Trunking & Containment Sizing Guide
Cable containment sizing for trunking and cable tray per BS 7671 Appendix 12 with cable factors
Trunking Fill Calculation
Trunking sizing is based on the 45% fill rule — the total cable factor must not exceed 45% of the trunking's usable internal area.
Fill % = (Sum of Cable Factors / Trunking Factor) x 100- Cable Factor
- = Cross-sectional area of each cable including sheath (mm²)
- Trunking Factor
- = Usable internal cross-sectional area of trunking (mm²)
BS 7671 Appendix 4, IET On-Site Guide
If the calculated fill exceeds 45%, select the next trunking size up. Always allow headroom for future cable additions on commercial installations.
Cable Factors — Common Sizes
Cable factors for flat T&E and singles
IET On-Site Guide, Table 4E| Cable Type | Size (mm²) | Cable Factor (mm²) |
|---|---|---|
| Singles (PVC) | 1.5 | 8.6 |
| Singles (PVC) | 2.5 | 11.9 |
| Singles (PVC) | 4.0 | 16.6 |
| Singles (PVC) | 6.0 | 22.9 |
| Flat T&E (2-core) | 1.0 | 16.6 |
| Flat T&E (2-core) | 1.5 | 21.2 |
| Flat T&E (2-core) | 2.5 | 29.3 |
| Flat T&E (2-core) | 4.0 | 40.4 |
Factors include overall sheath. 3-core T&E factors are higher — check tables.
Trunking Capacity (45% Fill)
Standard PVC trunking usable capacity
Manufacturer data| Trunking (mm) | Internal Area (mm²) | 45% Capacity (mm²) |
|---|---|---|
| 25 x 16 | 400 | 180 |
| 38 x 25 | 950 | 428 |
| 50 x 50 | 2500 | 1125 |
| 75 x 75 | 5625 | 2531 |
| 100 x 50 | 5000 | 2250 |
| 100 x 100 | 10000 | 4500 |
Internal areas are nominal — check manufacturer data for exact figures.
Worked Example
A commercial office run carries 8 x 2.5mm² singles and 4 x 1.5mm² T&E cables. Singles: 8 x 11.9 = 95.2 mm². T&E: 4 x 21.2 = 84.8 mm². Total = 180.0 mm². A 25x16mm trunking (capacity 180 mm²) is just adequate but leaves no room for expansion. Selecting 38x25mm (428 mm²) provides a 58% spare capacity for future cables.
Allow for Growth
Segregation Requirements