Cable Short Circuit Rating Calculator
Check that a cable or protective earth conductor can survive a fault until the protection operates, using the adiabatic equation from IEC 60364.
- S = I × √t ÷ k = 10,000 × √0.4 ÷ 115 = 55 mm²
The Adiabatic Equation
Imax = k × S ÷ √t
S is conductor size (mm²), I the fault current (A), t the disconnection time (s) and k a factor for conductor material and insulation (IEC 60364-5-54). Valid for t up to 5 s.
Worked example
A 10 kA fault cleared in 0.4 s on PVC insulated copper: S = 10,000 × √0.4 ÷ 115 = 55 mm², so use 70 mm².
Frequently Asked Questions
What is the k factor for copper PVC cable?
k = 115 for copper conductors with PVC insulation up to 300 mm², and 143 for XLPE or EPR.
When is the adiabatic equation valid?
For disconnection times up to 5 seconds, when the heat has no time to escape from the conductor.
Can I use this for protective earth conductors?
Yes. It is the standard method in IEC 60364 and BS 7671 to check that a CPC survives an earth fault.
Learn more on the blog
Cable sizing and ampacity explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article