What does the -2 suffix mean on cable markings like THWN-2?

The -2 suffix indicates a 90°C rated insulation system rather than the 75°C base rating. THWN-2 is rated 90°C in both wet and dry locations, while THWN without the -2 is only 75°C wet / 90°C dry. In NEC Table 310.16, you use the 75°C ampacity column for THWN-2 in most practical circuits (because most equipment terminals are only rated to 75°C), even though the insulation is physically rated to 90°C.

UL Cable Marking Explained: How to Read the Print Legend

Q: Is cable without UL markings illegal to use?

In most US jurisdictions, NEC 110.3(B) requires that all electrical equipment be used in accordance with its listing and labeling. Wire and cable installed in permanent building wiring is generally required to be listed by a nationally recognized testing laboratory (NRTL). Unlisted cable may be allowed in some industrial installations under special circumstances with the authority having jurisdiction (AHJ) approval, but it cannot be used in standard residential or commercial wiring without prior AHJ authorization.

Q: What does 'AWM' mean on a wire?

AWM stands for Appliance Wiring Material. It is a UL designation (covered under UL 758) for wires intended for use inside the enclosures of appliances, electronic equipment, and industrial machinery — not for building wiring in conduit or raceway. AWM wire has a UL style number (e.g., AWM Style 1007, 1015, 2651) that specifies its voltage, temperature, and environmental ratings. It is NOT the same as UL Listed building wire and cannot substitute for THHN or THWN in conduit wiring.

Why Cable Markings Matter

NEC 310.120 requires conductors 6 AWG and smaller to be durably marked at intervals not exceeding 24 inches with the AWG size, voltage rating, and insulation type. This isn't just housekeeping: installers, inspectors, and maintenance technicians rely on the print legend to verify that the cable installed in a raceway, conduit, or wall cavity actually meets the specification for that circuit's voltage, temperature, and location rating.

Getting the marking wrong during procurement — ordering cable that looks similar on a price sheet but has different ratings — is one of the most common ways incorrect material enters a project. Understanding the legend means you can verify compliance at the reel before it's pulled.

Anatomy of a THHN/THWN-2 Print Legend

A typical print legend looks like:

THHN/THWN-2 600V (UL) AWG 12 COPPER

Breaking this down letter by letter:

The Type Code: T-H-H-N

Letter	Meaning
T	Thermoplastic insulation
H	Heat-rated (75°C)
HH	Double H = 90°C heat rating
W	Rated for wet locations
N	Nylon outer jacket

So THHN = thermoplastic, heat (90°C), nylon jacket. THWN-2 = thermoplastic, heat (75°C wet / 90°C dry), wet location rated, nylon jacket, -2 suffix indicating 90°C wet rating. Most modern wire is dual-listed THHN/THWN-2 because the insulation meets both standards simultaneously.

Common Cable Type Codes

Type Code	Full Name	Max Temp	Location
THHN	Thermoplastic, Heat, Nylon	90°C dry / 75°C wet	Dry / damp
THWN-2	Thermoplastic, Heat-Wet, Nylon, 90°C	90°C wet & dry	Wet / dry
XHHW-2	XLPE insulation, Heat-Wet, 90°C	90°C wet & dry	Wet / dry
NM-B	Non-Metallic Sheathed Cable (Romex)	60°C	Dry only (inside walls)
UF-B	Underground Feeder Cable	60°C	Direct burial
USE-2	Underground Service Entrance, 90°C	90°C	Direct burial, wet
MC	Metal-Clad Cable (NEC Art. 330)	Depends on conductors	Exposed, concealed

Voltage Rating

The voltage rating printed on the cable is the maximum voltage the insulation is designed to withstand continuously. Common ratings are:

300V — control cable, low-voltage fixtures, appliance wiring
600V — standard building wire (THHN, THWN), tray cable, most commercial wiring
1kV (1000V) — medium-voltage distribution cable, some specialty constructions
5kV / 15kV / 35kV — medium-voltage power cable with thick insulation systems

The voltage rating does not mean you should run that voltage through the wire. It means the insulation safely withstands that voltage. A 600V-rated wire in a 120V circuit has substantial safety margin.

UL Listed vs UL Recognized vs UL Certified

UL Listed — the complete product (wire as used by the installer) has been tested and found to comply with UL standards. Building wire should always be UL Listed. The mark is the round UL logo on the jacket.
UL Recognized — a component intended to be incorporated into a larger listed product. AWM wire is typically UL Recognized, not Listed. The mark is a backward UR logo. Recognized wire cannot substitute for Listed building wire.
UL Certified — a newer program for certain product categories. For standard wire and cable, "Listed" remains the relevant status.

Frequently Asked Questions

What does the -2 suffix mean on cable markings?

The -2 suffix indicates a 90°C rated insulation system in both wet and dry locations. THWN-2 is rated 90°C wet / 90°C dry, while THWN without the -2 is only 75°C wet / 90°C dry. In practice you'll use the 75°C ampacity column from NEC Table 310.16 because most equipment terminals are only rated to 75°C.

Is cable without UL markings illegal to use?

In most US jurisdictions, NEC 110.3(B) requires equipment to be used in accordance with its listing. Wire for permanent building wiring must be listed by an NRTL. Unlisted cable can be used in some industrial applications with AHJ approval, but cannot substitute for listed wire in standard residential or commercial wiring.

What does AWM mean on a wire?

AWM stands for Appliance Wiring Material (UL 758). It is a UL Recognized designation for wires used inside equipment enclosures — not for building wiring in conduit. AWM wire carries a UL style number that specifies its ratings. It is NOT equivalent to UL Listed building wire and cannot substitute for THHN or THWN in conduit installations.