What is the maximum cable fill for conduit?

BS 7671 Appendix 5 and the IET On-Site Guide recommend a maximum space factor of 45% for conduit. This means the total cross-sectional area of all cables inside the conduit should not exceed 45% of the internal cross-sectional area of the conduit. This allows for cable pulling, heat dissipation, and future additions.

What is the maximum cable fill for trunking?

Trunking also has a recommended maximum space factor of 45% per the IET guidance. Because trunking is rectangular and cables can be laid flat, the actual arrangement can be more efficient than conduit. However, the 45% rule ensures cables are not packed too tightly and can dissipate heat properly.

Why does the number of bends matter for conduit?

Each bend in a conduit run makes it harder to pull cables through and effectively reduces the usable space. As a rule of thumb, each 90-degree bend reduces the effective capacity. It is generally recommended to have no more than two 90-degree bends between draw points. If more bends are needed, install additional draw-in boxes.

Can I mix different cable sizes in the same conduit or trunking?

Yes, you can mix different cable sizes, but you need to add up the cross-sectional areas of all cables and ensure the total does not exceed the 45% space factor limit. This calculator currently works with one cable type at a time, so add the CSA values for each cable type together manually if mixing sizes.

When should I use conduit versus trunking?

Conduit is typically used for individual circuit runs and where mechanical protection is needed in walls, floors and ceilings. Trunking is used where multiple circuits run along the same route, such as along walls in commercial premises. Trunking is easier to add cables to later and provides better separation between circuits when using compartmented trunking.

Conduit & Trunking Fill Calculator — BS 7671

Calculate cable fill for conduit and trunking installations. Check space factor compliance with BS 7671 18th Edition Appendix 5 and the IET On-Site Guide.

Enclosure Type

Round conduit or rectangular trunking

Conduit Size

Internal diameter of conduit

Cable Type & Size

Cable type determines overall cross-sectional area

Number of Cables

Total number of this cable type

How We Calculate This

This calculator checks whether the cables you want to install will fit within the chosen conduit or trunking size without exceeding the recommended space factor limit.

Space factor calculation

Space factor = (Total cable CSA / Enclosure internal CSA) x 100%. The recommended maximum is 45% for both conduit and trunking installations.

Step 1: Determine the internal cross-sectional area of the conduit or trunking from the manufacturer's data or standard sizes.

Step 2: Calculate the total cross-sectional area of all cables to be installed (including insulation and sheath).

Step 3: Compare the space factor against the 45% limit. If exceeded, upsize the conduit/trunking or split the cables across multiple runs.

Common conduit sizes

20mm: Most common for domestic circuits, 2-3 cables typically
25mm: Used for larger cable groups or longer runs
32mm: Commercial installations with multiple circuits

Frequently Asked Questions

Last updated: March 2026

All calculations are estimates. Verify with your supplier.

Conduit & Trunking Fill Calculator — BS 7671

How We Calculate This

Space factor calculation

Common conduit sizes

Frequently Asked Questions

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