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Mode {{ resultReady ? analysis.phase_label : '—' }} Secondary turns {{ resultReady ? formatTurns(analysis.secondary_turns) : '—' }} Secondary current {{ resultReady ? formatAmps(analysis.secondary_current_a) : '—' }}
Primary Core Secondary Load
The unit changes without changing the physical voltage.
Use the same RMS basis as the primary value.
VA—not watts—drives the full-load current calculation.
Three-phase expects balanced loading and line-to-line RMS voltage.
Whole turns from 1 to 1,000,000,000.
Neutral default: 0 turns (not measured).
Neutral ideal default: 100%.
%
Neutral default: 0% reserve.
%
The default is 3 decimals.
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Calculation trace:
Ideal RMS relationships; real hardware needs thermal, insulation, regulation, inrush, protection, and code review.
Vp / Vs = Np / Ns; I = S / (phase factor × V)
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A transformer changes alternating-current voltage through two windings linked by the same magnetic flux. A winding with more turns develops more root-mean-square (RMS) voltage, while a winding with fewer turns develops less. That relationship supports familiar jobs such as reducing mains voltage for controls, raising voltage for distribution, and providing isolation at roughly equal voltage.

Voltage alone does not describe the load a transformer can serve. Nameplates normally state apparent power in volt-amperes (VA), kilovolt-amperes (kVA), or megavolt-amperes (MVA). At a fixed VA rating, current rises as voltage falls. A 120 VA transformer carrying 0.5 A at 240 V can ideally supply 5 A at 24 V, so conductors and protection on the low-voltage side may need to handle much more current.

Quantity What it describes Why it matters
Turns ratio Primary turns compared with secondary turns. Sets the ideal voltage ratio and squared impedance ratio.
VA rating RMS voltage multiplied by RMS current under the selected phase model. Sets ideal full-load current; it is not interchangeable with watts.
Efficiency Output apparent power divided by estimated input apparent power. Raises estimated input current when losses are allowed for.
Design margin Extra VA reserved above the entered load. Provides planning reserve but does not certify a winding or installation.

Ideal ratio arithmetic is a starting point, not a transformer design. Core area, frequency, flux density, copper loss, temperature rise, insulation, voltage regulation, inrush, harmonics, cooling, enclosure, protection, and applicable electrical rules can all limit a real unit. Three-phase current also depends on using the intended line-to-line voltage convention and a balanced load.

How to Use This Tool:

Start with values from the same RMS operating condition, then add only the engineering assumptions you can justify.

  1. Enter Primary RMS voltage, Secondary RMS voltage, and their units. Choose Single-phase or Three-phase; the three-phase path expects balanced loading and line-to-line voltages.
  2. Enter the nameplate-style Transformer rating in VA, kVA, or MVA and the known Primary turns. The winding review will show the ideal secondary-turn target and full-load currents.
  3. Add Measured secondary turns when checking an existing winding. Leave it at zero when no measurement exists so a missing measurement is not treated as a zero-turn winding.
  4. Set Efficiency estimate, Design margin, and Display precision. Review the measured-turn error and engineering warning before using the rounded figures in a design note.

Interpreting Results:

Secondary turns is the ideal winding target implied by the voltage ratio; a fractional result must be reconciled with a practical whole-turn winding. Ideal secondary full-load current uses the entered VA rating, while the efficiency-adjusted primary current estimates greater input demand when efficiency is below 100%.

A positive measured-turn error means the measured secondary has more turns than the ideal target, and the corresponding ideal secondary voltage rises in the same proportion. Treat the margin result as reserved capacity, not proof that the core, wire, insulation, temperature rise, or protective devices are adequate.

Technical Details:

An ideal transformer has the same volts per turn on both windings. The voltage ratio therefore matches the turns ratio, and the impedance presented through the transformer changes with the square of that ratio. Apparent-power current uses a factor of 1 for single phase and the square root of 3 for a balanced three-phase system.

Formula Core:

The primary equations connect winding ratio, current, efficiency, margin, and reflected impedance without rounding intermediate values.

VpVs = NpNs =a
Ns=Np ×VsVp
I=Sk×V where k=1or3
Sin=Sη Smargin=S×(1+m)
ZpZs=a2
SymbolMeaningUnit
Vp, VsPrimary and secondary RMS voltage after unit conversionV
Np, NsPrimary and ideal secondary turnsturns
SEntered apparent-power ratingVA
ηEfficiency as a decimal fractionratio
mDesign margin as a decimal fractionratio
Zp/ZsIdeal primary-to-secondary impedance ratioratio

Rule Core:

Review messages use the first matching condition in this order.

OrderConditionReview cue
1Efficiency < 90%Review the low efficiency assumption.
2Design margin = 0%No reserve has been added.
3Primary-to-secondary turns ratio > 25 or < 0.04Review the extreme ratio and physical design constraints.
4Three-phase selectedConfirm balanced loading and line-to-line voltage.
5No earlier condition matchesContinue with full engineering review.

Measured winding review uses Vs,measured = Vp × Ns,measured ÷ Np. Its percentage error is the measured turns minus ideal turns, divided by ideal turns, multiplied by 100. Unit changes are normalized before these equations run, and display precision changes only the presented decimals.

Accuracy Notes:

The equations describe ideal RMS relationships and a planning allowance. They do not determine a safe or buildable transformer by themselves.

  • Confirm frequency, core flux density, winding resistance, regulation, thermal rise, insulation class, creepage, clearance, inrush, harmonics, cooling, and protection separately.
  • Use VA rather than load watts unless power factor and waveform effects have been accounted for.
  • For three phase, confirm the equipment connection and that both voltage entries use the line-to-line basis expected by the current equation.

Worked Examples:

A 240 V control supply

A single-phase 120 VA transformer with 1,000 primary turns and a 24 V secondary needs 100 ideal secondary turns. Its ideal full-load currents are 0.5 A on the primary and 5 A on the secondary. The 10:1 turns ratio also gives a 100:1 ideal primary-to-secondary impedance ratio.

Measured turns above the target

For 480 V to 120 V with 800 primary turns, the ideal secondary target is 200 turns. A measured winding of 210 turns is 5% high and corresponds to 126 V in the ideal ratio model, so the winding needs review before loading.

References: