Wire Derating 【Ultimate】

This is a concise guide to — why it’s needed, key factors, common rules of thumb, and how to apply them. 1. What Is Wire Derating? Derating means reducing the allowable current-carrying capacity (ampacity) of a wire below its standard-rated value because of environmental or installation conditions . Goal: Prevent overheating and insulation damage. 2. Why Derate? (Main Factors) | Factor | Reason | |--------|--------| | Ambient temperature | Higher surrounding temp = less heat dissipation | | Conductor bundling / raceway fill | Multiple wires trap heat | | Altitude | Thinner air = poorer cooling above 2000m (≈6600 ft) | | Termination temperature limits | Equipment terminals (e.g., breakers) may be rated 60°C or 75°C, even if wire is 90°C | 3. Common Derating Rules (based on NEC) A. Temperature Derating (NEC Table 310.15(B)(2)(a)) Start with wire’s insulation temperature rating (e.g., 90°C THHN). Multiply ampacity by factor below.

| 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 | Example: #12 THHN (90°C) rated 30A at 30°C ambient. At 45°C ambient → 30A × 0.82 = . B. Bundling / Raceway Fill (NEC 310.15(C)(1)) For more than 3 current-carrying conductors in a raceway or cable: wire derating

#12 THHN, 9 conductors in conduit, ambient 45°C. Base @90°C = 30A Bundling (9 wires) = 0.70 Temp (45°C) = 0.82 Final = 30 × 0.70 × 0.82 = 17.22A This is a concise guide to — why