What does THHN mean in wire?

THHN stands for Thermoplastic High Heat-resistant Nylon-coated. Decoded: PVC insulation, rated 90C in dry locations, wrapped in a thin nylon jacket that shrugs off abrasion and gasoline.

It is the default single conductor for commercial and industrial conduit work, and the slick nylon is why it pulls so easily.

Most modern spools are dual-rated THHN/THWN-2, adding a 90C wet rating; the W is the wet-location license. Without it, plain THHN stays indoors and dry.

Versus Romex: THHN is a loose conductor that must live in a raceway, while Romex is a complete cable assembly that runs bare through framing. Different products for different territories, and the sheathed conductors inside Romex are THHN-family anyway.

The THHN ampacity calculator shows what each size carries at its real temperature rating.