| | 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 |
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 | Standard assumes ( R_th = 2
[ 0.85 \times 0.70 \times 0.75 = 0.446 ] | Derating at 50°C | Derating at 60°C
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.
| | Thermal Resistivity | Derating factor | |------------------------|-------------------------|---------------------| | Damp clay/loam | 1.0 – 1.5 | 1.05 – 1.10 | | Standard (IEC assumed) | 2.5 | 1.00 | | Dry sand/gravel | 3.0 – 4.0 | 0.70 – 0.85 | 4. Depth of Burial (for buried cables) Deeper burial reduces heat dissipation (soil is cooler but more insulating).
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.