Conduit FillCalculator

BS 7671 (18th Edition Wiring Regulations) limits the total cross-sectional area of cables inside a conduit to 45% of the conduit's internal cross-sectional area. This limit applies where cables need to be drawn in or replaced after installation — it ensures enough room to pull cables through without damaging their insulation. The 45% rule is for three or more cables; for two cables the limit is 30%, and for a single cable it is 53% of the conduit bore. This calculator uses the 45% limit as the standard for multiple cables.

Cables generate heat when carrying current (I²R losses). When cables are bunched inside a conduit, their combined heat cannot dissipate as efficiently as cables clipped singly to a wall. BS 7671 requires derating of current-carrying capacity when cables are grouped — typically reducing the rated capacity by 0.6–0.8 for common grouping arrangements (Appendix 4, Table 4C1). Overfilled conduits both reduce mechanical protection (more force needed to pull) and increase thermal stress on insulation, shortening cable life.

Yes — BS 7671 permits mixed cable sizes in the same conduit, provided the total cable area still complies with the 45% fill rule. However, mixing sizes creates practical problems: different cables have different pulling tensions, so the smaller cables may be damaged during installation. Grouping derating must be applied to all cables collectively, which often means the smaller cables become the limiting factor. In practice, electricians often prefer to run separate conduits for significantly different cable sizes.

Conduit fill is calculated as a percentage of the conduit's circular cross-sectional area, with a 45% limit. Trunking fill is calculated as a percentage of the trunking's rectangular cross-sectional area, but the permitted fill percentage differs: BS 7671 Table 5A gives space factors for trunking depending on the number of cables. For trunking containing single-core cables, the fill factor is typically 45% for three or more circuits, the same as conduit. However, trunking is generally easier to fill and change than conduit because it has removable covers rather than requiring cables to be drawn through.

Cable derating is the reduction of a cable's current-carrying capacity (Iz) when it is installed in conditions worse than the BS 7671 reference method (typically single cable in free air or clipped to a surface). When multiple cables share a conduit, the grouping correction factor (Cg) from BS 7671 Table 4C1 applies. For example, two circuits in conduit: Cg = 0.8 (80% of the single-cable rating). Three circuits: Cg = 0.7. Four circuits: Cg = 0.65. Six or more: Cg = 0.57. These factors reduce the maximum permissible current and therefore the maximum load that can be connected.

BS 7671 Section 522 covers protection against mechanical damage and conduit installation. Key requirements: conduit internal diameter must be at least 1.5 times the outside diameter of the cable with the largest diameter; conduit ends must be reamed smooth to prevent cable damage; conduit must be installed with draw-in boxes at sufficient intervals for pulling (typically every 5–8 metres on a straight run or after two 90° bends); conduit must be of suitable type for the environment (PVC conduit is not suitable for temperatures above 60°C or where mechanical impact is likely — use steel conduit); all joints must be waterproof if the conduit is installed in a damp location.