Transformer Calculator

Primary RMS voltage:

Enter the primary winding RMS voltage.

Secondary RMS voltage:

Enter the secondary winding RMS voltage.

Transformer rating:

Enter the apparent power rating used for full-load current.

Phase configuration:

Select the winding system used by the rating and voltages.

Primary turns:

Enter the known primary winding turns.

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Measured secondary turns:

Optional. Compare a real secondary winding against the ideal voltage ratio.

Efficiency estimate:

Adjust only when estimating real input VA from a loaded secondary.

Design margin:

Optional reserve shown in the load plan.

Display precision:

Adjust output precision without changing the calculation.

Quantity	Value	Engineering note	Copy
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Load item	Value	Planning note	Copy
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Transformer calculations connect voltage, winding turns, current, and apparent power. The same device can step voltage down for controls, step voltage up for transmission, or keep voltage nearly unchanged for isolation, but the ratio rules still start from the windings on a shared magnetic core.

In the ideal model, each turn on the primary and secondary sees the same volts per turn. More turns produce more RMS voltage. Current moves in the opposite direction when the same VA rating is transferred, so a lower-voltage secondary can carry more full-load current than the primary even though the transformer rating has not changed.

Transformer planning terms and common interpretation mistakes
Term	Practical meaning	Common mistake
Turns ratio	Primary turns divided by secondary turns, matching the ideal primary-to-secondary voltage ratio.	Comparing turns or voltage values that were not entered on the same RMS basis.
Apparent power	The VA, kVA, or MVA rating used to calculate full-load current.	Using watts alone when power factor, nameplate VA, or rated current should be checked.
Line-to-line voltage	The voltage between two phases in a balanced three-phase system.	Mixing phase voltage and line voltage before applying the square-root-of-three current formula.
Reflected impedance	The ideal load impedance as seen from the other side of the transformer, scaled by the square of the turns ratio.	Treating the square-law result as a complete model of regulation, heating, fault duty, or protection.

Single-phase and balanced three-phase systems use different full-load current equations. Single-phase current is apparent power divided by RMS voltage. Three-phase current is apparent power divided by the square root of three times line-to-line RMS voltage. That distinction can change conductor sizing, protection choices, and whether the result agrees with a nameplate.

Primary and secondary transformer windings on a shared core, with voltage following turns and current moving inversely

The ideal ratio equations are useful for arithmetic checks, winding estimates, and first-pass load-current planning. They do not replace hardware design. Real transformers still need frequency, insulation, thermal rise, regulation, inrush, fault duty, grounding, enclosure, and code review before equipment is specified or energized.

How to Use This Tool:

Use rated RMS values and apparent power first, then add measured winding and planning assumptions if you have them.

Enter Primary RMS voltage and Secondary RMS voltage. For Three-phase AC, use line-to-line RMS voltage on both sides unless your source data has already been converted to phase voltage.
Set Transformer rating in VA, kVA, or MVA. This drives the ideal full-load current rows on both sides.
Choose Phase configuration. Single-phase AC uses S / V; Three-phase AC uses S / (sqrt(3) x VLL).
Enter Primary turns. The Winding Ratio Ledger reports the turns ratio, calculated secondary turns, volts per turn, current ratio, reflected impedance ratio, and ideal ampere-turns.
Open Advanced for Measured secondary turns, Efficiency estimate, Design margin, and Display precision. Efficiency affects estimated input VA and primary current only; design margin affects planning capacity rows only.
Review Load Current Plan and Transformer Side Map together so the voltage ratio, current direction, winding count, and planning note agree.
Correct validation messages before using the output. Voltages, rating, and primary turns must be positive; measured secondary turns must be zero or greater; efficiency must be above 0% and no more than 100%; design margin must stay from 0% to 400%.

Interpreting Results:

Start with the direction of change. A step-down case should show lower secondary voltage, fewer calculated secondary turns, and higher secondary full-load current. A step-up case should show higher secondary voltage, more calculated secondary turns, and lower secondary current. An isolation-like case should stay near 1:1 while still needing current and safety checks.

Transformer result cues and verification steps
Result cue	What it means	Verify before use
Step-down	Secondary voltage is below primary voltage, so ideal secondary current is higher at the same VA rating.	Secondary conductor size, overcurrent protection, voltage regulation, and heat rise.
Step-up	Secondary voltage is above primary voltage, so ideal secondary current is lower while insulation stress rises.	Insulation rating, clearance, enclosure limits, fault exposure, and protection rating.
Measured secondary check	A known secondary turn count is compared with the ideal calculated secondary turn count.	Turn rounding, winding tolerance, target voltage, and estimated measured secondary voltage.
Field review cue	The note flags low efficiency, no reserve, extreme ratio, or the need to confirm three-phase line voltage.	Datasheet current, load power factor, frequency, duty cycle, transformer class, and applicable electrical rules.

A clean ratio does not prove that a real transformer will run safely. Use the result as a consistency check, then compare it with the nameplate, datasheet, winding resistance, temperature-rise data, insulation system, protective device, and commissioning measurements.

Technical Details:

Ideal transformer math assumes the same changing magnetic flux links the primary and secondary windings. Equal volts per turn makes the voltage ratio follow the turns ratio. In the lossless model, apparent power is conserved, so the side with lower voltage carries higher full-load current.

Three-phase current uses the phase mode selected for the calculation. The single-phase factor is 1. The balanced three-phase factor is sqrt(3) when the entered voltages are line-to-line RMS values.

Formula Core:

The core equations use primary-to-secondary ratio a, apparent power S, RMS voltage, winding turns, efficiency estimate η, and design margin m.

\begin{array}{lcl} a & = & \frac{N_{p}}{N_{s}} = \frac{V_{p}}{V_{s}} \\ N_{s} & = & N_{p} \times \frac{V_{s}}{V_{p}} \\ I_{single} & = & \frac{S}{V} \\ I_{three} & = & \frac{S}{\sqrt{3} \times V_{LL}} \\ \frac{Z_{p}}{Z_{s}} & = & a^{2} \\ S_{in} & = & \frac{S}{η / 100} \\ S_{margin} & = & S \times (1 + m / 100) \end{array}

Transformer formula symbols and unit basis
Symbol	Meaning	Unit basis
`V_p`, `V_s`	Primary and secondary RMS voltage.	Converted to volts; three-phase current expects line-to-line voltage.
`N_p`, `N_s`	Primary and calculated secondary winding turns.	Turns; calculated secondary turns may be fractional before practical winding rounding.
`S`	Apparent power rating.	Converted to VA from VA, kVA, or MVA.
`η`, `m`	Efficiency estimate and design margin.	Percent entries used for estimated input VA and margin capacity rows.

For a 240 V to 24 V single-phase transformer rated 120 VA with 1000 primary turns, a is 10. The ideal secondary turns are 100. The ideal primary current is 120 / 240 = 0.5 A, the secondary current is 120 / 24 = 5 A, and reflected impedance ratio is 100:1.

Transformer calculation boundaries and review rules
Quantity or cue	Rule	Boundary or display note
Volts per turn	Primary voltage divided by primary turns.	Uses RMS primary voltage after unit conversion and the entered primary turns.
Measured secondary check	(Measured secondary turns - calculated secondary turns) divided by calculated secondary turns, then multiplied by 100.	A measured turn error within 2% receives the softer review badge; larger absolute error receives a warning badge.
Field review cue	Ordered caution check: efficiency below 90%, zero design margin, extreme ratio, then three-phase line-voltage reminder.	Extreme ratio means primary-to-secondary ratio above 25 or below 0.04.
Valid input bounds	Positive voltages, positive rating, positive primary turns, nonnegative measured turns, efficiency above 0% through 100%, margin from 0% through 400%.	Invalid entries hide calculated results until corrected.

The impedance ratio is an ideal load-reflection estimate. Leakage inductance, winding resistance, magnetizing current, core loss, saturation, frequency, load power factor, temperature, and construction geometry can all move measured behavior away from the ideal ratio result.

Accuracy Notes:

This is an ideal-ratio and load-current calculator for arithmetic review, not an approval method for building, selecting, or energizing a transformer.

Three-phase current assumes balanced loading and line-to-line RMS voltage.
Efficiency estimates input VA and estimated primary current only. It does not model voltage regulation, copper loss, core loss, heat rise, or magnetizing current.
Design margin changes planning capacity rows, not the base full-load current from the entered VA rating.
Shared or bookmarked URLs may carry entered settings with the URL, so avoid sharing sensitive project assumptions when that matters.

Worked Examples:

Small control transformer

With 240 V primary, 24 V secondary, 120 VA, Single-phase AC, and 1000 primary turns, Turns ratio Np:Ns reports 10:1. Calculated secondary turns is 100.00 turns, Ideal primary full-load current is 0.500 A, and Secondary full-load current is 5.000 A.

Three-phase line-voltage case

A 480 V to 120 V, 15 kVA three-phase transformer with 800 primary turns gives a 4:1 ratio and about 200 secondary turns. With line-to-line RMS voltage, Ideal primary full-load current is about 18.0 A and Secondary full-load current is about 72.2 A.

High ratio review

A 240 V to 5 V single-phase case with 120 VA, 1000 primary turns, and 20% design margin calculates about 20.83 secondary turns and 24 A secondary full-load current. Field review cue reports High ratio check, which points to insulation, creepage, winding, and regulation review beyond ideal math.

Measured winding mismatch

For the 480 V to 120 V example, entering Measured secondary turns as 210 reports roughly +5.0% in Measured secondary check and estimates about 126 V secondary. Treat that as a winding-plan warning, not proof that the finished transformer will hold 126 V under load.

FAQ:

Why does secondary current rise in a step-down transformer?

The ideal model preserves apparent power, so lower voltage means higher current at the same VA rating. The current ratio moves opposite the voltage ratio.

Should three-phase voltage be phase voltage or line-to-line voltage?

Use line-to-line RMS voltage for Three-phase AC. The current equation uses S / (sqrt(3) x VLL), and the field review cue reminds you to confirm the voltage basis.

Why is the calculated secondary turn count fractional?

The ideal ratio can produce a non-integer turn count. A real winding plan must round the turns and then check voltage error, winding tolerance, insulation space, and regulation target.

What does the efficiency estimate change?

Efficiency estimate changes Estimated input VA and estimated primary current. It does not change turns ratio, calculated secondary turns, or base full-load current from the entered VA rating.

Why do results disappear after an input change?

Read the validation message first. Required voltage, rating, and primary-turn fields must be positive; measured secondary turns must be zero or greater; efficiency must stay above 0% through 100%; and design margin must stay from 0% through 400%.

Glossary:

RMS voltage: Root-mean-square AC voltage, the basis used for transformer ratio and current calculations.
Turns ratio: Primary turns divided by secondary turns, matching the primary-to-secondary voltage ratio in the ideal model.
Apparent power: VA, kVA, or MVA rating used to calculate full-load current.
Line-to-line voltage: The voltage measured between two phases in a three-phase system.
Reflected impedance: The ideal load impedance transformation that changes by the square of the turns ratio.
Voltage regulation: The difference between no-load and loaded secondary voltage in a real transformer.

References:

15.6 Transformers, OpenStax, October 6, 2016.
DOE Fundamentals Handbook, Electrical Science Volume 4 of 4, U.S. Department of Energy, June 1992.
3 Phase Delta/Wye Calculator, DigiKey Electronics.