| Cross-section, mm² |
Circuit breaker rated current, A | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 2 | 4 | 6 | 10 | 16 | 20 | 25 | 32 | 40 | 50 | 63 | |
| Modular circuit breaker, type C | |||||||||||
| 1.5 | 300 | 150 | 100 | 60 | 38 | — | — | — | — | — | — |
| 2.5 | 500 | 250 | 167 | 100 | 63 | 50 | 40 | — | — | — | — |
| 4 | 800 | 400 | 267 | 160 | 100 | 80 | 64 | 50 | — | — | — |
| 6 | — | 600 | 400 | 240 | 150 | 120 | 96 | 75 | 60 | — | — |
| 10 | — | — | 667 | 400 | 250 | 200 | 160 | 125 | 100 | 80 | 63 |
| 16 | — | — | 1067 | 640 | 400 | 320 | 256 | 200 | 160 | 128 | 102 |
| Modular circuit breaker, type B | |||||||||||
| 1.5 | 600 | 300 | 200 | 120 | 75 | 60 | — | — | — | — | — |
| 2.5 | 1000 | 500 | 333 | 200 | 125 | 100 | 80 | — | — | — | — |
| 4 | 1600 | 800 | 533 | 320 | 200 | 160 | 128 | 100 | — | — | — |
| 6 | — | 1200 | 800 | 480 | 300 | 240 | 192 | 150 | 120 | — | — |
| 10 | — | — | 1333 | 800 | 500 | 400 | 320 | 250 | 200 | 160 | 127 |
| 16 | — | — | 2133 | 1280 | 800 | 640 | 512 | 400 | 320 | 256 | 203 |
How to use
Find the row with the cable cross-section and the column with the breaker rating - the cell at the intersection is the longest run at which the short-circuit current at its end is still high enough to trip the breaker instantly (magnetic release). A type B breaker trips at a lower current than type C, so its allowed length is twice as long - see the breaker characteristics for the trip thresholds.
Important
The values are calculated for copper conductors and a 230 V supply and only cover breaker tripping on a short circuit. The cross-section for continuous load current must be checked separately, and the run length for voltage loss - with the voltage drop table: the final limit is the smaller of the two. This is reference material - the actual power supply design should be done by a qualified electrician to current codes.
Why the length is shorter than a 5-10x/3-5x multiple would suggest
The calculation uses the strictest end of the guaranteed instant-trip range - 10x In for type C and 5x In for type B (the upper bound from breaker characteristics), and the conductor resistance at its operating temperature during a short circuit (≈0.023 Ω·mm²/m, not the "cold" 0.0175 Ω·mm²/m used for the voltage drop calculation). That is why the allowed length in the table is noticeably shorter than a calculation using the lower end of the trip range would give - a deliberate safety margin, not an error.