EngineerHub.orgWhat this calculator does
Enter the transformer rating and voltages, then select an operating load. The calculator applies standard apparent-power relationships and a simple constant-efficiency assumption to estimate input power and losses. Percent impedance is used to estimate the initial symmetrical fault current at the secondary terminals with an infinite upstream source.
Inputs
Results
Turns ratio: a = Vp / Vs
Single-phase current: I = S / V
Three-phase line current: I = S / (√3 × VLL)
Load current: Iload = Irated × load fraction
Real output: Pout = S × load fraction × power factor
Input power: Pin = Pout / η
Losses: Ploss = Pin − Pout
Terminal fault current: Isc ≈ Irated × 100 / Z%
The fault-current estimate assumes rated secondary voltage at the transformer terminals and negligible upstream source impedance. Actual fault current is normally lower once utility, cable, bus, and generator impedances are included.
Transformer relationship
How to interpret the results
- A ratio above 1 indicates a step-down transformer when the entered primary voltage exceeds the secondary voltage.
- For three-phase transformers, enter line-to-line voltages; the result is line current.
- Loading above 100% is flagged as overload. Whether a temporary overload is permissible depends on winding temperature, cooling class, ambient conditions, and the applicable loading guide.
- The calculated fault current is a transformer-only upper-bound estimate, not a protection study.
Model limitations
This calculator does not model winding connection, vector group, phase shift, tap position, excitation current, voltage regulation, harmonics, inrush, thermal aging, cooling class, or detailed copper and core losses.
Use the transformer nameplate and manufacturer data for final design. Verify conductor sizing, protection, earthing, insulation coordination, and short-circuit duties using the applicable codes and system study.