Transformer Voltage Drop Calculator
This transformer voltage drop calculator finds how much the secondary voltage falls when a transformer carries a load. Enter the kVA rating, the nameplate impedance, the X/R ratio, the load, and its power factor, and the transformer voltage drop calculator returns the drop in percent and volts, the voltage left at the load, and the rated and load currents.
Results
The Transformer Voltage Drop Calculator is part of our AC & Three Phase collection. Browse more ac & three phase to size, convert, and check your work alongside this tool.
What the transformer voltage drop calculator does
A transformer has internal resistance and leakage reactance, shown on the nameplate as a percent impedance. Under load, current through that impedance lowers the secondary voltage. How much depends on the load as a share of the rating and on the power factor, because inductive loads line up with the reactance. The transformer voltage drop calculator splits the impedance into resistance and reactance with the X/R ratio and applies the standard regulation formula.
Transformer voltage drop formula
The impedance splits into resistance and reactance:
With k = load ÷ rating, the voltage regulation is:
The second term is a small correction that matters at low power factors. Volts drop = secondary voltage × VD% ÷ 100.
Transformer voltage drop example
A 500 kVA, 480 V three phase transformer with 5% impedance and X/R = 4, carrying 400 kVA at 0.85 power factor:
The secondary sits at about 466 V under that load. The rated secondary current is 601 A and the load draws 481 A.
How to use the transformer voltage drop calculator
Enter the transformer kVA and the impedance from the nameplate, usually 2 to 6%. Keep an X/R ratio of 4 if you do not have one; small dry types run lower and large units higher. Enter the load in kVA, its power factor, and the secondary line voltage, then choose single or three phase. Press Calculate.
Reading transformer voltage regulation
A drop of 2 to 4% at full load is typical for distribution transformers. Add it to the cable voltage drop downstream: if the two together pass about 5%, motors start poorly and lights dim. Taps on the primary can raise the secondary by 2.5 or 5% to compensate. At light load the drop is small, so a tap set for full load can leave the voltage high at night.
When to use a transformer regulation calculator
- Checking voltage at a motor fed from a transformer
- Choosing a tap setting
- Comparing transformers with different impedances
- Adding the transformer drop to the cable drop
- Estimating voltage dip during motor starting
Common transformer voltage drop calculator mistakes
- Treating %Z as the drop. The drop depends on load and power factor.
- Ignoring the X/R ratio. It decides how power factor affects the drop.
- Forgetting cable drop. Add the feeder drop for the full picture.
- Using phase voltage in three phase. Enter line to line voltage.
Tips for transformer impedance voltage drop
- Read %Z from the nameplate, not a catalog average.
- Check regulation at the lowest expected power factor.
- Use primary taps to correct a steady low voltage.
Percent impedance: a trade-off
A low impedance gives a stiff supply with little voltage drop, but it also lets more current flow in a fault. A high impedance limits fault current but drops more voltage under load. That is why the same %Z appears in the fault current calculator: available fault current is roughly the rated current divided by %Z. Most designers accept 5 to 6% on medium transformers as a balance.
Secondary voltage drop from source to load
The transformer drop is only part of the total. Feeder and branch circuit cables add their own drop, which the voltage drop calculator works out from length and wire size. If the total is too high, a larger transformer lowers k, a lower-impedance unit lowers R and X, or a tap change raises the starting voltage. Size the rating with the transformer size calculator before checking regulation.
Transformer voltage drop at full load (X/R = 4)
| Impedance | PF 1.0 | PF 0.9 | PF 0.8 |
|---|---|---|---|
| 2% | 0.50% | 1.29% | 1.56% |
| 3% | 0.77% | 1.95% | 2.35% |
| 4% | 1.05% | 2.61% | 3.14% |
| 5% | 1.33% | 3.28% | 3.93% |
| 5.75% | 1.55% | 3.78% | 4.53% |
| 6% | 1.62% | 3.95% | 4.73% |
Transformer Voltage Drop Calculator FAQ
How do you calculate transformer voltage drop?
Split %Z into R and X with the X/R ratio, then VD% ≈ load fraction × (R cos φ + X sin φ). For 5% Z at 80% load and 0.85 PF it is about 2.9%.
Is transformer voltage drop the same as impedance?
Only at full load with a power factor matching the impedance angle. At unity power factor the drop is much smaller than %Z.
What is a normal transformer voltage regulation?
About 2 to 4% at full load for most distribution transformers.
Why does power factor change the drop?
Inductive load current lines up with the transformer reactance, so lagging power factor increases the drop.