| | 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).
[ 200 , A \times 0.446 = 89.2 , A ]
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