Parallel Transformer Load Sharing Calculator
Determine how two parallel-connected transformers divide total load based on their kVA ratings and percent impedances (%Z), and check for premature overloading.
Transformer 1 Nameplate
Transformer 2 Nameplate
Common Bus & Total Plant Load
Parallel Load Division Results
Frequently Asked Questions
What are the four essential requirements for paralleling transformers?
To safely parallel transformers without destructive circulating currents, both units must have: 1) identical secondary line voltages and turns ratios, 2) identical angular phase displacement (vector group e.g. Dyn11 with Dyn11), 3) identical frequency and phase sequence, and 4) closely matched percent impedances (%Z, ideally within ±7.5% to ±10%).
What happens if two parallel transformers have different percent impedances?
The transformer with the lower percent impedance (%Z) carries a disproportionate share of the load relative to its rating. It will reach 100% capacity long before the higher impedance unit is fully loaded, forcing the plant to de-rate the combined capacity of the substation.
What causes circulating currents in parallel transformers?
Even a minor 0.5% to 1.0% difference in secondary no-load voltage (caused by mismatched tap changer settings) drives continuous circulating current in a closed loop through the secondary windings. Because transformer internal impedance is very small, even a few volts of mismatch can produce dozens of amperes of continuous circulating heating.