Transformer Voltage Drop Calculator and Formula

A transformer voltage drop calculator estimates the voltage lost in the transformer’s windings when it supplies a load. Enter the transformer rating, secondary voltage, load current, and impedance percentage to get the approximate voltage drop and loaded secondary voltage; the result is an estimate, not a substitute for equipment specifications or a complete circuit study.

Formula

This calculator uses transformer percentage impedance as an approximate voltage-drop percentage at rated current. It scales that percentage in proportion to the entered load current and, as a simplified estimate, to the load power factor:

Rated current I_rated = (S × 1,000) / (n × V_s)

Approximate drop ΔV = V_s × (Z% / 100) × (I / I_rated) × PF

Approximate loaded voltage V_load = V_s − ΔV

• S is transformer rating in kilovolt-amperes (kVA).
• V_s is nominal secondary voltage in volts (V).
• Z% is nameplate impedance as a percentage.
• I is load current in amperes (A).
• PF is power factor, entered from 0 to 1.
• n is 1 for single phase or 3 for three phase. For a three-phase transformer, the voltage entered is line-to-line voltage.

The calculation treats the impedance percentage as a direct estimate of voltage drop at unity power factor, then applies a power-factor multiplier. Actual voltage regulation depends on the resistive and reactive parts of impedance and whether the load is leading or lagging. This simplified method is useful for a quick estimate, but it is not an exact regulation calculation.

Worked Example

Using the calculator’s defaults: a 25 kVA, three-phase transformer has a 480 V secondary, 5.75% impedance, a 30 A load, and a 0.8 power factor.

1. Find rated secondary current: 25 × 1,000 / (3 × 480) = 17.36 A.
2. Find the current as a fraction of rating: 30 / 17.36 ≈ 1.728.
3. Estimate the drop: 480 × 0.0575 × 1.728 × 0.8 ≈ 38.08 V.
4. Estimate loaded voltage: 480 − 38.08 = 441.92 V.

The calculator therefore displays an estimated drop of 38.08 V, loaded voltage of 441.92 V, and rated current of 17.36 A. The entered current is above the calculated rated current, so this example illustrates the arithmetic rather than suggesting that the transformer should be operated at that load.

How to Use the Calculator

• Enter the transformer’s kVA rating and the secondary voltage from its nameplate or documentation.
• Enter the expected load current, using the secondary side and the same phase system selected in the drop-down.
• Enter impedance as a percentage: for example, 5.75% is entered as 5.75, not 0.0575.
• Enter power factor as a decimal, such as 0.8 for 80%, and select single-phase or three-phase operation.
• Read the estimated voltage drop first, then the approximate loaded voltage and the transformer’s calculated rated secondary current.

For single-phase calculations, use the nominal voltage across the secondary terminals. For three-phase calculations, use line-to-line voltage and line current. Do not combine a phase-to-neutral voltage with a line-current formula or mix primary-side current with secondary voltage.

What the Result Means

The drop is the estimated difference between nominal secondary voltage and voltage under the specified load. A larger current generally means a larger drop in this simplified model. The loaded-voltage result is simply the entered nominal voltage minus that estimate; it does not account for voltage variation on the primary supply, cable and connection losses, temperature, harmonics, or changing loads.

Transformer nameplate impedance describes the voltage needed to circulate rated current under specified test conditions, expressed as a fraction of rated voltage. It is also important in fault-current calculations, but this calculator is not a fault-current or protection-sizing tool. The impedance percentage alone does not describe every aspect of voltage regulation.

Limits, Accuracy, and Common Mistakes

• Power factor is not the whole regulation calculation. For a more accurate estimate, use the transformer manufacturer’s resistance and reactance data, load power factor, and whether the load is leading or lagging. A detailed calculation resolves voltage drop into resistive and reactive components.
• Do not treat the output as a permissible voltage limit. Acceptable operating voltage depends on the connected equipment and applicable project requirements. Have a qualified electrical professional assess the complete installation.
• Check units and phase quantities. The calculator expects kVA, volts, amperes, and a decimal power factor. Entering VA instead of kVA or a percentage such as 80 instead of 0.8 gives an invalid input or misleading result.
• Account for the rest of the circuit. Conductors, connections, upstream supply variation, and other loads can add voltage change beyond the transformer’s own regulation.
• Use the correct transformer data. Impedance can differ between units, even at the same kVA rating. Use the nameplate or manufacturer documentation rather than assuming a typical percentage.

The calculator reports volts, so its arithmetic works in either metric or US customary installations once the voltage is expressed in volts and current in amperes. It does not calculate conductor voltage drop; that requires conductor length, material, size, temperature, installation conditions, and circuit configuration.

Frequently Asked Questions

How do you calculate transformer voltage drop?
For a quick estimate, multiply secondary voltage by impedance as a decimal and by the load-current fraction of rated current. This calculator also applies a simplified power-factor multiplier; an accurate regulation calculation needs resistance and reactance data.

What does transformer impedance percentage mean?
It is the percentage of rated voltage associated with producing rated current under the transformer’s specified impedance test conditions. It is useful for estimating regulation and fault current, but it is not by itself a complete description of loaded voltage.

Can this calculator estimate voltage drop on transformer cables?
No. It estimates the transformer’s own drop using its impedance percentage. Cable drop is a separate calculation that needs conductor and circuit details.

Should I use line-to-line or phase voltage for a three-phase transformer?
Use line-to-line secondary voltage with the three-phase selection and enter line current. Using phase-to-neutral voltage in that combination produces an incorrect rated-current result.

Why might measured voltage differ from the calculator?
The primary supply may not be at nominal voltage, and real regulation depends on impedance components, load power factor, temperature, conductors, connections, and load changes. The calculator is a simplified estimate, not a prediction of every operating condition.

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in kva | Transformer rating | kVA | 25 | 0.001
in vs | Secondary voltage | V | 480 | 0.001
in pct | Impedance | % | 5.75 | 0 | 100
in load | Load current | A | 30 | 0
in pf | Load power factor | - | 0.8 | 0 | 1
sel ph | Phase system | Single phase=1, Three phase=3
out drop | Transformer voltage drop | V | vs * pct / 100 * load / (kva * 1000 / (ph * vs)) * pf | 2
out loaded | Approximate loaded voltage | V | vs - drop | 2
out rated | Rated secondary current | A | kva * 1000 / (ph * vs) | 2