Wire Derating [best] May 2026
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.
| 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 | Does not include ground wires, neutrals that carry only imbalance (in multiwire branch circuits), or control wires not carrying continuous load. Apply both factors to the base ampacity (from the insulation temp column). Do not derate twice from the same table. wire derating
| 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: This is a concise guide to — why
#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 Why Derate
