Building Cable

Both are single-conductor building wire with thermoplastic insulation, and the two ratings are frequently combined as THHN/THWN-2 on the same conductor. THHN is rated 90°C in dry locations only; THWN adds a wet-location rating, typically 75°C wet / 90°C dry when dual-rated. Since most modern building wire carries both markings, the practical difference only matters if you find wire rated for just one.

NM-B is primarily permitted in one- and two-family dwellings and some multi-family and commercial applications, but many commercial occupancies and any exposed or damage-prone location require MC cable or conduit-run THHN instead per NEC Article 334. Always check the specific occupancy classification and local amendments before specifying NM-B for commercial work.

Match the conductor's ampacity (from NEC Table 310.16, based on insulation type and conductor material) to the circuit breaker or fuse rating, then apply any required derating for conduit fill, ambient temperature or continuous load. A 20A circuit typically needs 12 AWG copper THHN; a 15A circuit typically uses 14 AWG — but always verify against the applicable code edition and installation conditions rather than gauge alone.

Aluminum conductors are code-compliant and commonly used for larger feeders and services where the gauge upsizing needed to match copper's ampacity is cost-effective, but they require anti-oxidant compound at every termination and connectors specifically rated for aluminum (or AL/CU dual-rated) to prevent loosening from thermal cycling and resulting overheating.

The interlocked metal armor on MC cable provides physical protection against impact, abrasion and rodent damage that a plastic-jacketed NM-B cable doesn't have, which is why MC is permitted in more exposed locations, above suspended ceilings in commercial spaces, and in some cases as an equipment ground path itself, depending on the specific MC cable type and local code.