What is the maximum fill factor for conduit?

BS 7671 and industry practice limits conduit fill to 40% of the internal cross-sectional area. This ensures cables can be drawn in and out without damage, allows adequate air circulation for cooling, and provides space for thermal expansion. Exceeding 40% makes cable pulling extremely difficult and risks damage to cable insulation.

Why is cable OD used rather than conductor size?

The conduit fill calculation is based on the overall outer diameter (OD) of the cable, including insulation and sheath, not just the conductor cross-sectional area. A 2.5mm² single-core cable (6491X) has an OD of approximately 4.2mm, while a 2.5mm² twin and earth (6242Y) has an OD of approximately 11mm. The actual space occupied in the conduit depends on the full cable diameter.

Should I use round or flat cable OD for the calculation?

For flat cables like twin and earth (6242Y), the calculation uses the equivalent circular area based on the cable's width and depth dimensions. The cable factor tables in BS 7671 Appendix C provide pre-calculated values that account for the non-circular cross-section. This calculator uses these standard OD values.

What conduit size do I need for typical domestic circuits?

For singles (6491X) in conduit: 20mm conduit holds up to 9× 2.5mm² singles at 40% fill. For twin and earth cables, 20mm conduit can only fit 1× 2.5mm² T&E — you would need 25mm or 32mm for multiple T&E cables. Always check: a 20mm conduit is the minimum standard for domestic work, with 25mm being preferred for future-proofing.

Does cable derating apply when using conduit?

Yes — cables grouped in conduit must be derated per BS 7671 Table 4C1 (grouping factors). For example, 3 circuits in the same conduit have a grouping factor of 0.70, meaning each cable can only carry 70% of its tabulated current capacity. This is in addition to any other correction factors for ambient temperature or thermal insulation. Always check both fill ratio and derating.

TradeCalculator

Conduit Fill Calculator

Calculate cable fill ratio for conduit runs using cable outer diameters and conduit internal areas. Ensure compliance with the BS 7671 40% maximum fill factor for easy cable pulling.

Cable Type

Select cable construction type

Cable Size

Conductor cross-sectional area

Number of Cables

Total cables in the conduit

Conduit Size

Round PVC or steel conduit

How We Calculate This

Conduit fill is calculated by comparing the total cross-sectional area of all cables against the usable internal area of the conduit.

Fill % = (Total cable area ÷ Conduit internal area) × 100

Cable area = π × (OD/2)² per cable

Standard Conduit Sizes

16mm: Internal area 184mm² — light-duty, limited use
20mm: Internal area 290mm² — standard domestic
25mm: Internal area 456mm² — multiple circuits
32mm: Internal area 716mm² — sub-mains, commercial
40mm: Internal area 1,110mm² — heavy commercial
50mm: Internal area 1,764mm² — industrial

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.