
Ampacity Reference (3 Conductors in Raceway, 30°C Ambient):
| AWG / MCM | Structure | 75°C Rating
(Termination Limit) |
90°C Rating
(Derating Base) |
| 12 AWG | Solid | 25 A | 30 A |
| 10 AWG | Solid | 35 A | 40 A |
| 8 AWG | Stranded | 50 A | 55 A |
| 6 AWG | Stranded | 65 A | 75 A |
| 4 AWG | Stranded | 85 A | 95 A |
| 4/0 AWG | Stranded | 230 A | 260 A |
| 500 MCM | Stranded | 380 A | 430 A |
THHN/THWN-2 vs Alternatives
Different thhn thwn building wires may appear similar, but the correct choice depends on the installation environment, required flexibility, temperature rating, and code compliance.
THHN vs. MTW (Machine Tool Wire)
Most of our stranded cu THHN cables carry a dual MTW listing. However, “pure” MTW generally features a higher strand count for superior flexibility inside cramped control panels. If your application involves constant vibration or tight bending radii in automation cabinets, reviewing the specific differences in flexibility and jacket thickness in our MTW vs. THHN guide.
THHN vs. TFFN (Fixture Wire)
Think of TFFN as the “lighter duty” cousin of THHN. Cable THHN is typically available down to 14 AWG for branch circuits. When you need 16 AWG or 18 AWG for lighting fixtures or fire alarm control circuits, you must switch to TFFN. For a detailed breakdown of ampacity limits and insulation differences, refer to our comparison of TFFN vs. THHN.
THHN vs. Romex (NM-B) Wire
THHN/THWN-2 and Romex (Type NM-B) are different product categories and should not be treated as interchangeable. THHN copper wire is an individual building wire commonly pulled through conduit or raceway systems, while Romex is a nonmetallic-sheathed cable assembly mainly used for indoor residential wiring in normally dry locations. For conduit-based commercial, industrial, or wet-location applications, thwn electrical wire usually the more appropriate choice.
Buyers should also note that the conductors inside NM-B cable are not intended to be removed and used as standalone THHN conductors.
THHN vs. XHHW-2 (Cross-Linked)
THHN uses PVC/Nylon (Thermoplastic), while XHHW-2 uses XLPE (Thermoset). XLPE offers better resistance to leakage current and chemicals but is thicker and stiffer. For engineers deciding between cost and chemical resilience, our engineering note on THHN vs. XHHW provides the critical data needed for the decision.
XHHW vs. XHHW-2
XHHW and XHHW-2 are closely related XLPE-insulated building wires, but XHHW-2 provides the clearer designation for 90°C performance in both wet and dry locations. In practice, XHHW-2 is often preferred when buyers want stronger moisture and heat resistance with less specification uncertainty. When a project references XHHW without the “-2” suffix, the exact product marking and datasheet should always be checked before approval.
THHN vs. Solar PV Wire
A common mistake is attempting to use THHN for rooftop solar interconnections. Standard THHN is rated for 600V and often lacks long-term UV stability for direct exposure. Solar PV wire is rated for 1000V/2000V and has a much thicker jacket designed for intense sunlight. To avoid inspection failures, please read our guide on Choosing the Right Wire for Solar: PV Wire vs. THHN Wire.
Fast Selection Rule
Choose thhn wire type when the project requires a 600V copper building wire for conduit systems in commercial, residential, or industrial applications. Choose another construction when the job requires direct burial, prolonged exposed UV service, solar array cable, or a different insulation family specified by the project.
Need a formal quote or a full specification sheet for your project? We can also produce thhn aluminum wire. Contact the ZW Cable team for help with THHN wire by the foot selection, solid vs. thnn stranded wire construction, datasheets, export packaging, and quotation support.
