A derating factor is a multiplier (less than 1) applied to a cable’s rated current-carrying capacity to reduce its maximum allowable load. This ensures the cable operates within safe temperature limits under real-world conditions.
For cables in conduit/trunking, use similar or stricter factors. NEC Table 310.15(C)(1) provides detailed adjustments. Soil’s ability to dissipate heat varies. Standard assumes ( R_th = 2.5 , K·m/W ) (damp soil). Dry or sandy soil reduces ampacity. derating factor for cables
| | Reference Temp. | Derating at 50°C | Derating at 60°C | |----------------------|--------------------|----------------------|----------------------| | PVC | 30°C | 0.71 | 0.58 | | XLPE / EPR | 40°C | 0.89 | 0.77 | A derating factor is a multiplier (less than
Actual Cable Ampacity = Rated Ampacity (in ideal conditions) × Derating Factor Cables generate heat when carrying current. If ambient temperature is high, cables are bunched together, or buried in insulation, heat dissipation worsens. Without derating, insulation melts, causing short circuits or fires. Key Derating Factors (Per IEC & NEC Standards) 1. Ambient Temperature Derating Cables are rated at a reference temperature (e.g., 30°C for PVC, 40°C for XLPE). Above that, capacity drops. NEC Table 310
[ 0.85 \times 0.70 \times 0.75 = 0.446 ]