Transformer Calculator

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Calculate the rated secondary current, ideal voltages and turns ratios, or the kVA rating a load needs — with the basis of every result stated.Learn more ▾Show less ▴
Pick a task, enter only the values that task needs, and read the result with its assumptions beside it. All tasks share one transformer profile, so a value entered once is reused.
Phases?Single-phase uses I = S / V. Balanced three-phase uses I = S / (√3 × V) with the line-to-line voltage. Select the configuration from the transformer nameplate.

Example values

Rated secondary full-load current

– A

  • Rated primary full-load current– A
  • Line-voltage ratio (primary : secondary)–

This is the nominal full-load current from the nameplate values, not the current every connected load will draw.

Show assumptions and calculation

Example values

–

    Show assumptions and calculation
    Phases?Single-phase uses I = S / V. Balanced three-phase uses I = S / (√3 × V) with the line-to-line voltage. Select the configuration from the transformer nameplate.
    Add up several loads

    Adds apparent power arithmetically, which never understates the combined demand. It is not an exact calculation for loads with different power factors.

    Defaults: no growth allowance, no utilization margin — the base requirement.

    Illustrative list — edit to match the ratings actually available to you.

    Example values

    –

      The result means the rating covers the entered VA requirement under your assumptions — not an installation or code approval. Unbalanced single-phase loads can need a per-winding assessment.

      Show assumptions and calculation

      Each check states its model and assumptions next to the result. These are screening calculations on entered data, not an installation approval.

      Taps compensate supply-voltage variation; do not use them to push the secondary above its design voltage. Follow the manufacturer's instructions — off-circuit taps are changed only with the transformer de-energized.

      Uses the manufacturer's regulation figure at its stated conditions. The interpolation is linear in load fraction — an approximation, not a measured curve.

      Phases?Single-phase uses I = S / V. Balanced three-phase uses I = S / (√3 × V) with the line-to-line voltage. Select the configuration from the transformer nameplate.

      A line-to-neutral fault on a center-tapped secondary does not follow from the full-winding %Z — the half-winding impedance differs, so that case is refused rather than approximated.

      Example values

      –

        Show assumptions and calculation

        Scenarios

          No saved scenarios yet. Save the current inputs to compare configurations.

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          About This Tool

          This calculator handles the three transformer questions that come up most often: what current a transformer is rated for, how voltages relate to turns and connections, and what kVA rating a load needs. It is built to keep distinct quantities distinct. Rated values from the nameplate are labeled as rated, ideal-model results as ideal, and sizing results as valid only for the requirement you entered. When an answer is not determined by your inputs, the calculator says which value is missing instead of guessing.

          Sources: Schneider Electric (2003) — FAQ FA101694 · Schneider Electric (2024) — Why kVA Instead of kW

          Advanced checks — models and boundaries

          The advanced checks calculate against published, disclosed models. Fault current uses the infinite-source relationship I = I_rated / z, the same model Schneider and Bussmann publish, with no silent impedance default and no hidden worst-case factors. Voltage regulation uses the manufacturer's own figure and makes the denominator convention explicit, because published sources divide by different voltages. The loss model is the standard quadratic form P = P0 + x²·Pk with peak efficiency where copper loss equals core loss, verified against published textbook examples. The V/Hz indicator compares core flux against the design value. Every model refuses inputs it cannot honestly answer instead of substituting an assumption.

          Sources: Schneider Electric (2003) — FAQ FA105006 · Bussmann — Electrical Plan Review · Hammond Power Solutions — What is voltage regulation? · Kuphaldt — Lessons in Electric Circuits (AC), Voltage Regulation · Electronics Tutorials — Voltage Regulation · University of Anbar — Transformer Lecture Notes · Schneider Electric (2013) — FAQ FA174756 · Schneider Electric (2017) — FAQ FA330684 · IPTO (Greece) — Distribution Transformer Specification

          How to Use

          1. Pick the task: secondary current, voltage and turns, or load and size.
          2. Enter only the values that task asks for — the phase, rating, and voltage for a rated current.
          3. Read the result with its basis and assumptions, then save or share the scenario if useful.

          How to Use

          1. Pick the task: secondary current, voltage and turns, or load and size.
          2. Enter only the values that task asks for — the phase, rating, and voltage for a rated current.
          3. Read the result with its basis and assumptions, then save or share the scenario if useful.

          Methodology

          Rated currents use the standard nameplate relationships: I = S / V for single-phase and I = S / (√3 × V) with the line-to-line voltage for balanced three-phase transformers. Line-to-neutral input for wye secondaries converts with V(line-to-line) = √3 × V(line-to-neutral). Voltage and turns results use the ideal transformer relationships, with winding voltages derived from the connection: a delta winding sees the full line-to-line voltage and a wye winding sees it divided by √3, so the line-voltage ratio and the winding turns ratio are reported separately. Sizing multiplies the load by your growth allowance, divides by your utilization target, and selects the smallest candidate rating that covers the result. All arithmetic uses unrounded values; rounding happens only in the display.

          Sources: Schneider Electric (2003) — FAQ FA101694 · Megger — TTRU3 Turns Ratio FAQ · DV Power — Turns Ratio Tests

          Understanding Your Results

          Every result names its basis. "From nameplate" is a nominal value computed from rated quantities — a rated secondary current is the transformer's nominal maximum, not the draw of your actual load. "Ideal model" ignores losses and voltage drop, so it describes ratios and no-load behavior, not loaded terminal voltages. Sizing results mean one thing: the rating covers the VA requirement you entered, under the growth and utilization choices you made. They are not a statement about code compliance or installation safety. When inputs conflict — a rating, voltage, and current that disagree — the calculator shows the implied value and asks you to fix one input rather than silently overwriting anything.

          Sources: Schneider Electric (2003) — FAQ FA101916

          Sources

          Practical Examples

          Rated current: a 75 kVA three-phase transformer with a 208 V line-to-line secondary is rated for 75,000 / (1.732 × 208) ≈ 208.2 A per line. With a 480 V primary, the rated primary current is 90.2 A. Sizing with margins: a 60 kVA load with a 20% growth allowance needs 72 kVA. With an 80% utilization target instead, the same load needs 75 kVA. Applying both choices needs 90 kVA — the two margins are different decisions and multiply, which is why the calculator keeps them separate.

          Tips for accurate transformer calculations

          Read the rating and voltages from the transformer nameplate, not from the supply. Every formula here relies on the rated values that plate states. For three-phase units, keep the voltage basis straight: the standard formula uses line-to-line voltage, and 120 V line-to-neutral is not the same input as 208 V line-to-line. Enter watts only together with the true power factor. If you know the load in VA, use VA directly — no power factor is needed. State your margins explicitly. Growth allowance and utilization target are separate choices with different effects; the defaults are zero so you always see the base requirement. Specified low-voltage dry-type transformers are rated for continuous full load under their stated conditions, so an 80% utilization target is a design choice, not a rule. For dual-winding work, check the data sheet for permitted series and parallel connections before relying on the arithmetic.

          Sources: Schneider Electric (2003) — FAQ FA101916 · Hammond Manufacturing — 266L12B Data Sheet

          All calculations are performed locally in your browser. No data is sent to any server.

          Frequently Asked Questions
          How is the rated secondary current of a transformer calculated?
          For a single-phase transformer, divide the VA rating by the secondary voltage: I = S / V. For a balanced three-phase transformer, divide the total VA rating by √3 times the line-to-line voltage: I = S / (√3 × V). For example, a 75 kVA three-phase transformer with a 208 V line-to-line secondary has a rated secondary current of 75,000 / (1.732 × 208) ≈ 208.2 A per line, where 1.732 is the numerical value of √3. This is the standard nameplate relationship used by transformer manufacturers.
          Why are transformers rated in kVA instead of kW?
          A transformer rating states apparent power (kVA), the product of voltage and current, because the transformer must carry the full current regardless of the load's power factor. Real power (kW) equals apparent power only at a power factor of 1. This is why the calculator asks for the power factor whenever you enter a load in watts: without it, the current and the VA demand are not determined. It never assumes a power factor for you.
          Should I enter the line-to-line or the line-to-neutral voltage?
          For three-phase calculations, enter the line-to-line voltage by default — that is the voltage the standard three-phase formula uses. If your transformer has a wye-connected secondary, you can enter the line-to-neutral voltage instead and the calculator converts it with V(line-to-line) = √3 × V(line-to-neutral). For nominal systems, the two bases give slightly different exact results: 120 V line-to-neutral corresponds to 207.85 V line-to-line, not exactly 208 V. The calculator keeps the basis you selected and labels it in the result.
          What is the difference between the line-voltage ratio and the winding turns ratio?
          The line-voltage ratio compares the line-to-line voltages of the two sides. By contrast, the winding turns ratio compares the actual winding voltages, which depend on how each side is connected: a delta winding sees the full line-to-line voltage, while a wye winding sees the line-to-line voltage divided by √3. For a 480 V delta to 208 V wye transformer, the line-voltage ratio is about 2.31:1, but the winding turns ratio is about 4.00:1. The calculator reports both values separately whenever the connections are known, because stating only one of them is misleading for mixed delta-wye configurations.
          Is the calculated current the current my transformer will actually draw?
          No. The rated full-load current is the nominal maximum from the nameplate values — the current at the rated VA and rated voltage. What a transformer actually draws depends on the connected load, which is usually smaller and varies over time. For this reason, every result carries its basis label. Use the load and size task to compare an actual load against the rating; the result then shows the utilization as a percentage of the nameplate rating.
          How do center-tapped and dual secondary windings differ?
          A center-tapped winding is one winding with a fixed third terminal at its electrical midpoint: a 24 V center-tapped winding gives 12 V from either end to the center and 24 V end to end. Its two halves share one winding and cannot be connected in parallel. Two separate isolated windings are independent: two 12 V, 2 A windings give 24 V at 2 A in series or 12 V at 4 A in parallel, 48 VA either way — but only when the manufacturer permits the connection and the polarity is correct. A real example is the Hammond 266L12B, rated 12.6 V at 2 A in series and 6.3 V at 4 A in parallel. This calculator treats the two winding types as different objects and computes each correctly.
          Can this calculator tell me the breaker size or whether an installation is code compliant?
          No. This calculator computes electrical quantities from the values you enter — rated currents, ideal ratios, VA demand, and utilization. It does not select protective devices, verify code compliance, or approve an installation. Sizing results state that a rating meets the entered VA requirement, nothing more. Wire sizes, overcurrent protection, and code checks need the applicable electrical code and a qualified professional; our wire size and voltage drop calculators cover some of those calculations under the US NEC.
          Can a transformer change the supply frequency, for example 60 Hz to 50 Hz?
          No. A transformer is a passive device: whatever frequency energizes one winding appears on the other windings at the same frequency. Frequency conversion needs an active device such as a frequency converter, a variable-frequency drive, or a motor-generator set. Using a transformer at a frequency it is not rated for also has voltage limits set by the manufacturer, so check the nameplate and the manufacturer's instructions before connecting a 60 Hz unit to a 50 Hz supply.
          Does the calculator store or send my transformer data anywhere?
          No. All calculations run in your browser, and nothing you enter is sent to a server. Saved scenarios are stored only on your own device, and you can delete them at any time. Share links encode the input values you chose to include directly in the link itself, so share them only with people who may see those values.
          What do the result labels like "from nameplate" and "ideal model" mean?
          Each result carries a label for the model behind it. "From nameplate" means a nominal result computed from rated quantities. "Ideal model" means the ideal transformer relationships, which ignore losses and voltage drop. "Additional data needed" means the requested value is not determined by what you entered, and the message names the missing input. When only part of an answer is determined — for example the line-voltage ratio without the winding connections — the calculator shows the supported part and explains exactly what is missing rather than showing nothing.
          Why does the voltage-regulation check ask which voltage the percentage is divided by?
          Published sources genuinely disagree. Some divide the voltage change by the full-load voltage; others divide the same change by the no-load voltage. One pair of voltages therefore yields two different percentages, so a regulation figure is only meaningful together with its convention. The check therefore asks for the convention explicitly and defaults to the full-load denominator, which is the more common form in transformer literature. Entering a figure under the wrong convention shifts every calculated voltage.
          Is the fault-current result the short-circuit level of my installation?
          No. It estimates only what the transformer itself would limit a bolted terminal fault to, assuming a source with no impedance of its own, nominal voltage, and no cables between the transformer and the fault. Motor contribution and the asymmetrical peak are also outside the model. A real installation study adds the source and cable impedances and every contribution — the published point-to-point methods exist for exactly that. Use this result as a screening bound, and note that it needs the nameplate impedance: without %Z, no number is produced.
          Why does the calculator sometimes refuse an advanced calculation instead of estimating?
          Because for some questions a plausible-looking number would be wrong in a way you could not see. A line-to-neutral fault on a center-tapped winding does not follow from the full-winding impedance — the half-winding behaves differently, and published methods treat it with its own multiplier range. The percent impedance alone does not give the voltage drop at your power factor. And there is no "typical" impedance worth assuming silently. In each of these cases the refusal names the datum that is genuinely missing, which is more useful than a number with an invisible assumption inside it.