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BS 7671 Conduit & Trunking Space Factor Sizer

Calculate the minimum required steel or plastic conduit diameter and metallic trunking size for single-core cables. Implements the standard unit factor method from the IET On-Site Guide (Appendix E) and BS 7671 Appendix 5, accounting for run length, number of bends, and the mandatory 45% trunking space factor.

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1. Conduit Run GeometryRun length between inspection boxes (metres)Runs ≤ 3 m with 0 bends use OSG Table E1/E2. Runs with bends or > 3 m use Tables E3/E4.Number of right-angle (90°) bendsUnder BS 7671, runs with 3 bends are only tabulated up to 5 m before requiring a draw-in box.

2. Cable SpecificationsConductor constructionSolid conductors have slightly lower cable terms in short straight conduit runs.Insulation ratingLow Smoke Zero Halogen (LSZH) cables for public or commercial installations.

3. Cable Quantities (Single-Core)

Enter the quantity of each single-core cable running through the enclosure:

1.5 mm²2.5 mm²4.0 mm²6.0 mm²10.0 mm²16.0 mm²

Containment Sizing Result

Recommended Size–

Total Cable Factor–

Enclosure Factor–

Space Factor Fill–

Capacity Utilisation0%

Size Option	Capacity Factor	Space Factor Fill	Status

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BS 7671:2018+A4:2026

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Step-by-Step Calculation Working

Why the 45% trunking space factor is mandatory under BS 7671. Metallic and non-metallic cable trunking is restricted to a maximum 45% space factor (0.45 × internal cross-sectional area) to guarantee that heat generated by loaded conductors can dissipate through air convection, and to prevent mechanical damage to conductor insulation when pulling cables around offsets and bends.

⚠️ Pull Box Warning: Under the IET On-Site Guide, runs with three 90° bends have no tabulated factors for lengths exceeding 5 metres. Where cable pulls exceed these limits, an intermediate inspection elbow or draw-in box must be installed.

Regulation Reference: BS 7671:2018+A4:2026 Regulation 522.8 (Avoidance of damage during installation) and IET On-Site Guide Appendix E (Tables E1 to E6: Cable factors for conduit and trunking).

Frequently Asked Questions: Conduit & Trunking Capacity

What is the difference between Table E1 and Table E3 conduit cable factors?

Table E1 applies strictly to short, straight conduit runs up to 3 metres with zero bends. Because pulling tension is low, cables can be packed more tightly without snagging. Table E3 applies to any run containing one or more bends, or straight runs exceeding 3 metres, and uses lower cable factor multipliers to prevent insulation damage during installation.

Why is trunking space factor capped at 45% instead of 100%?

Cables are circular and cannot physically pack to 100% of rectangular space without crushing. Furthermore, current-carrying conductors generate thermal heat. The 45% space factor in BS 7671 Appendix 5 leaves adequate airspace for convective heat dissipation and ensures cables can be drawn in and future additions made without crushing existing circuits.

How many 2.5 mm² singles fit into a 20 mm conduit?

In a short straight run (≤ 3 m), each 2.5 mm² cable has a factor of 43 and 20 mm conduit has a factor of 460, permitting up to 10 cables (430 ≤ 460). In a 4 m run with 1 bend, each cable factor is 30 and 20 mm conduit capacity factor is 298, permitting up to 9 cables (270 ≤ 298).