The answer first: voltage regulation is how much your secondary voltage sags under load — typically 4–6% for a distribution transformer, and set almost entirely by the impedance voltage (Uk%)
Voltage regulation is the percentage by which a transformer's secondary voltage falls as you go from no load to full load, at a constant primary voltage and a constant power factor. For a standard oil-immersed distribution transformer that sag is normally 4–6%, and it is not something you can design away — it is the price the unit pays for having internal impedance (resistance plus leakage reactance). The useful part is that it is predictable: you can calculate it from the impedance voltage (Uk%) printed on the nameplate, the load current, and the load power factor, and you can compensate for it with tap changers. If your load is largely inductive — motors, compressors, pumps — expect the voltage to sag more than a purely resistive heating load, because it is the transformer's reactance (not its resistance) that does most of the dropping.
Key takeaways
- Voltage regulation is the no-load to full-load voltage drop, in percent. For distribution transformers it is typically 4–6%; the exact number is a design choice, not an accident.
- It is set by the impedance voltage Uk%. A unit with Uk% = 6% sags roughly 6% at full load at unity power factor — the nameplate impedance is your first-order prediction of regulation.
- Power factor decides the rest. Inductive loads (motors) drop voltage far more than resistive loads; a leading (capacitive) power factor can even raise the secondary voltage.
- Tap changers compensate, they do not eliminate. Off-load taps (±2×2.5%) and on-load taps (±10% or more) shift the whole ratio, but they correct the average level, not the load-dependent sag at the customer end.
- There is a trade-off with fault current. Lower Uk% gives tighter regulation but higher short-circuit current; higher Uk% limits fault current but worsens regulation — the same tension explained in our short-circuit calculation guide.
What voltage regulation actually measures
Voltage regulation is defined as the change in secondary voltage from no load to full load, expressed as a percentage of the full-load voltage, with the primary held at rated voltage and the power factor constant:
Voltage regulation % = (Vno-load − Vfull-load) ÷ Vfull-load × 100
A concrete example: a 400 V transformer that reads 400 V at no load and 380 V at full load has a regulation of (400 − 380) ÷ 380 × 100 = 5.26%. That number matters for two reasons. First, it tells you how much voltage your downstream equipment will actually see when the transformer is working hard. Second, it is a direct measure of the unit's internal impedance — which is why the definition is baked into the rated-voltage framework of IEC 60076-1, the general requirements standard for power transformers. In practice, most buyers do not compute regulation from scratch; they read the impedance voltage Uk% off the nameplate and use it as a stand-in, because the two quantities are nearly the same at unity power factor.
Why the voltage drops: resistance, leakage reactance, and the drop formula
Inside the transformer, every amp of load current has to push through two things: the winding resistance (R) and the leakage reactance (X) — the magnetic flux that escapes the core and links only one winding. Together they form the transformer's short-circuit impedance. The per-unit voltage drop caused by that impedance, for a load at power factor cos φ, is approximately:
ΔU% ≈ ukr% × cos φ + ukx% × sin φ
Here ukr% is the resistive component — equal to the load (copper) loss divided by the rated kVA, e.g. a 1000 kVA unit with 10.5 kW load loss has ukr% ≈ 1.05% — and ukx% is the reactive component, which is the dominant one. For a typical distribution transformer the reactance is 5 to 10 times the resistance, so most of the voltage drop is reactive. That single fact drives every practical consequence that follows.
The impedance voltage (Uk%): the one number that sets your regulation
The impedance voltage Uk% is the primary voltage, expressed as a percentage of rated voltage, that must be applied to circulate full-load current in a transformer whose secondary is short-circuited. It is measured in a routine test and stamped on the nameplate, and it is the single most useful number for predicting both voltage regulation and short-circuit current. Typical values for distribution transformers under IEC 60076 and IEEE C57.12.00 are:
- 4% — common for smaller distribution units (roughly up to 630 kVA).
- 6% — standard for larger distribution units (about 1000–2500 kVA).
- 8–10% — specified when you want to limit fault current (e.g. feeding a low-impedance network or multiple feeders) at the cost of looser regulation.
This is a genuine trade-off, and it is the reason a transformer cannot be "good at everything": a low impedance gives you a stiff, well-regulated voltage but lets a huge fault current flow, while a high impedance limits fault current but lets the voltage sag noticeably under load. The full logic of how Uk% caps the available short-circuit current is laid out in the three-step short-circuit calculation.
Power factor changes everything: resistive, inductive and capacitive load
Because the drop formula weights the resistive and reactive components by cos φ and sin φ, the kind of load matters as much as its size. The table below shows the approximate full-load regulation of a 6% Uk distribution transformer under different load power factors (indicative values; the resistive component is taken as ~1%):
| Load power factor | Typical load | Approx. full-load regulation |
|---|---|---|
| 1.0 (unity) | Resistive heating, lighting | ~1–2% |
| 0.9 lagging | Mixed commercial load | ~3–4% |
| 0.8 lagging | Motors, pumps, compressors | ~4.5–5% |
| 0.5 lagging | Motor starting (locked rotor) | ~6–7% |
Two consequences follow. First, a plant full of motors will see a noticeably softer voltage than a pure heating load on the same transformer — which is why the load's power factor, not just its kVA, belongs in your sizing decision (our sizing formula treats it as an input). Second, a leading (capacitive) power factor — from capacitor banks or long lightly-loaded cables — makes the reactive term negative, so the secondary voltage can actually rise above the no-load value at light load. That is why over-correcting power factor can push voltage out of the acceptable band.
How tap changers compensate — and why they are not a substitute for good design
A tap changer moves the effective turns ratio in steps, shifting the whole output-voltage curve up or down. An off-load tap changer (typically ±2 × 2.5%) is adjusted only with the unit de-energized and corrects for a fixed primary-voltage level; an on-load tap changer (OLTC) (typically ±10% or more in 1.25% steps) adjusts while energized and can track seasonal or daily variation. The distinction — and the 10–20% price premium an OLTC adds — is covered in detail in the on-load vs off-load tap changer guide.
What a tap changer cannot do is fix the load-dependent drop at the point of use. The voltage that reaches your motor is reduced twice: first by the transformer's own regulation, then by the voltage drop in the low-voltage feeder cable. A tap changer only corrects the first, at the transformer terminals. If your load sits at the end of a long feeder, or a large motor dips the bus on every start, the right fix is a dedicated automatic voltage regulator at the point of use — a device that holds the output voltage within a tight band regardless of load.
How to specify voltage regulation for your project
- State the impedance voltage you need. If voltage stiffness matters (motor starting, sensitive electronics), specify a lower Uk% (e.g. 4%) and accept the higher fault current; if fault limitation matters, specify 6–8%.
- Know your load power factor. A 0.8 lagging industrial load sags nearly twice as much as a resistive load on the same unit — put the real power factor, not unity, into your calculation.
- Add tap range for the primary-side variation. If your grid voltage varies, order a wider tap range (e.g. ±2 × 2.5% off-load, or on-load ±10%) so you can keep the secondary inside the IEC 60038 supply-voltage band (typically −10%/+6% or tighter, depending on the utility).
- Treat the feeder as part of the system. Calculate the combined transformer-plus-cable drop; a well-regulated transformer cannot save a feeder that is too long or too thin.
- Ask for the test report. The measured impedance voltage and load loss on the routine test report let you compute the exact regulation of your unit instead of relying on a catalogue value — the same discipline behind the transformer selection guide.
Sources
Source: IEC 60076-1 — Power transformers — Part 1: General (rated voltage, impedance voltage and regulation definitions)
Source: IEC 60076-5 — Power transformers — Part 5: Ability to withstand short circuit (impedance voltage and short-circuit withstand)
Source: IEC 60038 — IEC standard voltages (nominal and declared supply voltage bands)
Source: IEEE C57.12.00 — IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
Source: Industry experience — QDTB Engineering (typical impedance-voltage and regulation values, and selection guidance)