CWEA Electrical/Instrumentation (E/I) Grade 1 Practice Test 2026 - Free Electrical/Instrumentation Practice Questions and Study Guide

Ampacity is the maximum current a conductor can carry safely, based on its gauge and insulation rating. For copper #12 with THWN insulation, the NEC ampacity table uses the 75°C rating, which shows 25 amperes. That means the wire can carry up to 25 A under standard conditions. The 60°C column would be 20 A, and higher-temperature ratings (like THWN-2 at 90°C) would only apply if that insulation is used, along with appropriate conditions. In typical practice, you’ll see #12 circuits protected by 20 A breakers, even though the conductor’s rating allows up to 25 A.