Transformer Calculator
Calculate transformer kVA, primary and secondary line current, connection-aware winding ratio, or an infinite-source secondary fault-current screen from nameplate impedance.
How to Use
- Choose the transformer question - use rating mode for full-load currents, load mode for required kVA, or ratio mode for the ideal winding-voltage/turns relationship.
- Use line-to-line voltage for three-phase - enter the transformer nameplate line voltages. The calculator then converts those values to per-winding voltage from the selected Delta or Wye connection.
- Enter kW with power factor when that is your load data - load kW must be converted to apparent power before transformer sizing. Omitting PF can undersize the mathematical kVA requirement.
- Use %Z only when you have actual nameplate data - the optional fault-current result divides secondary full-load current by per-unit impedance and assumes an infinite primary source. It is not a complete short-circuit study.
- Check a candidate transformer if you have one - in load mode, the optional rating check compares the entered load and your planning target with an existing or proposed transformer kVA rating.
- Keep protection and conductor sizing separate - this page does not choose breakers, fuses, conductors, grounding, or equipment. Those remain separate design checks.
For a 75 kVA, 480 V Delta to 208Y three-phase transformer, line current is about 90.2 A on the primary and 208.2 A on the secondary. The line-voltage ratio is 2.3077:1, but the winding-voltage/turns ratio is about 3.997:1 because each Wye secondary winding sees 208/√3 ≈ 120 V. If the nameplate impedance is 5%, the infinite-source secondary fault-current screen is about 4.16 kA.
Line Values and Winding Values Are Not Always the Same
Transformer kVA and Full-Load Current
Transformer nameplates are commonly rated in kVA. Single-phase full-load current is kVA × 1000 ÷ volts. Balanced three-phase line current is kVA × 1000 ÷ (√3 × line-to-line volts). The same nameplate kVA therefore produces different primary and secondary line currents when the winding voltages differ.
Load mode reverses the relationship. If the secondary load is entered as amps, the calculator derives the minimum kVA from voltage, current, and phase. If the load is entered as real kW, it first divides kW by power factor to obtain apparent kVA. An optional user-selected margin can then be added to that mathematical minimum.
The margin is user-selected because there is no single percentage that fits every transformer application. Motors, nonlinear loads, medical equipment, duty, temperature, future load, voltage regulation, and manufacturer requirements can all change transformer selection.
For a long primary or secondary run, current alone is not the whole conductor check. After the operating current and conductor size are known, use the Voltage Drop Calculator to evaluate the run separately.
Example Transformer Full-Load Currents
| Transformer | Phase | Primary line current | Secondary line current |
|---|---|---|---|
| 25 kVA · 240→120 V | Single-phase | 104.17 A | 208.33 A |
| 45 kVA · 480→208 V | Three-phase | 54.13 A | 124.90 A |
| 75 kVA · 480→208 V | Three-phase | 90.21 A | 208.18 A |
| 112.5 kVA · 480→208 V | Three-phase | 135.32 A | 312.27 A |
Mathematical full-load currents only. Protection, conductor ampacity, inrush, harmonics, grounding, and product suitability are separate checks.
Worked Load-Sizing Example
| Step | Example | Result |
|---|---|---|
| Load | 60 kW at PF 0.80 | 75.00 kVA mathematical load |
| Planning margin | 20% selected by the user | 90.00 kVA planning target |
| Candidate transformer | 112.5 kVA | 66.7% loaded by the entered load |
| Remaining capacity | 112.5 - 75.0 kVA | 37.50 kVA before considering other project limits |
The candidate-rating check is arithmetic, not product approval. Starting current, harmonics, duty, temperature, voltage regulation, future load, protection, and manufacturer requirements can change the final selection.
Why Delta and Wye Matter for Turns Ratio
For an ideal single-phase transformer, Vp/Vs = Np/Ns. The same winding-voltage relationship applies to each phase of a three-phase transformer, but line voltage and winding voltage are not identical in every connection.
In a Delta winding, each winding is across the line-to-line voltage. In a Wye winding, each winding sees line-to-neutral voltage, which is line-to-line voltage divided by √3 in a balanced system. That means a 480Δ-to-208Y transformer has a line ratio of 480/208 ≈ 2.31, while the winding ratio is 480/(208/√3) ≈ 4.00.
The connection selectors are used only to explain winding voltage/current and turns ratio. They do not model vector-group phase displacement, grounding, taps, regulation, core design, saturation, or harmonics.
Optional Secondary Fault-Current Screen
When the transformer nameplate impedance is known, an initial maximum secondary fault-current screen can be made by dividing secondary full-load current by impedance expressed as a per-unit value. For 5% impedance, divide by 0.05.
This is the transformer-limited infinite-source case. Real available fault current also depends on the upstream source, feeder impedance, transformer tolerances, conductors, connections, and the location of the fault. Use it only as a screening number before a proper short-circuit study when equipment interrupting ratings are involved.
Do not guess transformer impedance. Use actual nameplate or manufacturer data for the specific unit.
Transformer Formulas Used
The calculator keeps line quantities and winding quantities separate:
Three-phase line current = I = kVA × 1000 ÷ (√3 × VLL)
Load amps to kVA, 1φ = kVA = V × I ÷ 1000
Load amps to kVA, 3φ = kVA = √3 × VLL × I ÷ 1000
Load kW to kVA = kVA = kW ÷ PF
Delta winding = Vphase = VLL; Iphase = Iline ÷ √3
Wye winding = Vphase = VLL ÷ √3; Iphase = Iline
Ideal winding turns ratio = Np ÷ Ns = Vphase,p ÷ Vphase,s
Infinite-source secondary fault current = Isc ≈ Isecondary,FL ÷ (%Z ÷ 100)
The equations are planning relationships, not a transformer design, protection study, or product selection.
Authoritative Technical Sources
- Eaton publishes the single-phase and three-phase transformer full-load-current formulas and the transformer-impedance multiplier used for an initial secondary short-circuit-current calculation.Eaton - Short-Circuit Current Calculations
- Schneider Electric gives the transformer kVA relationships used to convert load current or real kW with power factor into apparent power for single-phase and three-phase systems.Schneider Electric USA - Transformer kVA formula
- Schneider Electric documents a Delta-primary / Wye-secondary transformer connection used for common 480 V to 208Y/120 V systems, supporting the distinction between line and winding quantities.Schneider Electric USA - T2F transformer connection
- U.S. Department of Energy purchasing guidance treats distribution-transformer selection as more than a kVA arithmetic exercise, including intended loading and efficiency considerations.U.S. Department of Energy - Distribution Transformer Buyer's Guide
Next Steps
After transformer current is known, continue with the checks that use that current:
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Transformer calculations sit inside a larger electrical workflow. Browse the electrical calculator collection for load, current, conductor, protection, and raceway checks.