Conduit Fill Calculator - BS 7671 Compliant

Calculate if cables fit in conduit and check BS 7671 compliance with derating factors

Calculate Conduit Fill

Check if your cables fit in conduit according to BS 7671

Cables

Cable #1

How to Use the Conduit Fill Calculator

This calculator helps you determine if your cables will fit in a conduit and checks compliance with BS 7671 cable fill requirements.

Step-by-Step Guide

  1. Select Conduit Size - Choose the internal diameter of your conduit (16mm to 63mm). Use the "Suggest Size" button to automatically find the minimum size needed.
  2. Add Your Cables - For each cable type:
    • Select cable type (Single Core, Twin & Earth, or SWA)
    • Choose the cable size (cross-sectional area in mm²)
    • Enter the quantity of cables
  3. Calculate - Click the calculate button to see fill percentage, BS 7671 compliance, and derating factor.

Understanding the Results

  • Fill Percentage: The proportion of conduit space occupied by cables. Must not exceed BS 7671 limits.
  • BS 7671 Compliance: Pass/fail based on whether fill percentage is within regulations.
  • Derating Factor: Current capacity reduction due to grouping. More cables = more heat = lower capacity.
  • Remaining Space: Available space for future cables or easier pulling.

Pro Tips

  • • Allow extra space (aim for 30-35% fill) for easier cable pulling
  • • Consider future cable additions when sizing conduit
  • • Use larger conduit if many bends are required - cables harder to pull
  • • Remember to apply derating factor when sizing cables in grouped installations

BS 7671 Conduit Fill Requirements

BS 7671 Appendix 5 specifies maximum cable fill factors for conduit and trunking to ensure cables can be installed without damage and have adequate heat dissipation.

Maximum Fill Factors

Number of CablesMaximum Fill
1 cable53%
2 cables31%
3 or more cables40%

These limits ensure cables can be pulled without damage and allow for adequate heat dissipation.

Current Derating (Grouping)

When multiple cables are installed in the same enclosure, their current-carrying capacity must be reduced:

Typical Derating Factors:
  • 2-3 cables: 80-70%
  • 4-6 cables: 65-57%
  • 9-12 cables: 52-48%
  • 16-20 cables: 44-41%
Why Derate?
  • Heat buildup from grouped cables
  • Reduced cooling efficiency
  • Prevents overheating
  • Ensures safe operation

Common Installation Issues

  • Over-filling: Makes cable pulling difficult, risks damaging insulation, and causes overheating
  • Under-sizing: Not leaving room for future cables or making pulling impractical
  • Forgetting derating: Cables may overheat if current capacity not reduced appropriately
  • Mixing cable types: Different outer diameters make accurate calculation important

Reference: BS 7671:2018+A2:2022 Appendix 5 - Classification of external influences and Appendix 4 - Current-carrying capacity and voltage drop