Wire Insulation Types: THHN, THWN-2, XHHW and RHH
Compare wire markings, dry and wet temperature ratings, supported fill-area families and the without-outer-covering condition.
A wire marking identifies an insulation construction and rating; it does not supply a conductor gauge or finished outside diameter. Read every designation and suffix on a combined label.
Markings, conditions and fill data
Temperature entries below retain the UL Wire and Cable Marking and Application Guide, Table 1, page 14 reference. They do not verify the site's numeric Table 5 areas.
| Marking | Dry temperature | Wet temperature | Site fill data and chart |
|---|---|---|---|
| THHN | 90°C | No wet rating in this row | THHN-family chart |
| THWN | 75°C | 75°C | THWN chart; shared THHN-family areas |
| THWN-2 | 90°C | 90°C | THWN-2 chart; shared THHN-family areas |
| XHHW / XHHW-2 | Not reproduced here | Not reproduced here | Data status; areas and counts withheld |
| RHH | 90°C | No wet rating in this row | RHH chart; without outer covering only |
| RHW | 75°C | 75°C | RHH-family chart; without outer covering only |
| RHW-2 | 90°C | 90°C | RHH-family chart; without outer covering only |
Read a combined designation
T identifies thermoplastic insulation, HH the 90°C class, N a nylon jacket and W moisture resistance in UL Appendix A, page 30. UL discusses multiple designations on page 11. THHN/THWN retains the THWN wet 75°C rating; THHN/THWN-2 includes the THWN-2 wet 90°C rating. Standalone THWN remains 75°C dry and wet.
The nylon jacket belongs to the insulated-wire envelope. Bare conductor-metal area from an AWG or kcmil conversion is a different quantity. The site maps THHN, THWN and THWN-2 to one area family; equal stored areas do not make temperature ratings interchangeable.
XHHW data remains withheld
XHHW identifies a cross-linked thermoset insulation system, not a conductor material, gauge or conduit size. This site has no verified XHHW/XHHW-2 area dataset. Do not substitute THHN values at the same gauge. Construction, covering and compact-strand status must match the applicable dimension source.
RHH: without outer covering
The RHH, RHW and RHW-2 selections apply without outer covering. Covered conductors are unsupported by these area selections. The Table 5 guide lists supported insulated areas and missing columns; the ampacity guide is a separate lookup.
RELATED TOOLS
Data is transcribed from NEC 2023 Chapter 9 into versioned tables, with calculations performed by code. Automated checks cover selected outputs and consistency; not every cell has been independently reviewed. WireFillChart is not affiliated with or endorsed by NFPA. Verify against the code adopted by your local AHJ.