THHN THWN insulation specifications
The THHN/THWN insulation system combines a layer of polyvinyl chloride (PVC) with an outer nylon jacket, providing high thermal, mechanical, and chemical resistance. It is used on copper or aluminum conductors for electrical installations in commercial and industrial buildings, in both dry and wet environments.
Nomenclature and designation
Section titled “Nomenclature and designation”The acronyms describe the insulation properties according to the National Electrical Code (NEC):
| Abbreviation | Meaning | Condition of use |
|---|---|---|
| THHN | Thermoplastic High Heat-resistant Nylon-coated | Dry environment, 90 °C / 194 °F |
| THWN | Thermoplastic Heat and Water-resistant Nylon-coated | Wet environment, 75 °C / 167 °F |
| THWN-2 | THWN with extended rating | Wet environment, 90 °C / 194 °F |
The dual marking THHN/THWN-2 indicates that the conductor meets both conditions and can operate at 90 °C / 194 °F in dry and wet conditions.
Insulation composition
Section titled “Insulation composition”The insulation consists of two layers extruded over the metallic conductor:
- Primary PVC insulation – provides dielectric strength and base thermal resistance.
- Nylon jacket – thin outer layer that adds resistance to cutting, abrasion, and hydrocarbons, as well as facilitating installation in conduits.
Standard thicknesses for the most common sizes are shown below:
| AWG Size | PVC Thickness | Nylon Thickness | Approximate Outer Diameter |
|---|---|---|---|
| 14 | 0,38 mm / 0.015 in | 0,10 mm / 0.004 in | 2,30 mm / 0.091 in |
| 12 | 0,38 mm / 0.015 in | 0,13 mm / 0.005 in | 2,50 mm / 0.099 in |
| 10 | 0,38 mm / 0.015 in | 0,13 mm / 0.005 in | 3,20 mm / 0.126 in |
| 8 | 0,51 mm / 0.020 in | 0,13 mm / 0.005 in | 4,70 mm / 0.185 in |
Thermal characteristics
Section titled “Thermal characteristics”The temperature limits depend on the type of designation and the presence of moisture. The conductor must not exceed these temperatures in continuous operation:
| Condition | Designation | Maximum Temperature |
|---|---|---|
| Dry | THHN / THWN-2 | 90 °C / 194 °F |
| Wet | THWN | 75 °C / 167 °F |
| Wet | THWN-2 | 90 °C / 194 °F |
| Short-circuit (max. 5 s) | All | 150 °C / 302 °F |
When the ambient temperature differs from the base of 30 °C / 86 °F, the ampacity is corrected using the NEC formula:
I’ = I × √[(T_c – T_a’) / (T_c – T_a)]
| Variable | Description | Unit |
|---|---|---|
| I’ | Corrected allowable current | A |
| I | Nominal allowable current at 30 °C | A |
| T_c | Maximum conductor temperature (90 °C for THHN) | °C |
| T_a | Reference ambient temperature (30 °C) | °C |
| T_a’ | Actual ambient temperature of the installation | °C |
Electrical characteristics
Section titled “Electrical characteristics”The rated operating voltage of THHN/THWN conductors is 600 V, as established in UL 83. The dielectric strength of PVC ensures insulation between phases and between phase and ground under the intended installation conditions.
| Parameter | Value |
|---|---|
| Maximum service voltage | 600 V |
| Typical insulation resistance (per meter, 20 °C) | > 10⁹ Ω |
| Allowable overload capacity | 150 °C / 302 °F for short-circuit |
Chemical and moisture resistance
Section titled “Chemical and moisture resistance”The PVC‑nylon combination offers good resistance to common agents present in industrial and residential installations. The nylon jacket prevents degradation by oils, gasoline, and other light hydrocarbons.
| Chemical agent | Insulation behavior |
|---|---|
| Mineral oil and gasoline | Very good resistance; nylon does not dissolve or crack. |
| Diluted acids and alkalis | Good resistance at room temperature. |
| Water and moisture | Suitable for use in wet locations (THWN/THWN‑2). |
| Ozone and UV radiation | Surface degradation after prolonged exposure; outdoor use requires additional protection. |
| Aromatic solvents (benzene, toluene) | Can soften PVC; not recommended without prior evaluation. |
Applications and typical uses
Section titled “Applications and typical uses”THHN/THWN conductors are mainly installed in conduit, raceways, and cable trays within buildings. They are suitable for power, lighting, and control circuits in commercial and industrial installations.
| Application | Example |
|---|---|
| General supply | Service entrances in main and secondary panelboards. |
| Lighting and receptacle circuits | Interior wiring in office buildings and homes. |
| Industrial machinery | Connections in motors, pumps, and process equipment. |
| Wet locations | THWN-2 conductors for submersible pumps or condensation areas. |
| Underground installations | In buried conduits, provided the system’s watertightness is ensured. |
Reference standards
Section titled “Reference standards”| Standard | Description |
|---|---|
| UL 83 | Standard for Thermoplastic-Insulated Wires and Cables. |
| UL 1063 | Standard for Machine‑Tool Wires and Cables (applicable to some types). |
| NEC – NFPA 70 | National Electrical Code (Article 310 – Conductors for General Wiring). |
| NOM‑063‑SCFI | Safety requirements for electrical conductors (Mexico). |
| CSA C22.2 No. 75 | Standard for Thermoplastic‑Insulated Wires and Cables (Canada). |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum operating temperature of THHN insulation in dry environment?
Section titled “What is the maximum operating temperature of THHN insulation in dry environment?”The maximum operating temperature in dry conditions is 90 °C / 194 °F.
What temperature difference in wet environment exists between THWN and THWN‑2?
Section titled “What temperature difference in wet environment exists between THWN and THWN‑2?”THWN supports up to 75 °C / 167 °F in wet conditions; THWN‑2 reaches 90 °C / 194 °F under the same conditions.
What is the PVC insulation thickness on a 14 AWG THHN conductor?
Section titled “What is the PVC insulation thickness on a 14 AWG THHN conductor?”The nominal PVC thickness for 14 AWG is 0,38 mm / 0.015 in.
Up to what temperature can a THHN/THWN conductor withstand under short-circuit conditions?
Section titled “Up to what temperature can a THHN/THWN conductor withstand under short-circuit conditions?”It withstands up to 150 °C / 302 °F for a maximum of 5 seconds without permanent damage.
What is the rated service voltage of THHN/THWN cables?
Section titled “What is the rated service voltage of THHN/THWN cables?”The rated voltage is 600 V between phases, per UL 83 standard.
What minimum size of THHN conductor is used in 15 A residential circuits?
Section titled “What minimum size of THHN conductor is used in 15 A residential circuits?”The typical minimum size is 14 AWG (2,08 mm²), according to NEC ampacity tables for copper conductors without derating.