Cable Duct Size Calculator
Accurate cable duct sizing for electrical installations
Cable Duct Size Calculator
Compliant with NEC and IEC Standards
Results:
Total Cable Area: 0.00 mm²
Required Duct Area: 0.00 mm²
Recommended Duct Size: 0.00 mm
Duct Size (W x H): 0.00 x 0.00 mm
How to Use the Calculator
Power Cables
- Select "Power Cable" type
- Enter cable outer diameter
- Specify number of cables
- Choose appropriate fill factor
- Select duct shape preference
Control Cables
- Select "Control Cable" type
- Enter cable diameter including jacket
- Input total cable count
- Use standard 40% fill factor
- Choose optimal duct configuration
Data Cables
- Select "Data Cable" type
- Measure cable outer diameter
- Count all cable runs
- Consider future expansion needs
- Select appropriate duct shape
Cable Duct Sizing Fundamentals
Fill Factor
Fill factor determines cable density:
- 40% - Standard installations
- 50% - High density applications
- 60% - Maximum allowable fill
Duct Shapes
- Round - Most efficient space usage
- Square - Easy installation
- Rectangular - Multiple cable layers
Key Formulas:
Cable Area = π × (D/2)² × N
Duct Area = Cable Area / Fill Factor
Round Duct: D = √(4 × Area / π)
Calculation Formulas
Total Cable Area:
A_cable = π × (D/2)² × N
Required Duct Area:
A_duct = A_cable / (Fill Factor / 100)
Round Duct Diameter:
D_duct = √(4 × A_duct / π)
Example Calculation
Power Cable Installation:
Cable Diameter: 25mm
Number of Cables: 4
Fill Factor: 40%
Cable Area = π × (25/2)² × 4 = 1963.5 mm²
Duct Area = 1963.5 / 0.4 = 4908.7 mm²
Round Duct Diameter = √(4 × 4908.7 / π) = 79.0 mm
How to Calculate Cable Duct Size
Three quick checks to size your duct without guesswork.
1. Capture Cable Data
List each cable diameter and the quantity per size. The calculator sums every entry so you can mix feeders, controls, and comms in one run.
Total Cable Area = Σ π × (D/2)2 × Quantity
2. Apply Fill Factor
Select your target fill based on standards or site rules. Lower fills ease pulling tension and future additions; higher fills minimize duct size.
Required Duct Area = Cable Area / (Fill % / 100)
3. Convert to Duct Dimensions
Choose the shape to turn area into a real duct size. Rectangular outputs assume a practical 2:1 width-to-height ratio for stacked layers.
Round Duct = √(4A/π) | Square Duct = √A
Cable Duct Size Chart
Common bundles translated into practical duct picks.
| Cable Mix (OD × Qty) | Fill Factor | Estimated Duct Area (mm²) | Recommended Duct Size |
|---|---|---|---|
| 5 x 25 mm power | 40% | 6,136 | Round ≈ 88 mm | Rectangular ≈ 124 x 62 mm |
| 10 x 12 mm control | 40% | 2,827 | Round ≈ 60 mm | Rectangular ≈ 85 x 42 mm |
| 4 x 35 mm power + 6 x 16 mm control | 50% | 9,621 | Round ≈ 111 mm | Rectangular ≈ 157 x 79 mm |
| 8 x 20 mm instrumentation | 50% | 5,027 | Round ≈ 80 mm | Rectangular ≈ 114 x 57 mm |
| 12 x 10 mm data | 60% | 1,963 | Round ≈ 50 mm | Rectangular ≈ 71 x 36 mm |
Cable Duct Size FAQs
Straightforward answers for common duct sizing questions.
What fill factor should I pick?
40% is standard for most power and control runs to reduce pulling friction and allow future spares. Use 50–60% only when space is tight and pulling methods are well controlled.
Lower fill = easier pulls; higher fill = smaller duct.
How do mixed cable sizes affect the duct?
The calculator sums the cross-sectional area of every cable size you enter. A few large feeders can dominate duct size, so grouping similar diameters or splitting feeders into a parallel duct often optimizes space.
When should I choose rectangular over round?
Rectangular ducts suit stacked layers and tight ceilings; round ducts are easier to seal and typically stronger. Use the rectangular option when you need width-to-height control for tiered cable packs.
Rectangular recommendation assumes a 2:1 width-to-height ratio.
Do I need to include future expansion?
Yes. Add spare capacity by reducing the fill factor (e.g., 30–35%) or by entering placeholder cables matching your expected additions so the duct size accounts for tomorrow’s needs.
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