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
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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.