Wire Insulation Types Reference
Identification
Wire insulation types determine where + how a conductor can be installed. Wrong insulation in wet location = degraded over time + future shorts. Wrong insulation at high temp = melted + fire risk. This is the field card for wire identification - read the insulation print BEFORE installing.
Specifications
Most common residential / light commercial:
| Insulation type | Temp rating | Wet location? | Common use |
|---|---|---|---|
| THHN | 90 °C dry only | No, not on its own | Most common in conduit; nearly always sold dual-printed as THHN/THWN-2 |
| THWN-2 | 90 °C wet and dry | Yes | What the dual-printed spool actually gives you in a wet raceway |
| NM-B (Romex) | 90 °C dry, but circuit ampacity uses 60 °C | No (dry interior only) | Standard residential wiring in walls |
| UF-B (underground feeder) | 90 °C conductor, but circuit ampacity uses 60 °C | Yes, direct burial | Underground branch circuits |
| USE-2 | 90 °C wet/dry | Yes | Underground service entrance |
| SE / SER (service entrance) | 75 °C usually | Damp locations | Service entrance cables |
| MTW | 90 °C dry | Limited | Machine tool wiring |
| XHHW-2 | 90 °C wet/dry | Yes | Conduit installations, replacing THHN in some applications |
| PV (photovoltaic) | 90 - 105 °C wet | Yes | Solar installations |
| MC (metal-clad) cable | 90 °C dry typically | Limited | Commercial + some residential |
Materials
Conductor (the metal):
- Copper - standard; higher conductivity, no special connector requirements
- Aluminum - in practice, service entrance and large feeders. The NEC does not ban aluminum branch circuits outright; what failed was the AA-1350 alloy used in 12 and 10 AWG branch wiring in the late 1960s and early 1970s, which crept and oxidized at terminations and started fires. Modern conductors use the AA-8000 series alloy, and small-gauge aluminum branch wire has effectively left the residential market anyway. Some jurisdictions restrict it further, so check locally. Legacy AA-1350 branch wiring joined to copper requires COPALUM or AlumiConn; a standard wire nut on that joint is a fire hazard
- Tinned copper - copper plated with tin; common in older homes (1950s) + marine environments
Performance
Insulation color coding (separate from conductor function - see Wire Color Code Reference for that):
The OUTER jacket of NM-B / Romex is color-coded by gauge:
- 14 AWG (15A circuits): WHITE jacket
- 12 AWG (20A circuits): YELLOW jacket
- 10 AWG (30A circuits): ORANGE jacket
- 8 AWG (40A circuits): BLACK jacket (in newer cable; older may differ)
- 6 AWG (50A or larger circuits): BLACK jacket
Most other insulations don't have outer-jacket color conventions; the function (hot/neutral/ground) is identified by the INNER conductor color.
Safety
NM-B (Romex) is NOT rated for wet or damp locations. Installing Romex in a basement near a damp wall, in a crawl space exposed to moisture, OR underground = code violation + insulation degradation over years. UF cable IS rated for damp/wet/underground. The "looks the same to the customer" doesn't matter - these are different listings with different lifespans in different environments.
Notes
- THHN vs THWN vs THWN-2: the letters are the whole spec. T is thermoplastic, H is heat resistant (75 °C), HH is high heat resistant (90 °C), W means approved for wet locations, and N is a nylon outer jacket. So THHN is 90 °C dry only, THWN is 75 °C wet, and the trailing "2" means 90 °C in wet locations as well as dry. It is a temperature rating, not a revision year. Modern wire is almost always printed dual, THHN/THWN-2, which gives you 90 °C in both conditions, and that is what you want in any conduit that could ever see water. Read the print rather than assuming; a spool marked THHN only is a dry-location conductor.
- Reading the print on the insulation: every legitimate wire prints its specifications periodically along the length. If you can't find printing on a wire, it's either very old (replace) or counterfeit (rare but possible - verify supply chain).
- Re-using existing wire: in a renovation, existing wire in good condition can be re-purposed if the insulation rating supports the new use. Check the printing; verify the gauge supports the new circuit's amperage; visually inspect for insulation damage.
- Knob-and-tube wiring: pre-1950 individual conductors with cloth + asphalt insulation. NOT compatible with modern boxes or connectors. Insurance companies often refuse to insure homes with active knob-and-tube; remediation = re-pipe = significant quote.
References
- NEC Article 310 (Conductors for General Wiring)
- UL Wire + Cable Marking Standards
- Manuall internal: Conductor Ampacity Reference