Conduit fill calculator
Check how many cables fit in a conduit: conduit diameter, cable size and quantity — calculated by the 40% rule.
The 40% rule
Per IEC 61386 and NEC Chapter 9, the total cable area must not exceed 40% of the conduit area. This ensures the cables pull through without insulation damage and keep cool.
Reference table
| Conduit, mm | 1.5 mm² | 2.5 mm² | 4 mm² |
|---|---|---|---|
| 16 | 3 pcs | 2 pcs | 2 pcs |
| 20 | 5 pcs | 4 pcs | 3 pcs |
| 25 | 8 pcs | 6 pcs | 5 pcs |
| 32 | 13 pcs | 10 pcs | 8 pcs |
| 40 | 20 pcs | 16 pcs | 12 pcs |
Related calculators
- Cable size — cable-calculator
- Cable heating — cable-heating
- Breaker selection — breaker-selector
FAQ
How many wires in 16 mm conduit?
Up to 3 cables of 1.5–2.5 mm² by the 40% rule. More makes pulling hard and cooling worse.
What happens if a conduit is overfilled?
The cable won't pull, insulation gets damaged and the cable overheats — fire risk. Check fill before installation.
Is the 40% rule mandatory?
Yes — IEC 61386 / NEC Chapter 9 / PUE require it. A single conductor may use up to 53%.