Conduit fill calculator

Add the cables going into the conduit. It checks they fit and tells you the smallest conduit that works.

Conduit size

Cables (size and number)

Conduit fill

–%

Cable factor total–

Conduit factor–

Smallest conduit that fits–

Factors are for single-core thermoplastic (6491X) cables in conduit on short straight runs up to 3 m with no bends. Longer runs and bends use different tables. Check against your copy of BS 7671 or the On-Site Guide before relying on a result.

How it works

Sum of cable factors ≤ conduit factor

Each cable size has a cable factor, and each conduit size has a conduit factor. Add up the factors for every cable. If the total is no more than the conduit factor, the cables fit with enough space to pull them in without damage.

Worked example

Four 2.5 mm² stranded (4 × 43 = 172) and two 1.5 mm² stranded (2 × 31 = 62) give 234. 20 mm conduit has a factor of 460, so they fit, using about 51 % of the allowance. 16 mm conduit (290) would also work.

Factors come from the cable factor tables in Appendix E of the IET On-Site Guide, as reproduced in conduit manufacturers' data.