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 ✓ | |
| 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. It's a quick sanity check — but the factor tables, plus the grouping derating, are what you design and certify against.
Reference material for qualified electrical work. Figures follow BS 7671 and the IET On-Site Guide unless stated; always confirm against the current edition and the standard that applies to your installation.