Understanding Conduit & Trunking Fill Factors

During commercial fit-outs and industrial electrical installations, specifying the correct size of containment is crucial. Overfilling trunking or conduit is a direct violation of BS 7671 (The IET Wiring Regulations). Cables forced into undersized containment risk mechanical damage to their insulation during pulling, and they suffer from restricted heat dissipation, which drastically reduces their current-carrying capacity.

The 40% Conduit Fill Factor Rule

The primary rule for cable capacity in conduits is the 40% fill factor rule, which is established to prevent heat build-up and avoid cable damage during installation. To calculate the exact capacity for any combination of wires, you must use a standardized cable term unit factor system from BS 7671 Appendix 5.

Identify the Cable and Conduit Factors: Each cable size and conduit diameter is assigned a specific factor number based on its cross-sectional area and internal clearance space.

• 1.5 mm² Cable: Factor 22 (Straight) / 31 (Bends)
• 2.5 mm² Cable: Factor 30 (Straight) / 43 (Bends)
• 4.0 mm² Cable: Factor 43 (Straight) / 58 (Bends)
• 6.0 mm² Cable: Factor 58 (Straight) / 88 (Bends)

Standard 20 mm conduit factors change drastically depending on the physical installation parameters:

• Straight runs (under 3 metres): 460 Capacity Factor
• Runs with bends (or over 3 metres): 286 Capacity Factor

Apply the Calculation Formula

To determine the maximum number of allowable cables manually, divide the total conduit factor by the factor of your chosen cable size, always rounding down to the nearest whole number:

Example A: Straight Run Using 2.5 mm² Wires
Conduit Factor (Straight 20 mm): 460
Cable Factor (2.5 mm² Straight): 30
460 ÷ 30 = 15.33 → 15 Cables Maximum
(Note: While mathematically permitted for short, dead-straight test lengths, actual installation practicalities and heat dissipation factors limit real-world usage to significantly fewer wires).

Example B: Run with Bends Using 2.5 mm² Wires
Conduit Factor (20 mm with Bends): 286
Cable Factor (2.5 mm² with Bends): 43
286 ÷ 43 = 6.65 → 6 Cables Maximum

Cross-Sectional Area Method (Alternative Rule)

If you are mixing different cable sizes or using non-standard data cables, use the pure geometric area calculation. A standard rigid 20 mm PVC conduit has an internal diameter of approximately 17.4 mm.

Apply the 40% Safety Limit:
Maximum Usable Area = 237.8 mm² × 0.40 ≈ 95.1 mm²

Summary of Rules: The exact combination of cables must always satisfy the condition that the total cable factors do not exceed the assigned conduit factor, or the total outer area does not exceed 40% of the internal conduit space. For standard Trunking installations, this limit is slightly higher at 45%.