// FREE CALCULATORS
Transformer Sizing and Inrush Calculator
Size a three-phase transformer from design load, growth, redundancy, and derating, then check loading, secondary fault current, inrush check points, voltage regulation, and primary protection. NEC/ANSI and IEC.
// INPUTS
// LOAD AND SIZING
Apparent power at the worst-case power factor, after demand and diversity factors
0.3 is a common rule for ventilated dry-type units. Use the manufacturer's value, or 0 to ignore
1.00 for none. Enter a value below 1 for hot rooms or limited cooling
// SYSTEM AND TRANSFORMER
480 V (NEC/ANSI) or 400 / 415 V (IEC) are typical
From the nameplate or quote
Enter 0 for an infinite bus (no source impedance)
// REGULATION
Typical 3 to 10 for distribution units. Use test data if available
// LOSSES (optional, manufacturer data)
Design Load incl. Growth-kVA
Derating Factor-
Required Nameplate (per unit)-kVA
Selected Standard Size-kVA
Units Installed-
Day-One Loading (all units in service)-%
Planned Full Loading (worst case)-%
Primary Full-Load Current-A
Secondary Full-Load Current-A
Source %Z on Transformer Base-%
Secondary Fault Current, Infinite Bus-kA
Secondary Fault Current, with Source-kA
Fault Current at Low %Z Tolerance-kA
Inrush Check Point 1 (12 x FLA, 0.1 s)-A
Inrush Check Point 2 (25 x FLA, 0.01 s)-A
Voltage Regulation at Planned Full Load-%
Primary Protection-
Annual Energy Loss (all units)-kWh
Annual Loss Cost-
Assumptions and limits: three-phase unit; fault current is per transformer (paralleled units add); infinite-bus and source-impedance estimates ignore motor contribution and cable impedance; inrush is shown as two typical check points, not a computed value; regulation uses the first-order formula %R cos φ + %X sin φ. Use manufacturer data for critical work.
Disclaimer: For reference and preliminary estimation only. Engineering judgment is required. Verify results against the applicable code, project specification, and manufacturer data before use in design documents.
Please check inputs: load, voltages, impedance, power factor, and X/R must be positive, and N must be at least 1.