What is the difference between THHN and THWN?

THHN has a 90 °C dry and 75 °C wet rating; THWN is 75 °C in both. In practice most wire sold as THHN is dual-rated THHN/THWN-2, which holds 90 °C in dry and wet conditions — so the distinction has largely disappeared in new stock.

Can THHN be used in a wet location?

Yes, but only at its 75 °C ampacity. The insulation permits the location; the temperature rating that applies drops. Where the conductor will be wet and loaded near its 90 °C ampacity, XHHW-2 or THWN-2 is the correct choice.

Why does the termination rating matter?

Because ampacity is limited by the lowest-rated component in the circuit. A 90 °C conductor on a breaker marked 75 °C is limited to the 75 °C ampacity. Most standard terminations are rated 75 °C, which is why 90 °C wire rarely delivers its full ampacity in practice.

Can I use MTW wire in a building circuit?

Only where its listing permits it. MTW is machine-tool wire, and its ampacity comes from the machine-tool table rather than the building-wire table. Many MTW conductors are also dual- or triple-rated as THHN/THWN, in which case the building-wire ratings apply.

What wire should I use underground?

A conductor rated for wet locations at the temperature you need — USE-2 for direct burial, or XHHW-2, RHW-2 or THWN-2 inside conduit. A buried conduit is a wet location, so dry-rated conductors are not permitted even inside a raceway.

Wire Insulation Types

Twelve common wire insulation types with their dry and wet temperature ratings, voltage rating and the locations each may be used in.

Data verified 2026-09-29 · based on NEC 2023 (NFPA 70-2023)

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Wire Insulation Types and Ratings

Type^[1]	Full name^[2]	Temperature rating^[1]	Voltage^[1]	Permitted locations^[1]	Notes^[1]
THHN #	Thermoplastic High Heat-resistant Nylon-coated	90 °C dry / 75 °C wet	600 V	Dry and damp locations in conduit	The most common building wire. The nylon jacket resists abrasion and gasoline. In wet locations it is limited to 75 °C, which is why THWN-2 is often specified instead.
THWN #	Thermoplastic Heat and Water-resistant Nylon-coated	75 °C dry / 75 °C wet	600 V	Wet and damp locations	The older wet-rated version. Superseded in practice by THWN-2, which keeps the 90 °C dry rating.
THWN-2 #	Thermoplastic Heat and Water-resistant Nylon-coated, 90 °C	90 °C dry / 90 °C wet	600 V	Wet, damp and dry locations	THHN and THWN combined — 90 °C in both dry and wet conditions. This is what most modern THHN is actually dual-rated as, and it is the safe default.
THW #	Thermoplastic Heat and Water-resistant	75 °C dry / 75 °C wet	600 V	Wet and dry locations	Older and thicker than THHN, with no nylon jacket. Still found in existing installations.
THHW #	Thermoplastic Heat and Water-resistant, Heat-resistant	90 °C dry / 75 °C wet	600 V	Wet and dry locations	A heat-resistant version of THW.
XHHW #	Cross-linked polyethylene High Heat and Water-resistant	90 °C dry / 75 °C wet	600 V	Wet, damp and dry locations	Cross-linked polyethylene insulation, physically tougher than thermoplastic and with better moisture resistance. Common in larger sizes.
XHHW-2 #	Cross-linked polyethylene, 90 °C wet	90 °C dry / 90 °C wet	600 V	Wet, damp and dry locations	XHHW with a 90 °C wet rating. Preferred over THHN where the conductor will be wet and loaded near its ampacity.
RHW-2 #	Rubber Heat and Water-resistant, 90 °C	90 °C dry / 90 °C wet	600 V	Wet, damp and dry locations	Thermoset insulation, generally used in larger sizes and in underground conduit.
USE-2 #	Underground Service Entrance	90 °C	600 V	Direct burial and wet locations	Rated for direct burial without conduit. Often dual-rated as RHW-2/USE-2. Not for use in a building's interior unless also marked.
MTW #	Machine Tool Wire	60/75/90 °C depending on size	600 V	Machine tool and appliance wiring	Flexible stranded conductor for panel and machine wiring. Its ampacity comes from the machine-tool table, not the building-wire table.
TFFN #	Thermoplastic Flexible Fixture Wire, Nylon-coated	60/75/90 °C	600 V	Fixture and control wiring	Small fixture wire in 16 and 18 AWG. Used for control circuits and lighting fixtures.
RHH #	Rubber Heat-resistant	90 °C dry / 75 °C wet	600 V	Dry and wet locations	Thermoset, generally used in larger sizes.

The temperature rating that applies is the one for the condition the conductor is actually in. A THHN conductor is a 90 °C conductor in dry conditions and a 75 °C conductor in wet ones, so its ampacity in a wet location comes from the 75 °C column of the ampacity table — a real derating, not a technicality.

Conductor ampacity is limited by the lowest of three things: the insulation's temperature rating for the condition, the terminations' rating (most are 60 °C or 75 °C), and any adjustment for ambient temperature or conductor count. In practice the termination rating is often the binding constraint, which is why 90 °C wire on a 75 °C breaker is still limited to the 75 °C ampacity.

MTW is rated to 90 °C but its ampacity comes from the machine-tool table rather than the building-wire table, and the two are not interchangeable. Using building-wire ampacity for machine-tool wire — or the reverse — is a common error in panel work.

Dry, Damp and Wet Locations

The NEC distinguishes three conditions, and the distinction drives the temperature rating that applies.

Dry — no moisture contact. Most indoor wiring in heated space.

Damp — some moisture but not saturating, such as a basement or an outdoor location under a canopy. Rated conductors are required, but the wet rating does not apply.

Wet — in direct contact with water or earth, including underground conduit, outdoors exposed, and concrete slabs in contact with the ground. The wet temperature rating applies, and only conductors rated for wet locations may be used.

A buried conduit is a wet location regardless of how well it is sealed — groundwater eventually enters any buried raceway. That single fact is why direct-burial and wet-rated conductors exist, and why using dry-rated wire in an underground conduit fails inspection.

Frequently Asked Questions

What is the difference between THHN and THWN?: THHN has a 90 °C dry and 75 °C wet rating; THWN is 75 °C in both. In practice most wire sold as THHN is dual-rated THHN/THWN-2, which holds 90 °C in dry and wet conditions — so the distinction has largely disappeared in new stock.
Can THHN be used in a wet location?: Yes, but only at its 75 °C ampacity. The insulation permits the location; the temperature rating that applies drops. Where the conductor will be wet and loaded near its 90 °C ampacity, XHHW-2 or THWN-2 is the correct choice.
Why does the termination rating matter?: Because ampacity is limited by the lowest-rated component in the circuit. A 90 °C conductor on a breaker marked 75 °C is limited to the 75 °C ampacity. Most standard terminations are rated 75 °C, which is why 90 °C wire rarely delivers its full ampacity in practice.
Can I use MTW wire in a building circuit?: Only where its listing permits it. MTW is machine-tool wire, and its ampacity comes from the machine-tool table rather than the building-wire table. Many MTW conductors are also dual- or triple-rated as THHN/THWN, in which case the building-wire ratings apply.
What wire should I use underground?: A conductor rated for wet locations at the temperature you need — USE-2 for direct burial, or XHHW-2, RHW-2 or THWN-2 inside conduit. A buried conduit is a wet location, so dry-rated conductors are not permitted even inside a raceway.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	NEC Article 310 — Conductors for General Wiring	standard	NEC 2023 (NFPA 70-2023)
[2]	UL 83 — Thermoplastic-Insulated Wires and Cables	standard	n/a — UL does not year-stamp UL 83 in its designation; type designations are as currently published

Data Sources

Standard	Revision	What it covers on this page
NEC Article 310 — Conductors for General Wiring	NEC 2023 (NFPA 70-2023)	the temperature ratings and permitted locations
UL 83 — Thermoplastic-Insulated Wires and Cables	n/a — UL does not year-stamp UL 83; designations as currently published	the insulation type designations
NEC Article 100 — Definitions of dry, damp and wet locations	NEC 2023 (NFPA 70-2023)	the location definitions that select the rating

Cross-checked against:

Temperature ratings cross-checked against both the NEC conductor table and the UL 83 type designations
The dual-rated THHN/THWN-2 convention verified against manufacturers' published listings
Location definitions taken from NEC Article 100 rather than summarised from secondary sources

Type designations and ratings apply to the conductor as listed; a manufacturer's specific product may carry additional ratings marked on the insulation. Ampacity is limited by the lowest of the insulation rating for the condition, the termination rating, and any adjustment factors — see the ampacity chart.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

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Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Wire Insulation Types." https://refmanual.com/charts/wire-insulation-types/ (accessed 2026-09-29). Data from NEC Article 310 — Conductors for General Wiring, NEC 2023 (NFPA 70-2023).

BibTeX

@misc{wire-insulation-types,
  title        = {Wire Insulation Types},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/wire-insulation-types/}},
  note         = {Accessed: 2026-09-29},
  source       = {NEC Article 310 — Conductors for General Wiring, NEC 2023 (NFPA 70-2023)}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Wire Insulation Types
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/wire-insulation-types/
PY  - 2026
DA  - 2026-09-29
N1  - Source: NEC Article 310 — Conductors for General Wiring, NEC 2023 (NFPA 70-2023)
ER  -

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