Transformer Calculator
Step the ratio — kVA in, volts traded for amps, and the currents on both sides of the iron.
Transformer Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Ideal ratio — no impedance, regulation or losses modeled
- Single-phase arithmetic; three-phase named
In short: A 25 kVA transformer stepping 480 V to 120 V runs a 4:1 turns ratio: 208.333333 A available on the secondary, 52.083333 A drawn on the primary — volts stepped down 4×, amps stepped up 4×, kVA the same on both sides. At a 0.8 power factor that bank carries 20 kW of real work and 15 kvar of magnetizing burden. The ratio is a promise about current, not just voltage.
Formula
ratio = Vp ÷ Vs; Is = kVA × 1,000 ÷ Vs; Ip = Is ÷ ratio
A transformer trades voltage for current at fixed apparent power: the turns ratio is the primary voltage over the secondary, the secondary current is kVA times a thousand over the secondary volts, and the primary draws that current divided by the ratio. Real work still obeys the power triangle — at 0.8 PF a 25 kVA bank carries 20 kW and 15 kvar. Efficiency peaks where core loss meets copper loss, which is why transformers are happiest loaded 35–65% and worst loafing near zero.
Worked Example
- Enter the kVA rating and the two voltages.
- Read the turns ratio and the current on each side.
- Check the real work at your load's power factor.
Defaults: 4:1, 208.333333 A secondary. Drive the secondary to 240 V and the ratio halves to 2:1 while the primary stays pinned at 52.083333 A — the kVA sets the primary; the ratio only splits it.
Strengths & Limits Of This Model
Where this engine is strong
- Both currents printed — the ratio is about amps too
- The 35–65% loading doctrine named where it belongs
Where it stops
- Nameplate impedance sets the real voltage sag
Practical Use Cases
Panel feeds
the secondary amps the panel sees
Protection sizing
primary fuses from primary amps
Machine tools
480-to-120 control power
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Transformer Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why do amps go UP when volts go down?
Because the transformer only trades — power in equals power out (minus a percent or two of heat). Quarter the voltage and the current must quadruple to carry the same kVA. The 4:1 step-down is also a 1:4 step-up in current.
What does kVA actually size?
The windings: kVA is what heats the copper, on either side, regardless of power factor. A 25 kVA transformer is fully loaded by 25 kVA of reactive current doing no useful work — which is why correcting PF at the load buys real transformer capacity.
What loading is most efficient?
Transformers peak where core loss equals copper loss — typically 35–65% of nameplate. Sizing to the peak measured load, not the nameplate of the old unit, runs the iron in its efficient band.
How do I size the primary protection?
From the primary current this page prints: fuses or breakers at 125–250% of Ip per the scheme, per the electrical code in force. The 52.083333 A on the default bank is where that table starts.
Where does the 15 kvar come from?
The power triangle: at 0.8 PF, 25 kVA splits into 20 kW of real work and √(25²−20²) = 15 kvar of magnetizing exchange. The capacitor bank page prices the correction.
Single or three phase?
This page is the single-phase arithmetic (kVA = V × I ÷ 1,000). Three-phase banks carry √3 times the kVA per amp — the three-phase page carries that side; the currents here are the per-leg starting point.
What is regulation?
The secondary voltage sag from no-load to full load, a few percent on distribution transformers. It is a nameplate impedance property — this page prints the rated currents; the sag lives on the nameplate's impedance line.
Why 480 to 120 specifically?
It is the classic American machine-tool feed: plant power at 480 V stepped to 120 V for control circuits, task lighting and receptacles. The arithmetic is identical for any pair you enter.