Installation design
Adiabatic protective conductor calculation
- where
- minimum conductor cross-sectional area (mm²)
- fault current (A)
- disconnection time (s)
- material/temperature factor
Explain this equation
Gives the minimum cross-sectional area a protective conductor needs to carry a fault current without overheating. Use it to check that a CPC is big enough for the fault current and disconnection time.
I is the fault current in amperes. t is the protective device's disconnection time in seconds. k is a factor for the conductor material and its insulation, linking temperature rise to the energy allowed; 115 is typical for copper with 70 °C PVC. S is the minimum area in mm².
Heat produced during a fault is proportional to I²t. In a short fault effectively none escapes (that is what adiabatic means), so the conductor must absorb it without exceeding the insulation's temperature limit. What it can absorb rises with k²S², so setting I²t ≤ k²S² and solving for S gives this form.
1000 A for 0.1 s with k = 115 gives about 2.75 mm², so the next standard size up, 4 mm², is needed. Doubling the current doubles the area; doubling the time multiplies it by about 1.41.
Watch out: always round up to a standard size. The method is valid up to 5 seconds. For current-limiting devices clearing in under 0.1 s, use manufacturer let-through energy instead.
The adiabatic model checks whether a conductor can absorb fault energy during a short fault without exceeding the temperature basis of k. Square current, multiply by time, take the square root and divide by k.
The calculated area is a minimum thermal value. Actual selection also needs the available conductor sizes, mechanical minima and all other applicable requirements.
Enter your values to see the result.
Common mistake
Using I × t instead of I²t, or rounding conductor size down.
Limits
Use the correct k for conductor material, insulation and temperature limits within the adiabatic method validity. Current-limiting devices may require manufacturer let-through energy. Round up to an available size and check mechanical minima separately. Valid for fault durations up to 5 seconds; for current-limiting operation below 0.1 s, use verified manufacturer let-through data rather than assumed constant RMS current.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.