Underground duct bank conductor temperature & ampacity — Neher-McGrath / IEEE 835 method (IEC 60287 formulation), uniform or per-duct loading
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Project Info
Duct Banks in This Project
A project holds as many duct banks as you need — pick one to edit it. Changes autosave into the open project.
Duct Bank Configuration
Conductor & Load
Equipment Grounding Conductor
EGC is assumed to be the same material and insulation as the phase conductors (600 V THHN/THWN-2 insulation with 5 kV / 15 kV cable) — per NEC 250.118, PVC conduit requires a wire-type EGC. It counts toward conduit fill and duct air-gap crowding but carries no continuous current and adds no heat.
Thermal Environment
NEC 315.60 tables assume earth and concrete both at RHO 90 (Figure 315.60(D)(3): backfill “earth or concrete”), 20 °C, 100% load factor, 30 in to the top of the bank. This tool’s concrete default is 55, so it can read a few percent above the table.
Concrete resistivity varies with mix design, moisture, and aggregate — the presets are typical published ranges, not a substitute for mix-specific test data (ASTM D5334) on a critical project.
Advanced Parameters
Air gap constants U/V/Y per IEC 60287-2-1 for ducts in concrete (cable-to-duct air space: T₄′ = U / (1 + 0.1(V + Y·θm)·De), De in mm). Defaults shown are the fibre-duct-in-concrete values, commonly applied to PVC encased in concrete.
Results
Worst-Case Conductor Temp
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Uniform Load Scaling Limit
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Worst Duct
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Worst Conduit Fill
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Duct Bank Cross Section (click a duct to toggle spare)
Edit Duct
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EGC is assumed to be the same material as the phase conductors (per NEC 250.118, PVC conduit requires a wire-type EGC). It counts toward conduit fill and duct air-gap crowding but carries no continuous current and adds no heat.
Clipboard empty — select a duct and click Copy Settings.
Per-Duct Summary
Worst Duct — Calculation Detail
Methodology & assumptions. Conductor temperature per Neher-McGrath: TC = TA + I²·RAC·RCA′, with RCA′ = Rins + n·(Rair gap + Rconduit wall + Rearth). The calculation is written per duct so it also covers unequal loading (Different settings per duct): each duct's conductor temperature is TC,k = TA + Wk·(Rins + n·(Rair gap + Rwall + Rearth,self)) + Σj≠k n·Wj·Rmutual,kj + bank correction, where Rmutual,kj uses the Kennelly image ratio ln(d′kj/dkj) at concrete resistivity and the (ρearth − ρconcrete)·Gb envelope correction is driven by the total heat of all loaded ducts. All duct temperatures are solved simultaneously (coupled iteration) since each duct's losses depend on its own operating temperature. Daily loss factor (LF = 0.3·lf + 0.7·lf²) applies to mutual and far-field terms beyond Dx. AC resistance includes skin and proximity effect (IEC 60287-1-1); dielectric loss neglected (valid for 600 V cable; for 5 kV / 15 kV it is below the IEC 60287-1-1 thresholds of U0 ≥ 63.5 kV for EPR, 127 kV for XLPE); isothermal earth surface. 5 kV / 15 kV shielded cable: with shields bonded both ends, the shield circulating-current loss λ1 (IEC 60287-1-1, three cables in touching trefoil per duct) is added to the heat leaving each cable; the typical cable OD is built up from the conductor, strand shield, NEC Table 311.10(B) insulation wall, insulation shield, metallic shield and ICEA jacket. Circuit configuration: a balanced-neutral 4th wire adds no heat but enlarges the cable group in the duct air gap; a current-carrying neutral is counted as a 4th heat source. DC resistance and conductor diameters per NEC Chapter 9 Table 8; THHN/THWN-2 ODs per NEC Chapter 9 Table 5; XHHW-2 ODs per conductor diameter plus standard XLPE wall. NEC defaults: RHO-90 soil, 20°C earth ambient, 100% load factor. Verify results against NEC Annex B tables or a published Neher-McGrath worked example before relying on this tool for stamped deliverables.