NEC 310.4 · Conductor insulation · 2023 NEC

Wire insulation types explained: THHN, THWN-2, XHHW-2 and more

What the letters mean, how wet/dry and 60/75/90 °C ratings work, where each common type shows up on the job, and how to read NEC Table 310.4(1).

Need the full list? Look up Table 310.4(1) in the free NFPA viewer →

Decoding the letters (our own shorthand)

Letter(s)	Stands for	What it tells you
T	Thermoplastic	Insulation softens with heat (PVC-type)
X	Cross-linked polyethylene (XLP)	Thermoset — doesn't melt; tougher, better in heat and cold
R	Rubber / thermoset	Older designation, now covers modern thermoset compounds
H	Heat-resistant	Rated 75 °C
HH	High heat-resistant	Rated 90 °C (in the conditions named)
W	Wet	Suitable for wet locations
N	Nylon jacket	Thin outer nylon cover; resists abrasion and oil, pulls easier
U / USE	Underground / underground service entrance	Direct burial rated
-2 suffix	Enhanced rating	90 °C in both wet and dry locations

Our own summary of the 2023 NEC, not the code text. Look up 310.4 and Table 310.4(1) in the free NFPA viewer (nfpa.org/freeaccess), check the print on the conductor itself, and verify against the NEC edition adopted by your AHJ.

A few common types, decoded

Marking	Read it as	Typical field use
THHN	Thermoplastic, 90 °C, nylon jacket — dry/damp	Building wire in conduit indoors
THWN-2	Thermoplastic, wet-rated, nylon jacket, 90 °C wet or dry	Same wire, now allowed in underground and outdoor conduit
XHHW-2	XLP, 90 °C, wet-rated at 90 °C	Feeders and services, especially aluminum; cold-weather pulls
USE-2	Underground service-entrance cable, 90 °C	Direct-buried service laterals and PV source circuits outdoors
RHW-2	Thermoset rubber-type, 90 °C wet or dry	Often dual-marked with USE-2 so it can continue inside a building

Examples only — Table 310.4(1) lists many more types and additional details. Most building wire today is dual- or triple-marked (e.g., THHN/THWN-2); the marking on the jacket is what counts.

How to read Table 310.4(1), step by step

Step	What you do	Why it matters
1	Find the type letters printed on the conductor	The jacket print, not the reel tag, is what the inspector checks
2	Read the maximum operating temperature for that type	Sets which 310.16 column you may start from (60, 75, or 90 °C)
3	Read the application column: dry, damp, wet	A dual rating may give a lower temperature in wet locations
4	Check the outer covering / jacket requirement	Some types need a jacket; some have none
5	Confirm the location rule in 310.10 (dry, damp, wet, sunlight, direct burial)	Underground raceways are wet locations
6	Size the conductor from 310.16, then limit by termination temperature (110.14(C))	A 90 °C wire on 75 °C lugs is still used at 75 °C

Which location are you in?

Location	Examples	What the insulation needs
Dry	Conduit inside a heated building	Any general building wire rated for dry
Damp	Protected outdoor areas, some basements	Damp or wet rated
Wet	Underground or slab conduits, outdoors exposed to weather, concrete in contact with earth	A "W" type, and use its wet temperature rating
Direct burial	In the dirt without raceway	A type listed for direct burial (e.g., USE, UF)
Sunlight	Exposed outdoor runs	Sunlight resistant marking, or protection

Common field mistakes

Mistake	What to do instead
Pulling THHN-only wire into an underground conduit	Underground raceways are wet locations — use a W-rated type (THWN-2, XHHW-2)
Using the 90 °C ampacity because the wire is "90 degree"	Terminations and the wet rating can both hold you to 75 °C
Assuming XHHW is 90 °C when wet	Plain XHHW drops to a lower wet rating; XHHW-2 holds 90 °C wet
Running USE-2 inside the building	Not permitted for interior wiring unless it is also marked with a building-wire type like RHW-2
Treating a nicked nylon jacket as harmless on THWN	Damaged jackets can compromise the rating; inspect after rough pulls

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Reading the print on the wire

Look up Table 310.4(1) in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains the letter codes in our own words; it does not reproduce the table.

The letters printed on a conductor are a compact spec sheet. The first letter usually names the insulation material — T for thermoplastic, X for cross-linked polyethylene, R for rubber-type thermoset. H and HH are heat ratings, W means wet-location rated, N is a nylon jacket, and a -2 suffix means 90 °C in wet and dry locations alike.

Why it matters: the insulation’s temperature rating decides which column of Table 310.16 you can start from, and the location rating decides whether you can use the wire at all. An underground conduit is a wet location the day it’s installed — water will get in — so plain THHN doesn’t belong there. Heavily derated runs are where 90 °C insulation earns its keep: you start the correction and adjustment math from the higher 90 °C value, then cap the result at the termination rating.

Example. A reel marked THHN/THWN-2 can go in a slab conduit to an outdoor pedestal, and its 90 °C rating holds wet or dry. A reel marked THHN only cannot. A feeder in XHHW-2 aluminum is a common pick for services and long feeders — it pulls well in cold weather and holds 90 °C wet. USE-2 is for direct burial; if it’s also marked RHW-2 it can continue into the building.

Once you’ve picked a type, size the raceway with the conduit fill chart (insulation type changes the conductor area) and check buried runs against the burial depth chart. Not engineering advice; always read the marking on the actual product.

FAQ

What does THHN stand for?+

Thermoplastic insulation, High-Heat resistant (90 °C), with a Nylon outer jacket. Plain THHN is rated for dry and damp locations. Most wire sold today is dual-marked THHN/THWN-2 so it can also be used in wet locations.

What is the difference between THHN and THWN-2?+

THWN-2 is rated for wet locations at 90 °C; THHN alone is not wet-rated. Because most product is dual-marked THHN/THWN-2, the difference matters mainly when you pick up old stock or a reel marked THHN only for underground or outdoor conduit.

What does the -2 mean on XHHW-2 or THWN-2?+

The -2 suffix means the conductor is rated 90 °C in both dry and wet locations. Without it, many types drop to a lower temperature rating when wet.

Can USE-2 be used inside a building?+

Not by itself — USE is an underground service-entrance type. Most USE-2 on the market is also marked RHH/RHW-2, and that dual marking lets it continue inside as building wire. Check the print on the jacket and Table 310.4(1).

Where do I find the full list of insulation types?+

In NEC Table 310.4(1) (2023 NEC), which lists each type's trade name, temperature rating, allowed locations, insulation, and outer covering. Read it free in the NFPA viewer at nfpa.org/freeaccess.

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