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.
| | 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 | derating factor for cables
| | Arrangement | Derating factor | |----------------------|--------------------------------|---------------------| | 1 | Single circuit, free air | 1.00 | | 2–3 | Touching, in air | 0.80–0.85 | | 4–5 | Touching, in air | 0.70–0.75 | | 6–8 | Touching, in air | 0.60–0.65 | | >9 | Touching, in air | 0.50–0.55 | A derating factor is a multiplier (less than
Note: Always check manufacturer tables for exact values. When multiple cables run together, each heats its neighbors. The closer and more cables, the higher the derating. | Derating at 50°C | Derating at 60°C
[ 200 , A \times 0.446 = 89.2 , A ]
[ 0.85 \times 0.70 \times 0.75 = 0.446 ]
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.