A duct bank is an arrangement of conduits (ducts) encased in concrete, installed underground to carry power or control cables between two points. It is the standard method for routing medium-voltage and low-voltage cables in industrial plants, substations, and utility networks where direct burial is insufficient or cable protection is required.
The configuration — how many ducts, in how many rows and columns, at what spacing — directly affects cable ampacity, installation cost, civil works scope, and future capacity. Getting the sizing right at the design stage avoids costly rework during construction.
Before sizing a duct bank, the following must be established:
The conduit inner diameter must accommodate the cable outer diameter with sufficient clearance for installation. Per NEC Chapter 9, Table 1, a single cable in a conduit occupies no more than 53% of the conduit's cross-sectional area (31% for two cables, 40% for three or more). For multiple cables in one conduit, the fill percentages reduce further (31% for two cables, 40% for three or more).
For most underground power cable installations, one cable per conduit is the standard arrangement. The selection rule is:
Common practice adds one conduit size above the calculated minimum to ease cable pulling and allow for future recabling.
Once the number of conduits is established, the physical arrangement must be defined. Duct banks are typically configured in a rectangular grid — described as (rows) × (columns). Common configurations include 2×2, 2×3, 3×3, and 3×4, depending on the circuit count.
Standard spacing between conduit centrelines is 190mm (7.5") for 100mm conduits and 230mm (9") for 150mm conduits, though project specifications may vary. This spacing provides adequate concrete cover between ducts and allows heat to dissipate from each cable.
The arrangement should also consider:
Suppose you need to route 8 power circuits plus 2 spare ducts, using 120mm² MV cables with an OD of 55mm, in 150mm (6") PVC conduits.
Grouping cables in a duct bank creates a heat concentration zone. Each cable generates heat under load, and cables at the centre of a large duct bank are the worst-cooled. This is why cable ampacity in a duct bank is always lower than the same cable installed in free air or direct burial.
The derating calculation — applying group derating factors based on loaded circuit count, soil thermal resistivity, and ambient ground temperature — is covered in our cable ampacity derating article. For preliminary sizing, assume a group derating factor of 0.70–0.85 for duct banks with 4 or more loaded circuits.
DUCTBANK‑LMX is a two-part toolkit that provides fast, consistent, defensible duct bank dimensions for early-stage design, routing studies, and field coordination. It includes an Excel workbook for generating dimensions and an AutoCAD LISP/FAS utility that draws duct bank cross-sections with a single command. Available for immediate download.