Cable Trunking and Conduit Fill Calculator
Find the total external cross-sectional area of your cables from outer diameter and count, and see the required internal area and standard trunking size.
Formula used
A₁ = π × d² / 4 · A_total = N × A₁ · A_pathway = A_total / k_fill
Standard cable trunking cross-sectional area and usable fill table
Nominal internal cross-section (W × H) and usable cable area at 40% and 50% maximum fill ratios. Leave reserve space for heat dissipation, bending radius and future cable additions.
| Trunking size (mm) | Total area (mm²) | Usable area at 40% (mm²) | Usable area at 50% (mm²) |
|---|---|---|---|
| 25×16 | 400 | 160 | 200 |
| 40×16 | 640 | 256 | 320 |
| 40×25 | 1000 | 400 | 500 |
| 40×40 | 1600 | 640 | 800 |
| 60×40 | 2400 | 960 | 1200 |
| 80×40 | 3200 | 1280 | 1600 |
| 60×60 | 3600 | 1440 | 1800 |
| 100×40 | 4000 | 1600 | 2000 |
| 80×60 | 4800 | 1920 | 2400 |
| 100×60 | 6000 | 2400 | 3000 |
| 120×60 | 7200 | 2880 | 3600 |
Example: 8 cables with 12 mm outer diameter at 50% trunking fill
- Single cable area: A₁ = π × 12² / 4 = 113.10 mm².
- Total cable area for 8 cables: A = 8 × 113.10 = 904.78 mm².
- Required trunking area at 50% fill: 1809.56 mm²; first standard trunking size: 60×40 mm.
What should the cable trunking and conduit fill ratio be?
In conduits and pipes, a maximum fill factor of around 40% is commonly used to prevent jamming and jacket damage during pulling. In cable trunking and panel ducts, 50% fill leaves room for air circulation and future spare circuits.
Why is overall outer diameter used for trunking fill?
Nominal mm² only measures the metallic core; insulation and outer sheath make the physical cable much wider, so space occupancy is calculated from outer diameter d via A = π × d² / 4.
Why are conduit and trunking fill ratios limited to 40%–50%?
Limiting conduit fill to 40% and trunking fill to 50% prevents jacket damage during pulling, accounts for air voids between round cables and preserves heat dissipation.