In wire classification, what does a single H indicate?

• A. Heat resistance up to 167°F
• B. High-current rating
• C. High-heat tolerance with unspecified rating
• D. Heat-shrink insulation

Answer: (A)

A single H on wire insulation denotes its temperature tolerance. Specifically, it means the insulation is rated to withstand heat up to 167°F (75°C) without degrading. This rating tells you the maximum operating temperature the wire’s insulation is designed to handle safely. If the code had two H’s, that would imply a higher temperature rating, but a single H stays at that 75°C/167°F level. It isn’t about how much current the wire can carry—that depends on conductor size and overall design—nor is it about heat-shrink features, which relate to a different aspect of the insulation.