Duct Bank Design: How to Size Conduit Configurations

What Is a Duct Bank?

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.

Key Design Inputs

Before sizing a duct bank, the following must be established:

• Number of cables: total circuits to be routed, including spares
• Cable OD (outer diameter): determines minimum conduit size
• Conduit type and size: PVC, HDPE, or rigid steel; typically 100mm (4") or 150mm (6") trade size
• Spare capacity requirement: commonly 20–25% spare ducts for future use
• Soil thermal resistivity (ρ): affects heat dissipation and derating
• Applicable standard: NEC (NFPA 70), IEC 60364, or project specification

Conduit Sizing

The conduit inner diameter must accommodate the cable outer diameter with sufficient clearance for installation. Per NEC Chapter 9, Table 1, the fill limit is 53% of the conduit's cross-sectional area for one cable, 31% for two cables, and 40% for three or more cables.

For most underground power cable installations, one cable per conduit is the standard arrangement. The selection rule is:

• Calculate cable cross-sectional area: A_cable = π × (OD/2)²
• Minimum conduit area: A_conduit = A_cable / 0.53
• Select the next standard conduit size above this minimum

Common practice adds one conduit size above the calculated minimum to ease cable pulling and allow for future recabling.

Duct Bank Configuration

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 typically 190mm (7.5") for 100mm conduits and 230mm (9") for 150mm conduits. These are common practice, not NEC requirements, and 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:

• Grouping by voltage level: MV and LV cables should be segregated where possible
• Power and control separation: control and instrumentation cables should be kept away from high-current power cables
• Spare duct placement: distribute spares across the grid rather than clustering at one edge
• Depth of cover: NEC Table 300.5 specifies minimum cover by voltage and location; IEC 60364-5-52 requires adequate depth and protection against damage, with specific depths set by national rules and the project specification

Worked Example

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.

• Total conduits required: 8 circuits + 2 spares = 10 conduits
• Cable fill check: A_cable = π × 27.5² = 2,376 mm². A_conduit for 6 in Schedule 40 PVC (ID ≈ 154mm) ≈ 18,640 mm². Fill = 2,376 / 18,640 = 12.7%: well within 53% limit.
• Configuration: 10 conduits → use a 2×5 arrangement (2 rows, 5 columns)
• Duct bank width: 4 × 230mm spacing + 168mm duct OD + 2 × 75mm concrete cover = ~1,240mm wide
• Duct bank depth (2 rows): 1 × 230mm spacing + 168mm duct OD + 2 × 75mm concrete cover = ~550mm deep (excluding burial depth); the 75mm concrete cover is a typical minimum, so use the project specification value

Thermal Considerations

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 typically lower than the same cable installed in free air or in single 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 about 0.65–0.80 for duct banks with 4 or more loaded circuits.

Summary

• Size conduits based on cable OD and the 53% fill rule (one cable per conduit)
• Add 20–25% spare ducts and distribute them across the grid
• Use standard conduit spacing (190mm for 100mm, 230mm for 150mm conduit)
• Segregate MV/LV and power/control cables within the bank
• Account for thermal derating: do not size cables at free-air ratings for duct bank installation

Disclaimer: This guide is for general information and preliminary reference only. Engineering judgment is required. Verify methods, values, and code requirements against the edition adopted for your project, the project specification, and manufacturer data before use in design documents.

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