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Trunking Fill Calculator

Check cable capacity for trunking against BS 7671 45% fill limit

Reference Info & Formulas
Fill Factor

Trunking: 45% maximum fill

Based on BS 7671 Appendix 5

Common Sizes

50x25: 1,250mm² internal

75x50: 3,750mm² internal

100x50: 5,000mm² internal

150x75: 11,250mm² internal

Trunking Size
Select trunking dimensions
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

Trunking fill calculations verify that the total cable area does not exceed 45% of the trunking's internal cross-sectional area.

Each cable type has a published cable factor from Table 5C of the IET On-Site Guide. The calculator sums the cable factors for every cable in the trunking and compares this against the space factor of the selected trunking size. The space factor represents 45% of the trunking's usable internal area.

Fill % = (Σ Cable Factors / Space Factor) × 100
Σ Cable Factors
= Sum of individual cable factors for all cables in the trunking
Space Factor
= 45% capacity figure for the chosen trunking size

IET On-Site Guide, Appendix E, Table 5C

If the fill percentage exceeds 100% of the space factor (i.e. the cables exceed 45% of the internal area), a larger trunking size must be selected. The calculator also recommends the smallest trunking that accommodates the cable selection with adequate margin.

Quick Reference — Trunking Space Factors

Common trunking sizes and their space factors

IET On-Site Guide Table E3
Trunking Size (mm)Internal Area (mm²)Space Factor (45%)
25 × 16400180
38 × 16608274
38 × 25950428
50 × 381,900855
50 × 502,5001,037
75 × 503,7501,687
75 × 755,6252,531
100 × 505,0002,250
100 × 757,5003,375
100 × 10010,0004,500
150 × 507,5003,375
150 × 10015,0006,750

Space factors assume standard metal or PVC trunking with no internal compartment dividers. Compartmentalised trunking has a reduced usable area per compartment.

Quick Reference — Cable Factors

Cable factors for common single-core PVC stranded cables

IET On-Site Guide Table E6
Conductor (mm²)Cable Factor
1.07.1
1.58.6
2.512.6
4.016.6
6.021.2
10.035.3
16.047.8
25.074.2

Values are for single-core PVC insulated, stranded conductors. T&E and SWA cables have different (larger) cable factors due to their outer sheathing.

Practical Notes

Why the 45% Rule Exists

The 45% space factor serves two purposes. First, it ensures adequate air circulation around cables for heat dissipation — packed cables run hotter and must be derated further. Second, it limits friction when drawing cables in, preventing insulation damage during installation and allowing future cables to be added.

Segregation in Compartmentalised Trunking

When using compartmentalised trunking, the 45% fill calculation must be applied to each compartment individually, not to the total trunking area. A 100 × 50mm trunking with a central divider gives two compartments of approximately 50 × 50mm each, not one large space. Each compartment has its own reduced space factor.

Trunking vs Conduit — When to Use Which

Trunking is preferred for long straight runs with many cables, particularly in commercial risers and distribution routes. Conduit suits smaller numbers of cables with frequent bends and drops to accessories. As a rule of thumb, if you have more than 4-5 circuits on the same route, trunking is more practical and cost-effective than multiple conduit runs.

Grouping Derating Still Applies

Meeting the 45% fill limit does not remove the need for grouping correction factors. Table 4C1 derating applies based on the number of circuits: for example, 6 circuits require a factor of 0.57. Cable sizes must be checked against both the fill calculation and the derated current-carrying capacity.

Frequently Asked Questions

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