Example: #12 THHN (90°C) rated 30A at 30°C ambient. At 45°C ambient → 30A × 0.82 = .
But what happens when you run ten of those wires through a hot attic in the middle of July? wire derating
The default current-carrying capacity () of a wire is calculated for ideal conditions—usually a single cable in open air. In reality, several factors hinder heat dissipation: Example: #12 THHN (90°C) rated 30A at 30°C ambient
| Ambient Temp (°C) | 60°C wire | 75°C wire | 90°C wire | |------------------|-----------|-----------|-----------| | 21–25 | 1.00 | 1.00 | 1.00 | | 26–30 | 0.91 | 0.94 | 0.96 | | 31–35 | 0.82 | 0.88 | 0.91 | | 36–40 | 0.71 | 0.82 | 0.87 | | 41–45 | 0.58 | 0.75 | 0.82 | | 46–50 | 0.41 | 0.67 | 0.76 | The default current-carrying capacity () of a wire