What is RHH and RHW Wire?
RHH, RHW, and RHW-2 are rubber-insulated building wires used in branch circuits and feeders. NEC Article 310 and Table 5 govern dimensions and ampacity.
RHH and RHW are rubber-insulated single conductors used for branch-circuit and feeder wiring. RHH stands for Rubber, Heat-resistant (90°C dry). RHW adds water resistance — Rubber, Heat- and Water-resistant — rated 75°C wet and 90°C dry. The modern RHW-2 is rated 90°C in both wet and dry locations. All three are listed in NEC Table 310.4(A) and sized using NEC Chapter 9 Table 5.
Construction
- Conductor: Soft-drawn copper, solid or stranded
- Insulation: Cross-linked synthetic rubber (or thermoset elastomer)
- Jacket: Optional moisture- and flame-resistant outer covering on the W variants
- Temperature ratings:
- RHH — 90°C dry only
- RHW — 75°C wet / 90°C dry
- RHW-2 — 90°C wet and dry
The defining feature of the RH family is its thick rubber insulation, which makes the finished conductor noticeably larger than a comparable THHN/THWN-2 of the same AWG.
Cross-sectional area (NEC Chapter 9 Table 5)
This is the practical takeaway for conduit fill calculations — RHW-2 takes up far more space:
| AWG | RHW-2 area in² | THHN area in² | Ratio |
|---|---|---|---|
| 14 | 0.0209 | 0.0097 | 2.15× |
| 12 | 0.0260 | 0.0133 | 1.96× |
| 10 | 0.0333 | 0.0211 | 1.58× |
| 8 | 0.0556 | 0.0366 | 1.52× |
| 6 | 0.0726 | 0.0507 | 1.43× |
| 4 | 0.0973 | 0.0824 | 1.18× |
| 2 | 0.1333 | 0.1158 | 1.15× |
| 1/0 | 0.2026 | 0.1825 | 1.11× |
A 1" EMT (interior area 0.864 in²) holds 9 of 12 AWG THHN at 40% fill, but only 6 of 12 AWG RHW-2. Always cross-check on the wire fill chart when substituting insulations.
NEC code reference
- NEC 310.4(A) — conductor insulation type listings
- NEC Chapter 9 Table 5 — actual dimensions for fill math
- NEC Chapter 9 Table 8 — conductor properties (DC resistance, area)
- NEC Table 310.16 — ampacity at 60/75/90°C
Ampacity termination is still limited by the 75°C column for most equipment under 100 A (NEC 110.14(C)) — the 90°C rating is used only for ambient and derating corrections.
Common applications
- Service entrance conductors (RHW, RHW-2)
- Branch circuits and feeders in damp or wet locations
- Industrial control wiring where rubber's flexibility helps in cold weather
- Mining and quarry installations (rubber survives abrasion)
- Replacement in older buildings already wired with rubber-insulated conductors
Trade-offs vs THHN/THWN-2
| Property | RHW-2 | THHN/THWN-2 |
|---|---|---|
| Insulation | Thermoset rubber | Thermoplastic (PVC + nylon) |
| Cold flexibility | Excellent | Stiffens below freezing |
| Outer diameter | Large | Compact |
| Cost per foot | Higher | Lower |
| Fill efficiency | Poor | Good |
For most new commercial work, THHN/THWN-2 wins on cost and fill. RH-family wire is chosen when flexibility at low temperatures or specific listings (mine power cable, industrial heat exposure) are required.
Quick reference
| Type | Wet rating | Dry rating | Use |
|---|---|---|---|
| RHH | — | 90°C | Dry only |
| RHW | 75°C | 90°C | Wet/dry, 75°C wet |
| RHW-2 | 90°C | 90°C | Wet/dry, full 90°C |
Related
FAQ
RELATED TOOLS
This page's figures are transcribed from NEC 2023 (NFPA 70) Chapter 9, Tables 1, 4 and 5, and checked by a version-controlled automated test suite — they are not signed off by a licensed professional. WireFillChart is an independent reference, not affiliated with or endorsed by NFPA. Always verify against the code edition adopted by your local authority having jurisdiction (AHJ).