Transformer Sizing Calculator
Calculate load kVA, apply a planning allowance, select the next published transformer size, and check primary and secondary full-load current.
Calculator is for informational and preliminary sizing purposes only. Terms and Conditions
For three-phase current-based sizing, use line-to-line voltage and line current. The selected capacity allowance is applied after the load kVA is calculated.
Choose the sizing basis
Select the system phase and the load information you already know.
Choose single-phase or balanced three-phase.
Use current, real power and power factor, or known apparent power.
Enter the load and capacity allowance
Required load fields change with the selected sizing basis. Secondary voltage stays available so the calculator can also report transformer full-load current. Values update automatically.
Enter the transformer secondary/load voltage for current outputs. For three-phase systems use line-to-line voltage. Then enter the load using current, kW with power factor, or known kVA. The capacity allowance is a user-selected planning input, not an automatic code requirement.
Transformer sizing result
The primary result separates the exact design requirement from any nominal transformer selection.
Sizing details
- Calculated load—
- Design requirement—
- Transformer loading—
Show calculation steps Review load kVA, allowance, nominal selection, current, and checks
- Enter valid values to see the complete calculation.
Capacity and voltage flow
Compare actual load with selected transformer capacity and see the primary-to-secondary relationship.
Method, Sources, and Assumptions
Calculation basis, published nominal ratings, scope limits, and final checks.
Current-based sizing uses \(S=VI/1000\) for single-phase and \(S=\sqrt{3}V_{LL}I/1000\) for balanced three-phase. kW-based sizing uses \(S=P/PF\). The selected allowance is applied after the load kVA is calculated.
References- Schneider Electric — Required transformer kVA rating and amperage capacity
- Schneider Electric — Published standard single- and three-phase transformer kVA ratings
- Schneider Electric — Maximum possible transformer-secondary available fault current
- The published low-voltage rating sequence is manufacturer-specific product guidance, not a universal promise that every rating is available for every voltage, enclosure, winding material, temperature rise, efficiency class, or application.
- The capacity allowance is not treated as a code-required percentage. Use the actual project load calculation, future-growth basis, continuous-load treatment, motor starting requirements, harmonic considerations, and applicable design criteria.
- Primary and secondary full-load current are based on the selected nominal transformer kVA. Actual operating load current can be lower.
- The optional fault-current result is a transformer-limited estimate using an effectively infinite primary source. A final short-circuit study must include source, conductor, bus, and other applicable impedances at the actual equipment location.
- Final transformer selection must be checked against the currently adopted electrical code, manufacturer data, available voltage/configuration, grounding, protection, temperature/altitude conditions, nonlinear or motor loads, and the authority having jurisdiction where applicable.
Calculator guide
How to Size a Transformer
A transformer is sized from the apparent power required by the load. Use voltage and current for single-phase or balanced three-phase loads, use kW with power factor when real power is known, or enter known kVA directly. The calculator above converts that load into kVA, applies the planning allowance you choose, and can select the next published low-voltage transformer rating. It then checks transformer loading and primary and secondary current at the entered voltages.
The most useful way to read the result is as three separate quantities: connected load, design requirement, and selected nominal transformer capacity. Those numbers answer different questions and should not be treated as interchangeable.
- Start with
- Load current and voltage, kW and power factor, or known kVA
- Main result
- Required kVA and, when enabled, the next published nominal transformer rating
- Verify next
- Voltage, loading, motor or harmonic duty, fault current, protection, conductors, and manufacturer availability
Transformer Sizing Formulas and Calculation Method
The calculator uses exact steady-state apparent-power relationships for the selected input basis, then applies the entered capacity allowance. When published nominal-size selection is enabled, the result is compared with Schneider Electric’s listed low-voltage transformer ratings for the selected phase.
Calculate the connected load in kVA
Plain language: multiply volts by amps for single phase; multiply line-to-line volts by line amps and by √3 for balanced three phase; or divide kW by power factor when the load is known in real power. The first two expressions divide by 1,000 to convert VA to kVA.
Schneider Electric publishes these same single-phase and three-phase kVA relationships and states that kW-based sizing also requires load power factor. Schneider Electric transformer kVA formula.
Apply the selected allowance and check transformer current
The first expression converts connected load into the calculator’s design requirement. The current expression converts a selected transformer kVA rating back into rated-side current at the specified voltage.
When no published nominal transformer has been selected, a current calculated from the exact required kVA is a current at that result capacity, not a manufacturer nameplate full-load current.
- \(S_{\text{load} }\)
- Connected apparent load Apparent power required by the represented load before the user-selected planning allowance.
- \(S_{\text{req} }\)
- Design requirement Connected load after the selected design or growth allowance is applied.
- \(S\)
- Transformer apparent-power capacity The nominal transformer rating, or the exact result capacity when nominal selection is disabled.
- \(V\)
- Single-phase voltage Voltage across the single-phase load or transformer side represented by the calculation.
- \(V_{LL}\)
- Line-to-line voltage RMS voltage measured between phase conductors for the balanced three-phase system.
- \(I\)
- Load current Maximum load current represented by the sizing calculation; for three phase this is line current.
- \(P\)
- Real load power Active power when the kW plus power-factor sizing mode is used.
- \(PF\)
- Power factor Ratio of real power to apparent power for the load represented by the entered kW.
- \(m\)
- Design or growth allowance User-selected additional percentage applied after connected load is calculated.
- \(I_{\text{FL} }\)
- Transformer full-load current Rated-side current calculated from a selected nominal transformer kVA and the corresponding side voltage.
Worked Example: 208 V, 150 A Three-Phase Load
This example reproduces the calculator’s default current-based case: a balanced three-phase 208 V load drawing 150 A, with a 20% user-selected planning allowance and a 480 V primary.
Calculate load and design requirement
The connected load is about 54.04 kVA. With the selected 20% allowance, the calculator’s design requirement becomes about 64.85 kVA.
Select the nominal rating and calculate current
The next Schneider Electric published three-phase low-voltage rating above 64.85 kVA is 75 kVA. At 75 kVA, the rated-side current is about 90.21 A at 480 V and 208.18 A at 208 V.
Result
75 kVA nominal transformer
The original 54.04 kVA connected load uses about 72.05% of the 75 kVA nominal capacity, leaving about 20.96 kVA of nominal capacity above that connected load. The 20% allowance is already reflected in the 64.85 kVA design requirement; it should not be applied a second time.
How to Interpret the Transformer Sizing Result
The headline kVA is only part of the answer. Check whether it is an exact requirement or a published nominal selection, then review loading, remaining capacity, and current at both transformer voltages.
Connected load is not transformer FLA
A 150 A load at 208 V three phase is about 54.04 kVA. If a 75 kVA transformer is selected, its 208 V full-load current is about 208.18 A. The 150 A entered load and the transformer’s 208.18 A nameplate-side capacity describe different quantities.
Sizing is linear until a nominal-size threshold
With phase, voltage, and allowance held constant, a 10% increase in load current produces a 10% increase in connected kVA and design kVA. The displayed nominal size changes only when that requirement crosses the next published rating.
Use a fast reasonableness check
For current-based sizing, connected kVA must rise when either voltage or current rises. For kW-based sizing, lowering power factor must increase required kVA. A result that moves in the opposite direction usually indicates the wrong phase, voltage convention, unit prefix, or power-factor entry.
From Calculated kVA to a Transformer Selection
A calculated kVA requirement is a minimum mathematical capacity for the represented load and allowance. A real transformer selection also has to match an available product rating, primary and secondary voltage, phase, frequency, winding arrangement, installation environment, and application duty.
Calculated requirement
This is the exact result of the load model after the user-selected allowance. For example, 64.85 kVA is a valid design requirement even though it is not a listed nominal rating in the calculator’s published low-voltage series.
Selected nominal capacity
With published-rating selection enabled, the calculator chooses the first Schneider Electric low-voltage rating in the selected phase series that is not below the requirement. Schneider’s published selection guidance also requires matching load voltage, system voltage, phase, frequency, and mounting or installation needs. Schneider Electric transformer selection criteria.
Motor Loads, Harmonics, and Other Real-World Factors
The basic kVA relationship is necessary, but some loads can require additional checks before the transformer is finalized. The calculator does not automatically convert those application effects into a universal oversizing factor.
Motor starting
Schneider Electric notes that motor starting current can be many times running current and can create excessive transformer voltage drop during acceleration. A transformer that is adequate for steady-state kVA can therefore still need a motor-starting voltage-drop check. Schneider Electric motor-load transformer guidance.
Nonlinear and harmonic loads
Electronic and other nonlinear loads can produce harmonic currents that increase transformer heating. Schneider describes K-factor as a weighting of harmonic load currents based on their heating effect. Do not assign K-4, K-13, or another harmonic-duty rating from kVA alone; verify the load characteristics and transformer manufacturer’s application guidance. Schneider Electric K-factor guidance.
Transformer impedance
Percent impedance does not replace the steady-state kVA sizing equation, but it materially affects short-circuit current and voltage behavior. When %Z is entered in the calculator, the optional fault-current output is only a transformer-limited terminal estimate.
Installation and product constraints
Available voltage combinations, enclosure, mounting location, ventilation, temperature, altitude, winding arrangement, taps, sound level, efficiency class, and manufacturer product scope can eliminate an otherwise plausible kVA selection. Verify the actual product data before procurement.
Transformer kVA Sizes and Full-Load Amps
The calculator’s published-rating option uses Schneider Electric’s current low-voltage rating lists. These are manufacturer-scoped reference values, not a universal claim that every transformer manufacturer offers every rating for every voltage and application.
| Phase | Published ratings (kVA) |
|---|---|
| Single phase | 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 200, 250, 333 |
| Three phase | 3, 6, 9, 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750 |
Source scope: Schneider Electric states that these lists apply to Square D / Schneider Electric low-voltage transformers. Published transformer kVA ratings.
| Transformer (kVA) | 208 V (A) | 240 V (A) | 480 V (A) | 600 V (A) |
|---|---|---|---|---|
| 15 | 41.6 | 36.1 | 18.0 | 14.4 |
| 30 | 83.3 | 72.2 | 36.1 | 28.9 |
| 45 | 124.9 | 108.3 | 54.1 | 43.3 |
| 75 | 208.2 | 180.4 | 90.2 | 72.2 |
| 112.5 | 312.3 | 270.6 | 135.3 | 108.3 |
| 150 | 416.4 | 360.8 | 180.4 | 144.3 |
| 225 | 624.5 | 541.3 | 270.6 | 216.5 |
| 300 | 832.7 | 721.7 | 360.8 | 288.7 |
| 500 | 1,387.9 | 1,202.8 | 601.4 | 481.1 |
| 750 | 2,081.8 | 1,804.2 | 902.1 | 721.7 |
The current table is derived from the transformer current relationship published by Schneider Electric. A kVA rating does not correspond to one fixed amp value; phase and voltage determine the current. Schneider Electric transformer current formula.
Common Transformer Sizing Mistakes
Most large sizing errors come from using the wrong electrical quantity or applying the same adjustment twice, not from the arithmetic itself.
Using line-to-neutral voltage in the three-phase formula
The calculator’s balanced three-phase current mode uses line-to-line voltage. Entering 120 V for a 208Y/120 V system or 277 V for a 480Y/277 V system understates the three-phase apparent load if the current represents the full three-phase line load.
Dividing voltage-current kVA by power factor
When kVA is calculated from voltage and current, apparent power has already been determined. Power factor belongs in the kW-to-kVA path; applying it again would overstate the transformer requirement.
Confusing panel rating with calculated load
A 200 A panel bus does not prove that the load is 200 A. Size from the load calculation or measured/design current that actually represents the equipment demand, then separately verify panel, feeder, and protection ratings.
Double-counting spare capacity
If the load has already been increased by a justified planning allowance, do not add the same margin again simply because the exact requirement falls between nominal product ratings. The next nominal size already creates additional capacity above the calculated requirement.
Treating transformer FLA as actual operating current
Full-load current corresponds to the transformer’s rated kVA at the stated voltage. A partially loaded transformer normally carries less current. Compare connected load current with transformer full-load current rather than substituting one for the other.
Using terminal fault current as downstream fault current
The impedance option gives a transformer-limited terminal screening result. Feeder, busway, connection, and finite-source impedance can materially change the current at a downstream panel or equipment location.
Assumptions, Limits, and Final Design Checks
This calculator is appropriate for preliminary transformer capacity selection and electrical cross-checking, but it is not a complete equipment specification, protection study, short-circuit study, motor-starting study, or code-compliance determination.
Balanced three-phase relationship
Three-phase current-based sizing assumes the represented load can be modeled with the standard balanced line-to-line voltage and line-current relationship. Significant phase unbalance needs a phase-by-phase review rather than only one \(\sqrt{3}\) calculation.
Power factor must match the entered kW
In kW mode, the power factor must describe the same load and operating condition as the real-power value. A stale or assumed power factor changes apparent power directly because \(S=P/PF\).
Published ratings are product-scoped
The nominal-size list used by the calculator comes from Schneider Electric low-voltage transformer guidance. Other manufacturers, medium-voltage equipment, special-purpose transformers, or custom designs may use different available ratings.
Final electrical design requires additional checks
Transformer voltage/configuration, frequency, grounding, overcurrent protection, conductor ampacity, voltage drop, interrupting duty, SCCR, installation environment, temperature/altitude, harmonics, inrush, motor starting, and manufacturer application limits are outside the basic kVA equation.
Sources and Calculation Verification
The calculator method, nominal rating lookup, motor-load caution, harmonic-load discussion, and transformer-limited fault-current relationship were checked against current Schneider Electric technical guidance. The worked example was also recomputed independently and reverse-checked from kVA back to current.
- Schneider Electric — Transformer kVA formula — supports single-phase, three-phase, and kW-with-power-factor sizing relationships.
- Schneider Electric — Standard transformer kVA ratings — supports the calculator’s Square D / Schneider Electric low-voltage nominal rating lists.
- Schneider Electric — Transformer current rating formula — supports conversion from transformer kVA and voltage to single-phase or three-phase rated current.
- Schneider Electric — Transformer selection for motor loads — supports the need to consider motor starting current and transformer voltage drop.
- Schneider Electric — K-factor and harmonic loading — supports the relationship between nonlinear-load harmonic current and transformer heating.
- Schneider Electric — Transformer available fault current — supports the simplified infinite-source secondary fault-current relationship based on full-load current and percent impedance.
Calculation check: the 208 V, 150 A three-phase example gives \(54.039985\ \text{kVA}\); multiplying by 1.20 gives \(64.847982\ \text{kVA}\). The next listed three-phase rating is 75 kVA. Reversing the current equation at 208 V gives \(208.179\ \text{A}\), which returns essentially 75 kVA when substituted back into the three-phase apparent-power equation.
Transformer Sizing Calculator FAQ
These questions address the transformer-size, kVA, current, power-factor, and impedance distinctions that most often change the practical answer.
What size transformer is needed for a 200 A, 208 V three-phase load?
The connected apparent load is \(\sqrt{3}(208)(200)/1000\approx72.05\ \text{kVA}\). A 75 kVA three-phase transformer is above that raw connected-load value in Schneider Electric’s published low-voltage rating series. If your project requires an additional planning allowance, apply it before selecting the nominal size; for example, a 20% allowance raises the design requirement to about 86.46 kVA, so 75 kVA would no longer meet that chosen requirement.
How many amps can a 75 kVA three-phase transformer supply?
It depends on the line-to-line voltage. Using \(I=75{,}000/(\sqrt{3}V_{LL})\), a 75 kVA three-phase transformer is approximately 208.2 A at 208 V, 180.4 A at 240 V, 90.2 A at 480 V, and 72.2 A at 600 V.
Do I always need to add 20% or 25% when sizing a transformer?
No universal planning percentage is automatically correct for every application. Use an allowance only when it is justified by the project load basis, expected growth, uncertainty, operating duty, or other design criteria. Motor starting, nonlinear loads, special equipment, and manufacturer requirements may require separate checks rather than a blanket percentage.
Why is transformer capacity sized in kVA instead of only kW?
Transformer electrical loading depends on voltage and current, so apparent power in kVA is the useful capacity quantity. When only real power is known, convert it with \(kVA=kW/PF\). For example, 100 kW at 0.80 power factor represents 125 kVA of apparent load.
Does transformer impedance change the required kVA size?
Percent impedance is not part of the basic steady-state kVA equation used to convert load voltage/current or kW/power factor into apparent power. It does matter for voltage behavior and available fault current, so it becomes important when the selected transformer is evaluated as part of the complete electrical system.
Can I size a transformer from the panel’s amp rating?
Only if that amp value genuinely represents the load you intend the transformer to serve. A panel bus or breaker frame rating is an equipment rating, not automatically the actual connected or calculated demand. Use the project load calculation or representative design current, then separately verify panel, conductor, and overcurrent-device ratings.