THHW Wire

Q: What is the temperature rating of THHW?

THHW is rated 90°C in dry locations and 75°C in wet locations. The ampacity values in NEC Table 310.16 use the 90°C column for conductors rated at or above 90°C, but actual usable ampacity is limited by the termination temperature (typically 75°C per 110.14(C)).

THHW stands for Thermoplastic High Heat-resistant Water-resistant. General purpose in raceway. Dual-rated: 90C dry (90C column), 75C wet (75C column) in Table 310.16.

Dry rating90°C

Wet rating75°C

Voltage600VV

LocationsDry, Wet

Specifications

Property	Value
Full name	Thermoplastic High Heat-resistant Water-resistant
Insulation material	PVC
Outer covering	None (single layer)
Temperature rating (dry)	90°C (194°F)
Temperature rating (wet)	75°C (167°F)
Voltage rating	600VV
Approved locations	Dry, Wet

Advantages and limitations

Advantages: ['Dual-rated', 'Available in large sizes']

Limitations: ['Less common than THHN/THWN-2 for new work', 'Thicker than THHN']

When to use THHW wire

General purpose in raceway. Dual-rated: 90C dry (90C column), 75C wet (75C column) in Table 310.16.

Common applications for THHW:

Specialized applications - THHW is used where its specific temperature and environment ratings are required
See manufacturer specifications for approved installation methods

NEC code references for THHW

The NEC defines wire insulation types in Table 310.4(1) (formerly Table 310.104(A)). Key sections that apply:

Table 310.4(1) - lists THHW with its temperature rating, insulation material, and approved locations
Table 310.16 - ampacity values for conductors in raceway, cable, or earth (the values shown in our ampacity table above)
Section 110.14(C) - termination temperature limits that determine real-world usable ampacity
Section 310.15(C)(1) - adjustment factors for more than 3 current-carrying conductors in a raceway

Frequently asked questions about THHW wire

Where can I use THHW wire?

THHW is approved for use in dry, wet per NEC Table 310.4(1). It has a dry temperature rating of 90°C and a wet rating of 75°C. Always verify that your specific installation meets the approved location requirements.

What is the temperature rating of THHW?

THHW is rated 90°C in dry locations and 75°C in wet locations. The ampacity values in NEC Table 310.16 use the 90°C column for conductors rated at or above 90°C, but actual usable ampacity is limited by the termination temperature (typically 75°C per 110.14(C)).

Other wire insulation types

THHN 90°C THWN 75°C THWN-2 90°C XHHW 90°C XHHW-2 90°C RHH 90°C RHW-2 90°C USE-2TW 60°C NM-B 60°C UF-B 60°C MC Cable 90°C AC Cable (BX) 90°C SER/SEU 75°C PV Wire 90°C THW 75°C THW-2 90°C RHW 75°C MI 90°C UF 60°C XHH 90°C FEP 90°C SIS 90°C

Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.

Important: For reference only. Not a substitute for a licensed electrician. Electrical work can cause serious injury, death, fire, or property damage if performed incorrectly. Always hire a licensed electrician for electrical work, especially for service upgrades, panel work, and 240V circuits. Values are derived from NFPA 70 (NEC) for educational purposes. Always verify against your locally adopted NEC edition and amendments - local jurisdictions may enforce stricter requirements. WireRef provides reference information only and is not responsible for work performed based on this content. NEC® is a registered trademark of the National Fire Protection Association. Free NEC access via NFPA · OSHA Electrical Safety · Terms of use.

NEC 2023 references verified April 2026