What insulation types are common for conductors used in commercial and residential wiring and rated for 90°C?

• A. Romex NM cable.
• B. PVC insulated.
• C. XHHW only.
• D. THHN/THWN-2 insulated conductors.

Answer: (D)

The key idea is that 90°C-rated conductors used in both commercial and residential wiring are most commonly THHN or THWN-2 insulated conductors. THHN is a thermoplastic insulation designed for dry locations, and THWN-2 adds a wetter rating, giving a true 90°C rating in both dry and wet locations. Because these are individual conductors intended for use in conduits and raceways, they are routinely sized and installed in commercial buildings and in residential settings that use conduit, allowing the higher 90°C ampacity where permitted.

Other options describe cable types or insulation materials that don’t consistently provide the 90°C rating in all required conditions, or aren’t the standard choice for 90°C applications in both environments. NM cable (Romex) is common in homes but its conductors are not the go-to for 90°C ampacity in typical installations, and PVC or XHHW-only choices don’t reflect the widely used, versatile 90°C paired insulation of THHN/THWN-2.