What is the maximum fill for cable trunking?

The IET On-Site Guide limits fill by cross-sectional area — typically 45% for trunking and lower for conduits depending on cable count and diameter.

How do I check multiple cables in one run?

Enter each cable CSA and count. The calculator sums total cable area against the internal area of the selected containment.

Does grouping factor affect containment sizing?

Grouping affects cable current-carrying capacity (derating), not physical fill — but both must be checked on the same route.

BuildFlo Design Guide · Electrical · IET On-Site Guide

Cable containment fill calculator

Check trunking, conduit and cable tray fill per IET On-Site Guide. Free cable containment capacity calculator.

What this calculator does

Cable containment helps UK building services engineers with trunking, conduit, and tray fill check per iet on-site guide. The calculation follows IET On-Site Guide methods used in everyday MEP design — enter your project inputs below for a first-pass result.

Whether you searched for "cable containment", "cable containment calculator", or "how to cable containment", this page gives you a working tool plus the context to use it on a real job.

When to use it

Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.

How to use it

1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.

Typical output unit: %.

Cable containment calculator

Trunking, conduit, and tray fill check per IET On-Site Guide

CalculationContainment type

IET On-Site Guide space factor limits

Containment shapeInternal width (mm)Internal depth / height (mm)Internal diameter (mm)

For circular conduit — internal diameter, not nominal size

Spare capacity (%)

Extra allowance on top of the fill limit when sizing or checking

Fill limit override (%)

Optional — leave blank to use IET default for the containment type

Number of bends

Conduit only — 2+ bends prefers the IET cable factor method

Cables

All calculators →Save results in BuildFlo →

Tips

Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
Need the regulation citation? Open BuildFlo chat in the app and ask about cable containment under UK standards.

---

*Sign up for BuildFlo for the full calculator suite, saved results, and project workspace.*

Frequently asked questions

What is the maximum fill for cable trunking?: The IET On-Site Guide limits fill by cross-sectional area — typically 45% for trunking and lower for conduits depending on cable count and diameter.
How do I check multiple cables in one run?: Enter each cable CSA and count. The calculator sums total cable area against the internal area of the selected containment.
Does grouping factor affect containment sizing?: Grouping affects cable current-carrying capacity (derating), not physical fill — but both must be checked on the same route.