How to use this calculator
- Enter transformer data. Use transformer kVA, secondary voltage, phase and nameplate percent impedance.
- Set X/R and source mode. Use infinite bus for a conservative transformer-only screen, or enter known upstream source fault current and X/R on the same voltage base.
- Add downstream impedance. Enter equivalent R and X for a feeder, bus or equipment point if the fault location is not at the transformer terminals.
- Check utilization. Compare available fault current with the entered interrupting rating or SCCR screen value.
How it works
A transformer short-circuit screen starts from full-load current. For
three phase, I_FL = kVA x 1000 / (sqrt(3) x V_LL). For
single phase, I_FL = kVA x 1000 / V. The transformer-only
infinite-bus fault current is then I_SC = I_FL / (Z% / 100). For the
exact transformer secondary search path, use the
transformer impedance fault current calculator.
The short-circuit current calculator turns transformer impedance, optional source fault current and optional downstream feeder impedance into R/X components and adds them as series impedance on the same voltage base. Use the transformer kVA calculator when you still need source apparent power, the voltage drop calculator for normal load voltage drop, and the conduit fill calculator or conduit pull tension calculator after the physical raceway path is known.
Worked example
Verified against the live calculator
A 75 kVA, three-phase transformer at 480 V with
5.75% impedance has full-load current of about
90.2 A. Infinite-bus transformer fault current is
90.2 / 0.0575 = 1,570 A. With no upstream or downstream
impedance entered, available fault current at the secondary terminals is
the same value. A feeder or limited upstream source lowers the available
current at the entered point.
Reference data
Use this worksheet to keep the voltage base and impedance terms explicit. All optional source and downstream impedance values must be converted to the same secondary-voltage base before the available-current result is meaningful.
| Worksheet step | Formula | Use the result for |
|---|---|---|
| Full-load current | I_FL = kVA x 1000 / (phase factor x V) | Transformer load current |
| Transformer impedance | Z_T = V / (phase factor x I_SC) | Magnitude from kVA and Z% |
| R/X split | R = Z / sqrt(1 + (X/R)^2), X = R x X/R | Series impedance terms |
| Known source | Z_source = V / (phase factor x I_source) | Optional upstream Thevenin impedance |
| Fault point | I_fault = V / (phase factor x |Z_total|) | Available fault current |
Source: Formula-only worksheet; transformer nameplate impedance, utility/generator source data, conductor impedance and equipment interrupting ratings must come from project or manufacturer data.
Frequently asked questions
How do I calculate transformer short-circuit current?
First find transformer full-load current: I_FL = kVA x 1000 / (phase factor x V). Then divide by percent impedance as a decimal: I_SC = I_FL / (Z% / 100).
What does infinite bus mean?
Infinite bus assumes the upstream source has no impedance, so transformer impedance is the only source limitation. It is a conservative first-pass transformer-secondary screen when utility or generator fault data is not known.
How do I include known source fault current?
Choose known source fault current and enter the upstream source current converted to the same secondary-voltage base. The calculator converts that current to a Thevenin impedance and adds it in series with transformer impedance.
How do I include a downstream feeder or bus run?
Enter equivalent downstream resistance and reactance in milliohms. For three-phase faults use per-phase equivalent impedance; for single-phase faults use the circuit loop equivalent.
Does this handle NEC 110.24, interrupting rating or SCCR?
It supports the available-fault-current worksheet step, but it does not approve NEC marking, interrupting ratings, equipment SCCR, current-limiting protection or installation code compliance. Use qualified electrical study data, listed equipment instructions and AHJ review for final work.
Is this an arc-flash calculator?
No. Available fault current is one input to many electrical studies, but arc-flash incident energy also needs clearing time, equipment type, enclosure data, working distance and a current standard/method.
Method & assumptions
- Three-phase mode uses line-to-line voltage and balanced three-phase fault-current relationships.
- Single-phase mode uses the entered voltage and loop-equivalent impedance.
- Known-source mode assumes source fault current has been converted to the secondary-voltage base before transformer impedance is added.
- Downstream R/X inputs are equivalent impedance values, not conductor-size or conduit lookup tables.
- Does not calculate arc flash, breaker/fuse clearing time, current limitation, equipment SCCR, series ratings, selective coordination, DC fault current or permit/code approval.