Conduit Fill & Cable Capacity
Work out how many cables fit in a conduit without overheating or damaging insulation on the pull.
IET On-Site Guide (cable/conduit factors)BS 7671 Appendix 4 (grouping)
Overfilling a conduit does two things: it makes cables impossible to pull without stripping insulation, and it traps heat so the cables can't carry their rated current. The IET On-Site Guide gives a 'factor' method that turns the geometry into simple arithmetic: add up the cable factors, and pick a conduit whose factor is larger.
The factor method
Every cable size has a 'cable factor' (a number proportional to its space demand) and every conduit size/length has a 'conduit factor' (its usable capacity). The rule is simple:
Σ (cable factors) ≤ conduit factorSum the factors of all cables in the run; choose a conduit whose factor is equal or greater.
Two sets of tables
Short straight runs (≤ 3 m, no bends) use one set of factors; longer runs and runs with bends use a second, more conservative set that accounts for the extra pulling force and reduced usable area. Always use the table that matches your run.
Worked example
Say you're running eight 2.5 mm² single-core thermoplastic (solid) cables through a 6 m conduit that has two bends. Using the running-system cable factors (illustrative figures: always read the current tables):
| Item | Factor | Qty | Subtotal |
|---|---|---|---|
| 2.5 mm² solid cable factor | ≈ 30 | 8 | 240 |
| Sum of cable factors | 240 | ||
| 20 mm conduit, 6 m, 2 bends | ≈ 260 | 260 (acceptable) | |
| 25 mm conduit, 6 m, 2 bends | ≈ 460 | 460 (spare capacity) |
240 ≤ 260, so 20 mm just works; 25 mm gives headroom for future cables. Figures illustrate the method: use the current On-Site Guide tables for real designs.
Don't forget the heat
Fitting the cables in is only half the job. The more current-carrying conductors share a conduit, the less heat each can shed, so a grouping factor (Cg) reduces the current each cable may carry.
| Circuits grouped | Approx. grouping factor Cg |
|---|---|
| 2 | 0.80 |
| 3 | 0.70 |
| 4 | 0.65 |
| 6 | 0.57 |
Enclosed in conduit, BS 7671 Appendix 4. A neutral+line pair is one circuit; count circuits, not cores.
Rule of thumb, then verify
A common working target is not to exceed about 40% of the conduit's cross-sectional area. Treat that only as a quick check. Use the applicable factor tables and grouping correction for the design.
Reference material for competent electrical work. Use the sources and jurisdiction stated in this guide, and always confirm the current rules, equipment instructions, and site conditions before relying on a value or procedure.