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Conduit & Trunking Fill Calculator

Check cable capacity against BS 7671 fill factors

Reference Info & Formulas
Fill Factors

Conduit: 40% maximum

Trunking: 45% maximum

Based on BS 7671 Appendix 5

Why Fill Limits?
  • Allow cable pulling
  • Prevent overheating
  • Enable future additions
  • Reduce mechanical stress
Enclosure
Select conduit or trunking size
Cables
Add cables to calculate fill

Data: BS 7671:2018+A2:2022 — Appendix 5, Table 5A

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Conduit and trunking fill calculations ensure cables can be safely installed and adequately cooled within their containment.

The 45% fill rule limits cables to 45% of the containment's internal cross-sectional area. Each cable has a published cable factor representing its overall cross-sectional area including insulation. The sum of all cable factors must not exceed the space factor (usable capacity) of the chosen trunking or conduit.

Fill % = (Σ Cable Factors / Space Factor) × 100
Σ Cable Factors
= Sum of individual cable factors for all cables
Space Factor
= Usable capacity of the trunking or conduit

IET On-Site Guide, Appendix E

For conduit, the approach is the same but uses conduit capacity tables. The cable factors account for the outer diameter of each insulated conductor, ensuring the calculation reflects the actual space each cable occupies.

Quick Reference — Cable Factors

Single-core PVC stranded cable factors

IET On-Site Guide Table E6
Conductor (mm²)Cable Factor
1.58.6
2.512.6
4.016.6
6.021.2
10.035.3
16.047.8

Factors are for single-core PVC insulated, stranded conductors. Solid conductors and other insulation types have different values.

Practical Notes

Why 45% — Heat and Friction

The 45% limit exists for two critical reasons: heat dissipation and drawing-in friction. Cables packed beyond this threshold cannot shed heat effectively, causing temperature rise that derates their capacity. Physically, cables become extremely difficult to draw in, risking damage to the insulation during installation.

Grouping Derating in Trunking

Remember that grouping derating applies in addition to the fill calculation. Table 4C1 factors reduce cable capacity based on the number of circuits: 2 circuits = Cg 0.80, 4 circuits = 0.65, 9 or more circuits = 0.50. A trunking within the 45% fill limit can still require upsized cables due to grouping.

Segregation Is Mandatory

Power circuits and data, telecommunications, or fire alarm cables must be segregated. Use trunking with an internal divider or entirely separate containment. This prevents electromagnetic interference and is a requirement of both BS 7671 and BS 5839 for fire alarm circuits.

Frequently Asked Questions

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