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

Set the Conduit and Run - Choose the conduit size (16 mm to 63 mm), the run length in metres, and the number of bends (0, 1 or 2). Use the "Suggest" button to find the smallest conduit that accommodates your cables.
Add Your Single-Core Cables - The IET cable-factor method is for single-core PVC conductors ("singles" / 6491X). For each conductor:
- Choose the conductor size (1.0 mm² to 25 mm²)
- Enter the quantity
Calculate - The calculator returns the total cable factor, the conduit factor for the run, BS 7671 compliance, and the spare capacity.

Understanding the Results

Total Cable Factor: The sum of the IET cable factors for every conductor in the run.
Conduit Factor: The IET capacity of the conduit for this size, run length and number of bends. It falls as the run gets longer and as bends are added.
BS 7671 Compliance: Pass when the total cable factor is not greater than the conduit factor.
Utilisation: Total cable factor ÷ conduit factor. Lower values leave headroom for future cables and easier pulling.
Spare Capacity: Remaining factor units before the conduit limit is reached.

Pro Tips

• Aim for utilisation well below 100% — cables pull easier
• Consider future cable additions when sizing conduit
• Break runs longer than 10 m or with more than 2 bends with a draw-in box (IET recommended practice)
• Cable grouping derating is a separate consideration — size the conductors with the cable-size calculator

BS 7671 Conduit Fill Requirements

The IET cable-factor (unit) method — On-Site Guide Appendix E and Guidance Note 1 — checks whether cables fit in conduit. Each conductor has a cable factor and each conduit a conduit factor; the run is acceptable when the sum of the cable factors does not exceed the conduit factor.

How the cable-factor method works

Add up the cable factor of every conductor in the run.
Find the conduit factor for the conduit size — short straight runs (3 m or less, no bends) and longer runs or runs with bends use different factor tables.
The cables fit when the total cable factor is not greater than the conduit factor.

The conduit factor falls as the run gets longer and as bends are added, because the cables become harder to pull. Runs with more than two bends should be broken with a draw-in box.

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

Conduit Fill Calculator - BS 7671 Compliant

Calculate Conduit Fill

Single-Core Cables