However, you are allowed to use the 90°C rating for the derating calculation itself (to get a higher starting number before applying the penalties), provided the final result does not exceed the termination temperature rating.
Excessive heat turns flexible insulation brittle, causing it to crack and expose live wires. wire derating
more than three current-carrying conductors in a single conduit or bundle. Number of Conductors Adjustment Factor (Multiplier) 4–6 80% 7–9 70% 10–20 50% 21–30 45% Pro Tip: Grounding wires typically don't count as current-carrying conductors, but neutral wires often do if they carry unbalanced loads! How to Handle It If your project requires a lot of wires in one run, you have two main choices: Upsize the Wire: Use a thicker gauge (like jumping from #12 to #10 AWG) to lower resistance and heat. Split the Run: Use two separate conduits so you don't exceed the three-conductor threshold in each. Ignoring derating is one of the most common ways to fail an inspection—and more importantly, it's a major safety risk. Stay cool, stay compliant! Are you planning a project with However, you are allowed to use the 90°C
(or ampacity adjustment ) is the practice of reducing the maximum current a wire can safely carry based on environmental and installation factors. Its primary purpose is to prevent overheating , which can damage insulation and lead to electrical fires. 💡 Core Concept: Why Derate? Ignoring derating is one of the most common
| Number of conductors | Derate factor | |----------------------|---------------| | 4–6 | 0.80 | | 7–9 | 0.70 | | 10–20 | 0.50 | | 21–30 | 0.45 | | 31–40 | 0.40 |
Example: #12 THHN (90°C) rated 30A at 30°C ambient. At 45°C ambient → 30A × 0.82 = .
We look at NEC Table 310.16.