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Duct bank thermal model

Cross-section of a direct-buried duct bank (PVC conduits in concrete encasement). Each conduit circle is colored by conductor temperature. Center conduits can't shed heat as easily as edge ones — the hotter they run, the lower the safe ampacity. Based on a simplified Neher-McGrath thermal circuit model.

Conduit grid

Cable size (Cu XLPE 90 °C)

Load current

100%

0 A — 120% rated

Soil thermal resistivity

0.90

0.5 moist K·m/W 2.0 dry

Ambient soil temp

25 °C

10 °C 40 °C

Summary

Rated current —

Hottest conduit —

Coolest conduit —

Temp spread —

Position	Conductor (°C)	Rise above ambient	Mutual factor	Status

< 50 °C

50–65 °C

65–80 °C

80–90 °C

> 90 °C — exceeds rating

NEC 310.15 / Neher-McGrath simplified thermal circuit