Conductors for General Wiring

Article 310 answers the everyday question: what wire do I use? It defines insulation types (THHN, THWN-2, XHHW and others), their wet and dry temperature ratings, and the ampacity tables that tie conductor size to allowable current — then adjusts those numbers for ambient temperature and for bundling several current-carrying conductors together.

The heart of the article is heat management. Insulation is the weak link; overload a conductor and its insulation cooks, cracks and eventually arcs. The derating factors for temperature and conductor count exist because heat that can't escape is what actually destroys wiring.

When it applies

Article 310 is where conductors get their ampacity, insulation type, and temperature rating. Any general wiring run leans on these tables. You match the conductor to the load it carries, then correct for how many current-carrying conductors share the raceway and for the ambient temperature around them, because heat is what ultimately limits a wire.

Common mistakes

• Reading ampacity off the highest temperature column when the terminals are only rated for a lower one.
• Skipping the derating for bundled conductors or a hot attic environment.
• Confusing the insulation type letters and using a conductor not rated for a wet or high-heat location.

In the field

I always size to the terminal temperature rating, not the fattest number in the table. The lug is the weak link on heat, and that is the column that governs the connection.