The calculation, without hidden assumptions
Estimate load division between two already-compatible transformers connected to the same bus. The calculation converts each nameplate percent impedance to a common-voltage admittance weight using kVA ÷ Z%, then shows each assigned kVA and which unit reaches its rating first.
How to use this calculator
Enter each transformer nameplate kVA and percent impedance.
Enter the total apparent-power load to be supplied by the parallel bank.
Use this load-sharing model only after confirming compatible voltage ratio, polarity, phase sequence/vector group, frequency and connection.
Compare each assigned kVA and loading percentage; the lower-impedance unit can reach its rating before the bank reaches the sum of both nameplates.
For ratio mismatch, unequal X/R, circulating current, protection or fault-duty work, use a complete engineering study rather than this magnitude-only screen.
Where people use it
- •Checking load division between unequal-kVA transformers with different percent impedances.
- •Finding which parallel transformer becomes the limiting unit first.
- •Estimating usable bank kVA before either transformer reaches nameplate loading.
- •Explaining why equal nameplate Z% lets unequal-kVA transformers share approximately in proportion to their ratings.
Example: 1000 kVA at 6% + 630 kVA at 6%, 1300 kVA load
Weights are 166.67 and 105.00. The 1300 kVA bank load divides to about 797.55 kVA and 502.45 kVA, so both transformers operate at about 79.75% of nameplate.
What the result does not assume
- •This calculator assumes equal secondary voltage ratios and compatible polarity, phase sequence/vector group and frequency. Do not use it to decide whether unknown transformers are safe to parallel.
- •The kVA/Z% weighting assumes impedance angles are sufficiently similar for an apparent-power sharing screen. Different X/R ratios require complex-impedance analysis.
- •A tap or turns-ratio mismatch can create circulating current even at no external load; that current is not calculated here.
- •Percent impedances should come from nameplate/factory test data and be interpreted on each transformer own kVA base.
- •The maximum-bank-load result is a nameplate loading screen, not an overload authorization; thermal limits, cooling, ambient conditions and manufacturer loading guidance still apply.
- •Do not use this result as breaker, relay, arc-flash or fault-duty approval.
Frequently asked questions
How do parallel transformers share load?+
For transformers with equal voltage ratio and similar impedance angle, load divides according to branch admittance. Using each transformer own kVA base, a practical weight is kVA divided by nameplate percent impedance.
Do equal percent impedances split kVA equally?+
Only when transformer kVA ratings are also equal. With equal Z% but unequal ratings, load shares approximately in proportion to the kVA ratings.
Why can the bank not always carry the sum of both nameplates?+
If percent impedances differ, one transformer can take a larger percentage of its own rating and reach 100% before the other does.
Can transformers with different voltage ratios be paralleled?+
Ratio mismatch can drive circulating current and needs explicit engineering evaluation. This calculator intentionally assumes compatible ratios rather than hiding that risk.
Does vector group matter?+
Yes. Three-phase transformers must have compatible phase displacement, polarity and phase sequence. The arithmetic here is not a substitute for those checks.
Does this calculate circulating current?+
No. Circulating current requires ratio/tap mismatch and complex impedance information; a magnitude-only Z% load-share calculator cannot determine it reliably.
Semantic next steps
Continue the calculation
These links move to a different input, formula or project stage rather than a keyword variation of this page.
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