The insulation type printed on a conductor decides three things: how hot it may run, whether it may get wet, and which ampacity column applies to it. Here is every common type side by side, with a full page for each.
| Type | Dry rating | Wet rating | Ampacity column | Installation | Direct burial | Typical use |
|---|---|---|---|---|---|---|
| THHN / THWN-2 | 90°C | 90°C (THWN-2) | 75°C | Raceway required | No | Conduit branch circuits and feeders |
| NM-B (Romex) | 90°C | Not permitted | 60°C | Exposed / concealed indoors | No | Residential branch circuits |
| XHHW / XHHW-2 | 90°C | 90°C (XHHW-2) | 75°C | Raceway required | No | Feeders, services, large aluminum |
| USE-2 | 90°C | 90°C | 75°C | Direct burial | Yes | Underground services and feeders |
| UF-B | 90°C | Yes | 60°C | Direct burial | Yes | Outdoor branch circuits |
| SOOW / SJOOW | 90°C | Yes (W rating) | Table 400.5 | Portable only | No | Tools, temporary power, machine drops |
| Welding cable | 105°C | Jacket dependent | Manufacturer | Not NEC building wire | No | Welding leads, battery and inverter runs |
| Speaker wire | n/a | Burial-rated types only | n/a | CL2 / CL3 in wall | Rated types only | Loudspeaker runs, Class 2 circuits |
The ampacity column is the one that catches people out. NM-B and UF-B contain 90°C conductors but are restricted to the 60°C column by NEC 334.80 and 340.80 respectively, which is why 6 AWG Romex will not carry a 60 A load while 6 AWG THHN in conduit will.
THHN/THWN-2 is a 600 V building wire rated 90°C in dry and wet locations, with PVC insulation and a nylon jacket. It must be run in conduit or a …
Romex (NM-B) ampacity comes from the 60°C column: 14 AWG is 15 A, 12 AWG is 20 A, 10 AWG is 30 A, 6 AWG is 55 A. Full size chart with sheath colo…
XHHW-2 is a 600 V building wire with cross-linked polyethylene insulation rated 90°C wet and dry. Full ampacity table, how it differs from THHN, …
USE-2 is a 90°C wet-rated single conductor listed for direct burial without a raceway. It has no flame rating, so it may not be run inside a buil…
UF-B is grey direct-burial branch circuit cable rated 60°C for ampacity: 14 AWG is 15 A, 12 AWG is 20 A, 10 AWG is 30 A. Burial depth requirement…
SOOW is 600 V oil-resistant, weather-resistant portable cord; SJOOW is the 300 V junior version. Ampacity from NEC Table 400.5, what every letter…
Welding cable is 600 V, 105°C, extremely fine-stranded EPDM cable built for flexibility, not for building wiring. Ampacity by duty cycle, Class K…
Speaker wire is unshielded low-voltage cable in 12 to 18 AWG. Which gauge for your run length and impedance, what CL2 and CL3 in-wall ratings mea…
Wire type designations are built from a fixed set of letters. Once you know them, an unfamiliar marking parses on sight:
So THWN-2 reads as thermoplastic, heat resistant, wet rated, nylon jacketed, 90°C wet. XHHW-2 is cross-linked, high heat resistant, wet rated, 90°C wet. USE-2 is underground service entrance, 90°C wet.
THHN is a single insulated conductor that must be pulled through conduit. Romex (NM-B) is a complete cable assembly with two or three insulated conductors and a bare ground inside a sheath, run exposed or concealed in dry indoor locations. THHN uses the 75°C ampacity column in practice; NM-B is restricted to the 60°C column.
USE-2 for service and feeder conductors, and UF-B for branch circuits. Both are listed for direct burial without a raceway. THHN, THWN-2, XHHW-2 and NM-B may not be direct buried — the first three require a raceway and NM-B is prohibited underground entirely.
NM-B and UF-B are restricted to the 60°C column by NEC 334.80 and 340.80. THHN, THWN-2, XHHW-2 and USE-2 use the 90°C column as the derating basis, but are limited to 75°C at the terminations by NEC 110.14(C). Flexible cords use NEC Table 400.5 instead of Table 310.16 entirely.
T is thermoplastic, H is heat resistant to 75°C, HH is high heat resistant to 90°C, W is wet location rated, N is a nylon jacket, X is cross-linked polyethylene, and a -2 suffix means 90°C in wet locations as well as dry. So THWN-2 is thermoplastic, high heat, wet rated, nylon jacketed, 90°C wet.