Cable Short-Circuit Withstand Calculator
Minimum cable cross-section for a fault current and clearing time, and how long a given cable survives it.
Results
What this tool does
A cable sized only for its normal load can still be destroyed by a fault it has to carry for a fraction of a second, because the energy goes with the current squared. This is the check that decides whether the protective device clears fast enough for the cable behind it — and it is why a slow device sometimes forces a larger cable than the load ever needed.
Formula
S = I √t ÷ k (adiabatic equation)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ik | Fault current | kA |
tt | Clearing time | s |
kk | K factor | — |
ss | Conductor cross-section | mm² |
SR | Conductor cross-section | mm² |
TM | Clearing time | s |
IM | Fault current | kA |
I2 | Let-through energy | ×10⁶ A²s |
UT | Utilisation | % |
Worked example
- Fault current10 kA
- Clearing time0.2 s
- K factor115
- Conductor cross-section50 mm²
- Conductor cross-section38.888 mm²
- Clearing time0.33063 s
- Fault current12.8574 kA
- Let-through energy20.0000 ×10⁶ A²s
- Utilisation77.78 %
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
What is the k factor?
A constant that bundles the conductor's heat capacity, its resistivity and the temperature rise its insulation can survive. Published values in IEC 60364-5-54 include 115 for copper with PVC insulation, 143 for copper with XLPE, 76 for aluminium with PVC and 94 for aluminium with XLPE. The equation is called adiabatic because it assumes the fault clears so fast that no heat escapes the conductor — true below about five seconds, and conservative above it.