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, 75 °C wet | Yes (dual rated as THWN-2) | Most common in conduit; residential branch circuits |
| THWN-2 | 90 °C wet/dry | Yes | Modern equivalent of THHN+THWN |
| 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) | 60 °C wet/dry | 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 - service entrance only in modern installs (banned for branch in residential); requires special connectors (COPALUM, AlumiConn) where it joins copper
- 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 THHN-2 vs THWN-2: modern wire is typically marked DUAL - THHN/THWN-2 means it has both ratings. The "2" indicates the post-2008 standard. Either suffix lets you use it in dry conduit (THHN function) or wet conduit (THWN function).
- 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