Calculator inputs
Example data table
| Trunking (mm) | Cables (diameter × qty) | Reserve | Allowance | Design fill | Status at 45% |
|---|---|---|---|---|---|
| 100 × 50 | 10×6, 16×2 | 10% | 5% | ≈ 24% | Within limit |
| 150 × 50 | 20×8, 10×10 | 20% | 10% | ≈ 55% | Over limit |
| 75 × 50 | 6×10, 10×6 | 0% | 0% | ≈ 31% | Within limit |
Formula used
- Trunking area: AT = W × H
- Cable area per cable: Ac = π × (d/2)²
- Total cable area: AΣ = Σ(Ac × qty)
- Allowance factor: k = 1 + (Allowance% / 100)
- Usable area: AU = AT × (1 − Reserve% / 100)
- Design fill: Fill% = (AΣ × k) / AU × 100
- Raw fill: Raw% = AΣ / AT × 100
How to use this calculator
- Measure the trunking internal width and height in millimetres.
- Enter your project fill limit and optional reserve/allowance values.
- Add each cable diameter and quantity; remove unused rows.
- Press Calculate to see raw and design fill results.
- If over limit, increase trunking size or reduce cable count.
- Use the download buttons to export the report as CSV or PDF.
Professional guide to trunking fill checks
1) Why trunking fill matters on site
Trunking fill is a practical indicator of whether cables can be installed, dressed, and maintained without damage. Overfilled trunking increases installation time, raises the risk of insulation abrasion, and makes future additions difficult. A clear fill check supports planning, reduces rework, and improves long-term serviceability.
2) What “area” represents in this method
This calculator uses a cross‑sectional area approach. Trunking area is calculated from internal width and height. Each cable is approximated as a circle using its outer diameter. The summed cable area is compared with trunking capacity to estimate occupancy. It is a reliable planning tool when consistent measurements are used.
3) Allowance and reserve for realistic design
Real installations are not perfectly packed. Bends, couplers, segregation requirements, and cable stiffness can reduce the usable packing efficiency. The Allowance option adds a margin to cable area to reflect practical packing. The Reserve option deliberately holds back capacity for future circuits, reducing the usable trunking area.
4) Interpreting raw fill vs design fill
Raw fill compares the geometric cable area with the full trunking area. Design fill applies allowance and reserve, giving a more conservative figure for decision‑making. If design fill exceeds your limit, the run is likely to be difficult to install or expand, even if raw fill appears acceptable.
5) Example data and quick interpretation
Example: trunking 100×50 mm, limit 45%, reserve 10%, allowance 5%, cables 10 mm×6 plus 16 mm×2. The calculator typically reports design fill near 24%, which is comfortably within limit and leaves usable spare area for changes. Increase cable count or diameter and the fill rises quickly, especially with reserve.
6) Good measurement practice
Use internal dimensions, not nominal catalogue sizes, and confirm the cable outer diameter from manufacturer data. Include sheath and any fire‑rated coating thickness where relevant. For mixed cable types, list each diameter group separately to keep the calculation transparent and auditable.
7) Typical actions when you are over limit
If the run is over limit, consider upsizing trunking, splitting circuits into parallel routes, or switching to a different containment system. Review segregation, bend points, and access requirements. Re‑calculate after each design change and keep a downloaded report with your drawings for traceability.
8) Using exported reports for approvals
The CSV export is useful for QA logs, material take‑offs, and peer review. The PDF summary is suitable for submittals, method statements, and handover packs. Consistent fill reporting strengthens coordination between electrical, HVAC, and architectural teams during congested corridor planning.
FAQs
1) What cable diameter should I enter?
Use the outer diameter including insulation and sheath. If a manufacturer provides an overall diameter, use that value for the most accurate area estimate.
2) Should I use internal or external trunking dimensions?
Always use internal width and height. External dimensions include wall thickness and will overstate capacity, leading to optimistic fill percentages.
3) What does the allowance percentage represent?
Allowance adds margin for imperfect packing, bends, couplers, and cable stiffness. It increases “effective cable area” to provide a conservative design fill.
4) Why use a reserve percentage?
Reserve protects capacity for future circuits or changes. It reduces usable trunking area, helping you avoid designing a route that is full on day one.
5) How do I treat mixed cable types?
Enter each distinct outer diameter as a separate row with its quantity. This keeps the breakdown clear and makes reviews and audits easier.
6) Is this method valid for non-circular cables?
It is best for round cables. For flat or irregular shapes, use an equivalent diameter or an area from product data, then apply a higher allowance.
7) What should I do if the result is over limit?
Increase trunking size, split routes, or reduce cable count. Re-check reserve and allowance, then export the report to document the updated design decision.
Accurate fill checks keep installations safe and serviceable today.