Wire Insulation Types
Each letter in a wire designation tells you something: T = thermoplastic, X = cross-linked polyethylene, H = heat-resistant (75°C), HH = high heat-resistant (90°C), W = wet-rated, N = nylon jacket. Here is every common type:
| Type | Full Name | Dry °C | Wet °C | Locations |
|---|---|---|---|---|
| THHN | Thermoplastic High Heat-resistant Nylon-coated | 90 | - | dry, damp |
| THWN | Thermoplastic Heat-resistant Water-resistant Nylon-coated | 75 | 75 | dry, damp, wet |
| THWN-2 | Thermoplastic Heat-resistant Water-resistant Nylon-coated (90°C wet) | 90 | 90 | dry, damp, wet |
| XHHW | Cross-linked polyethylene High Heat-resistant Water-resistant | 90 | 75 | dry, damp, wet |
| XHHW-2 | Cross-linked polyethylene High Heat-resistant Water-resistant (90°C wet) | 90 | 90 | dry, damp, wet |
| RHH | Rubber High Heat-resistant | 90 | - | dry |
| RHW-2 | Rubber Heat-resistant Water-resistant (90°C wet) | 90 | 90 | dry, damp, wet |
| USE-2 | Underground Service Entrance (90°C) | - | 90 | underground, direct burial |
| TW | Thermoplastic moisture-resistant | 60 | 60 | dry, wet |
| NM-B | Nonmetallic Sheathed Cable (Romex) | 60 | - | dry |
| UF-B | Underground Feeder Cable | 60 | 60 | dry, damp, wet, direct burial |
| MC Cable | Metal Clad Cable | 90 | 75 | dry, damp, wet (if listed) |
| AC Cable (BX) | Armored Cable | 90 | - | dry, damp |
| SER/SEU | Service Entrance Cable (Round/Unarmored) | 75 | 75 | dry, damp, wet |
| PV Wire | Photovoltaic Wire | 90 | 90 | dry, wet, exposed to sunlight |
| THHW | Thermoplastic High Heat-resistant Water-resistant | 90 | 75 | Dry, Wet |
| THW | Thermoplastic Heat-resistant Water-resistant | 75 | 75 | Dry, Wet |
| THW-2 | Thermoplastic Heat-resistant Water-resistant (90C wet) | 90 | 90 | Dry, Wet |
| RHW | Rubber Heat-resistant Water-resistant | 75 | 75 | Dry, Wet |
| MI | Mineral-Insulated (Metal-Sheathed) | 90 | 90 | Dry, Wet, Hazardous, High temperature |
| UF | Underground Feeder (single conductor) | 60 | 60 | Dry, Wet, Direct burial |
| XHH | Cross-linked polyethylene High Heat-resistant | 90 | - | Dry only |
| FEP | Fluorinated Ethylene Propylene (Teflon) | 90 | 90 | Dry, Wet, Plenum |
| SIS | Switchboard Insulation Silicone | 90 | - | Dry only, Switchboard wiring |
Requirements vary by state. NEC edition, licensing, permits, and GFCI rules differ by jurisdiction. Check your state or pick it below for personalized info across WireRef.
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NEC 2023 references verified April 2026