Underground Duct Bank Ampacity

Calculate underground electrical duct bank ampacity, mutual heating derate factors, soil RHO, and conduit configurations per NEC Annex B.

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

When multiple conduits carrying heavy power cables are grouped in a trench or concrete-encased duct bank, heat generated by each conductor cannot easily dissipate into the surrounding earth. Under NEC §310.14(B) and Informative Annex B (Neher-McGrath method), mutual heating deratings must be applied.

Duct Bank Thermal Factors:

  • 1. Duct Bank Configuration (Rows × Columns): Conduits located in the center of a 3×3 or 4×3 duct bank run significantly hotter than perimeter conduits. The hottest interior conduit sets the ampacity rating for all identical circuits.
  • 2. Soil Thermal Resistivity (Earth RHO, °C-cm/W): The resistance of soil to heat flow:
    • RHO 60: Wet/damp soil, high water table (excellent heat dissipation)
    • RHO 90: Standard design earth (average soil)
    • RHO 120: Dry sandy / gravel soil (poor heat dissipation — requires extra derating).
  • 3. Conduit Spacing: Standard 7.5-inch (190 mm) center-to-center spacing maintains adequate concrete/earth between adjacent raceways.
  • 4. Concrete Encased vs. Direct Buried: Concrete encasement distributes heat more evenly across the duct bank footprint compared to loose soil backfill.

Formulas & equations

Duct Bank Derated Ampacity = Base Table Ampacity × F_mutual × F_soil_RHO × F_ambient Mutual Heating Derate Factor (F_mutual) ≈ 0.65 to 0.88 (lookup from NEC Annex B) Neher-McGrath Equation: I = √[ (Tc − (Ta + ΔTd)) ÷ (Rdc × (1 + Yc) × Rca) ]

NEC reference table

Duct Bank Arrangement Total Conduits Mutual Heating Factor (F_mutual) Typical 500 kcmil Cu Ampacity
1 Row × 2 Columns (1×2)2 Conduits0.88 (88%)334 Amps (vs 380A base)
2 Rows × 2 Columns (2×2)4 Conduits0.78 (78%)296 Amps
3 Rows × 2 Columns (3×2)6 Conduits0.72 (72%)273 Amps
3 Rows × 3 Columns (3×3)9 Conduits0.65 (65%)247 Amps (Center conduit)
4 Rows × 3 Columns (4×3)12 Conduits0.60 (60%)228 Amps

*Source: NEC Annex B Table B.310.2(1) & (2) (7.5" spacing, RHO 90, 75°C conductors).

Worked example, step by step

Example: Sizing 500 kcmil Cu Feeder in a 2×2 Concrete Encased Duct Bank

Setup: 4 conduits in 2×2 arrangement, 75°C THHN/XHHW Copper 500 kcmil (Table 310.16 base = 380A), RHO 90 earth.

  • 1. Mutual Derate Factor (2×2): Annex B Table B.310.2 specifies F_mutual = 0.78.
  • 2. Derated Conductor Ampacity: 380 Amps × 0.78 = 296.4 Amps per phase.
  • 3. Total Bank Capacity: 4 three-phase feeders @ 296.4A each = 1,185 Amps total transmission capacity.

Frequently asked questions

In a duct bank, each conduit generates heat from I²R electrical resistance losses. Because soil is a poor thermal conductor, the surrounding earth acts as thermal insulation, trapping heat and raising conductor operating temperatures unless current is reduced.

Soil thermal resistivity (RHO) is the measure of how strongly soil resists heat conduction (°C-cm/Watt). It is measured on-site using a thermal needle probe. Low RHO soils (e.g. engineered fluidized thermal backfill FTB) allow conductors to carry up to 25% more current without overheating.

A duct bank is a group of underground conduits, typically encased in concrete or direct-buried in a common trench, that route electrical conductors between a utility source, switchgear, or buildings. Grouping raceways this way concentrates heat, which is why duct bank ampacity is lower than a single buried conduit per NEC Annex B.

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