Fault MVA and fault current at a bus, combining the upstream source with the transformer impedance. This is the number every downstream protection setting and equipment rating depends on.
Verified against published standard
System Data
Transformer (optional)
Leave the transformer blank to get the fault level at the source bus itself.
Earth Fault
Enter system data and calculate.
About This Calculation
Short circuit (fault) current calculations size every piece of protective equipment in an electrical system — breaker interrupting ratings, busbar bracing, cable withstand, and relay pickup settings. IEC 60909 is the international standard method: it uses an equivalent voltage source at the fault point rather than trying to track the actual pre-fault voltage and load state, which makes the calculation repeatable and conservative. Getting this number right matters directly for safety: a breaker rated below the actual available fault current can fail catastrophically instead of clearing the fault.
Frequently Asked Questions
What is the difference between symmetrical and asymmetrical fault current?
Symmetrical current is the steady-state RMS value after the initial DC offset decays. Asymmetrical current includes that DC offset and is higher in the first few cycles — breakers are rated for both a symmetrical interrupting capability and often an asymmetrical (or "close and latch") rating.
Why does the X/R ratio matter?
X/R ratio sets how large the DC offset is and how long it takes to decay. A high X/R (common near large transformers and generators) means a bigger asymmetrical peak in the first cycle, even at the same symmetrical fault current — this is what breaker asymmetrical/close-and-latch ratings are checking against.
Is three-phase or single-phase-to-ground fault current usually higher?
It depends entirely on the system’s grounding. On a solidly grounded system, single-phase-to-ground fault current is often close to or above the three-phase value. On a resistance-grounded or ungrounded system, it is normally far lower — sometimes deliberately limited to tens of amps by a neutral grounding resistor.
Do I need the full IEC 60909 correction factors for a quick check?
For final equipment specification, yes — IEC 60909 includes voltage factor c, impedance correction factors, and near/far-from-generator distinctions that this calculator does not fully replace. For an early sizing estimate it is normally the right order of magnitude.