Sizing a feeder breaker or protective device starts with the winding current. A transformer calculator converts kVA and voltage ratings into current, turns ratio, and fault levels.
Calculate Transformer Current, Turns Ratio, and Fault Levels
This tool calculates primary and secondary full-load current, turns ratio, and estimated fault current for a three-phase transformer from its kVA rating and winding voltages. Electrical engineers, protection coordination specialists, and electricians use it to select conductors, breaker sizes, and protective device ratings before installation.
Using the Transformer Calculator
Enter apparent power (S) in kVA, primary voltage (Vp) in volts, and secondary voltage (Vs) in volts, both line-to-line. The calculator returns primary and secondary full-load current in amperes, turns ratio, estimated downstream fault current, active/reactive load capacity, and thermal dissipation at full rated load.
Transformer Current and Turns Ratio Formulas
Full-load current on each winding is:
$$I = \frac{S \times 1000}{\sqrt{3} \times V}$$
Where $I$ is full-load current in amperes, $S$ is rated apparent power in kVA, and $V$ is the line-to-line rated voltage of that winding. This is the standard rated-current definition per IEC 60076-1, Clause 4, applied separately to the primary and secondary voltages. Turns ratio follows directly from the voltage ratio: $a = V_p / V_s$.
Estimated downstream fault current, assuming an infinite (stiff) primary source, is:
$$I_{sc} = \frac{I_{rated}}{Z\%}$$
This simplified single-transformer fault current method, and the typical percentage-impedance (Z%) values it depends on, are described in IEEE C57.12.00 and IEC 60909-0. Common mistake: entering phase voltage instead of line-to-line voltage on a three-phase system, which understates current by a factor of $\sqrt{3}$.
Primary and Secondary Winding Diagram
When 125% Breaker Sizing Doesn’t Apply
The common “125% of full-load current” breaker sizing rule (NEC Table 450.3(B)) only governs transformers rated 1000V or less. A transformer with a primary above 1000V — for example, a 4160V primary — falls under NEC Table 450.3(A) instead, which permits primary-only overcurrent protection at up to 300% of primary FLA for fuses (or 600% for circuit breakers) when transformer impedance is 6% or less, subject to supervision and coordination requirements. Applying the 125% figure to a medium-voltage primary significantly undersizes the allowable protective device rating compared to what the Code actually permits.
Fault current and thermal loss figures on this page assume a 5% transformer impedance and roughly 2% full-load loss, both common default assumptions rather than nameplate values. Actual percentage impedance (Z%) and loss figures vary by manufacturer and design and are printed on the transformer nameplate — always substitute the real nameplate Z% for protective device coordination studies, since fault current is inversely proportional to Z%.
Transformer Calculator FAQs
Why is primary current lower than secondary current?
A step-down transformer has a higher primary voltage and lower secondary voltage. Since apparent power is constant across an ideal transformer, current is inversely proportional to voltage — the higher-voltage side always carries less current.
Is the fault current estimate accurate enough for protective device coordination?
No. This is a simplified single-transformer estimate assuming an infinite primary source and an assumed impedance. Real coordination studies must include actual nameplate Z%, upstream source impedance, and cable impedance, typically via IEC 60909 or ANSI/IEEE short-circuit methods.
Why does the calculator assume a 0.8 power factor for load capacity?
0.8 lagging is a common planning assumption for mixed industrial loads. Actual usable real power (kW) depends on your facility’s true power factor; a higher PF yields more kW from the same kVA rating, and a lower PF yields less.
Does turns ratio equal voltage ratio exactly in a real transformer?
Approximately, under no-load ideal conditions. Under load, winding resistance and leakage reactance cause a small voltage drop, so the measured voltage ratio deviates slightly from the nameplate turns ratio, particularly at low power factor.
How does transformer impedance affect fault current?
Fault current is inversely proportional to percentage impedance (Z%). A transformer with 5% impedance produces roughly twice the fault current of an otherwise identical unit with 10% impedance, which directly affects breaker interrupting rating requirements.