THHN Wire

Q: What does THHN stand for?

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. The "T" means thermoplastic insulation (PVC), "HH" means high heat-resistant (rated 90°C), and "N" indicates a nylon jacket over the insulation for physical protection and lubrication when pulling through conduit.

Q: Is THHN the same as THWN-2?

Most modern THHN wire is dual-rated THHN/THWN-2, meaning it meets both specifications. THWN-2 adds a "W" for wet locations and "2" for 90°C wet rating. When you buy THHN at a supply house, it is almost always stamped THHN/THWN-2 on the jacket. For practical purposes they are the same product.

Q: Can I use THHN outdoors?

Yes — if it is in conduit. THHN/THWN-2 is rated for wet locations when pulled through conduit. It is not rated for direct burial without conduit. For underground runs without conduit, use UF-B cable or USE-2 individual conductors.

Q: What is the ampacity of 12 AWG THHN copper?

12 AWG THHN copper has a 90°C ampacity of 30A per Table 310.16. However, the usable ampacity is typically 25A due to the 75°C termination temperature limit per 110.14(C). Most residential breakers and devices are rated 75°C. See our 12 AWG copper ampacity page for the full derating chain.

NEC Quick Answer

The maximum voltage rating for THHN and THWN conductors is 600V. THHN is rated 90C for dry and damp locations. THWN is rated 75C for wet locations. The dual-rated THHN/THWN-2 conductor is the most common type installed today, rated 90C dry and 90C wet, with a 600V maximum.

NEC 2023: Table 310.4(1)

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. Most common building wire for conduit/raceway. Usually dual-rated THHN/THWN-2.

Dry rating90°C

Voltage600V

Locationsdry, damp

Specifications

Property	Value
Full name	Thermoplastic High Heat-resistant Nylon-coated
Insulation material	PVC (thermoplastic)
Outer covering	Nylon jacket
Temperature rating (dry)	90°C (194°F)
Voltage rating	600V
Approved locations	dry, damp

Advantages and limitations

Advantages: Thin insulation = more conduit fill capacity. Easy to pull. Inexpensive.

Limitations: Not rated for wet alone. PVC can stiffen below -10°C.

Ampacity by gauge

The table below shows NEC Table 310.16 ampacity for THHN conductors. Values shown reflect the 110.14(C) termination temperature limit, which determines the real-world usable ampacity - not just the insulation rating. Click any gauge to see the full derating chain.

Gauge	Copper	Aluminum
14 AWG	See page	-
12 AWG	See page	See page
10 AWG	See page	See page
8 AWG	See page	See page
6 AWG	See page	See page
4 AWG	See page	See page
3 AWG	See page	See page
2 AWG	See page	See page
1 AWG	See page	See page
1/0 AWG	See page	See page
2/0 AWG	See page	See page
3/0 AWG	See page	See page
4/0 AWG	See page	See page

When to use THHN wire

Most common building wire for conduit/raceway. Usually dual-rated THHN/THWN-2.

Choose THHN when:

You're pulling wire through conduit (EMT, PVC, or rigid) - THHN is the standard pull wire
The run is exposed or in a commercial building where cable isn't permitted
You need individual conductors for a subpanel feeder in conduit
The installation is outdoors in conduit (the THWN-2 dual rating covers wet locations)

Don't use THHN when:

Direct burial without conduit - THHN is not rated for direct burial even though THWN-2 is wet-rated
Exposed runs without raceway - individual THHN conductors must be in conduit or raceway
As a substitute for cable in residential walls - use NM-B for concealed residential wiring

Common applications for THHN:

Residential and commercial branch circuits in conduit - THHN is the most commonly pulled individual conductor in metallic and plastic raceways
Panel-to-subpanel feeders - individual conductors in conduit are often more economical than cable for larger gauges
Service entrance wiring - used in rigid conduit from meter base to main panel
Outdoor and underground in conduit - the "W" designation (THWN-2) means wet-rated for conduit exposed to moisture

NEC code references for THHN

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 THHN 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 THHN wire

What does THHN stand for?

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. The "T" means thermoplastic insulation (PVC), "HH" means high heat-resistant (rated 90°C), and "N" indicates a nylon jacket over the insulation for physical protection and lubrication when pulling through conduit.

Is THHN the same as THWN-2?

Most modern THHN wire is dual-rated THHN/THWN-2, meaning it meets both specifications. THWN-2 adds a "W" for wet locations and "2" for 90°C wet rating. When you buy THHN at a supply house, it is almost always stamped THHN/THWN-2 on the jacket. For practical purposes they are the same product.

Can I use THHN outdoors?

Yes - if it is in conduit. THHN/THWN-2 is rated for wet locations when pulled through conduit. It is not rated for direct burial without conduit. For underground runs without conduit, use UF-B cable or USE-2 individual conductors.

What is the ampacity of 12 AWG THHN copper?

12 AWG THHN copper has a 90°C ampacity of 30A per Table 310.16. However, the usable ampacity is typically 25A due to the 75°C termination temperature limit per 110.14(C). Most residential breakers and devices are rated 75°C. See our 12 AWG copper ampacity page for the full derating chain.

Other wire insulation types

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 THHW 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

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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