Estimates the temperature rise of a buried duct bank above ambient soil temperature from the total heat loss generated inside it, by approximating the duct bank's rectangular cross-section as a circular cylinder of equal area and applying the standard buried-cylinder external thermal resistance formula used in cable thermal rating work. Heat Rise mode finds the temperature rise for a given loss; Maximum Losses mode works backwards from a target maximum temperature.
Standards followed
Neher-McGrath / IEC 60287-2-1 — buried-cylinder external thermal resistance method (equal-area equivalent-diameter simplification for a rectangular duct bank)
What it can do
- Equivalent cylinder diameter from duct bank width and height (equal cross-sectional area).
- External thermal resistance from soil resistivity, burial depth to bank center, and equivalent diameter.
- Heat Rise mode: bank-to-ambient temperature rise and resulting duct/bank surface temperature from total losses per meter.
- Maximum Losses mode: maximum permissible total losses per meter for a target maximum allowable temperature.
Example problem
ρ=1.0 K·m/W, 1.0m burial depth to center, 1.2m×0.6m duct bank, 20°C ambient soil, 60 W/m total losses.
- D_eq = 2×√((1.2×0.6)/π) = 2×√0.2292 ≈ 0.957m
- R_ext = (1.0/2π) × ln(4×1.0/0.957) = 0.1592 × ln(4.178) = 0.1592 × 1.430 ≈ 0.228 K·m/W
- ΔT = 60 × 0.228 ≈ 13.7K → bank surface ≈ 20 + 13.7 = 33.7°C
≈13.7K rise, ≈33.7°C bank surface temperature.
This is a simplified equal-area equivalent-cylinder screening estimate, not the full Neher-McGrath method — it omits the internal geometric factors for individual ducts/cables inside the bank, mutual heating between circuits, and the concrete/duct internal thermal resistance. Use it for preliminary sizing only; a heavily loaded duct bank or a design near its thermal limit needs a full thermal study (e.g. a finite-element or full Neher-McGrath calculation) with the specific duct/cable arrangement.