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Voltage Regulation

Voltage regulation measures how much a source's voltage drops when you start drawing current from it. It compares the no-load voltage (when nothing is connected) with the full-load voltage (when the rated load is connected). A small percentage means the voltage stays steady, which is good. For example, if a generator outputs 240 V at no load and 230 V at full load, the regulation is about 4.3%.

Written by the Vorzep Team

Power Systems Transformers

Voltage Regulation Calculator

Voltage regulation tells you how much a supply's output voltage falls when the load is connected. Enter any two of VR, no-load voltage VNL and full-load voltage VFL and the third is solved. Add the full-load current to estimate the source resistance and losses.

VR = (VNL − VFL) / VFL × 100%Rs = (VNL − VFL) / IFL
Solve for
Full-load current IFL
A
0 or more.
Acceptable regulation limit
%
0 or more.
—
Voltage regulation
—

Step-by-step solution

Companion quantities

Equivalent circuit

A real supply behaves like an ideal source at VNL in series with an internal resistance Rs. The load current drops voltage across Rs.

Terminal voltage versus load current

Reference data

Typical voltage regulation (approximate)
SourceTypical VRNote
Common service voltage tolerances
Standard / caseNominalAllowed range
ANSI C84.1 Range A (US service)120 V114 to 126 V (±5%)
ANSI C84.1 Range B (US, temporary)120 V110 to 127 V
EN 50160 (European supply)230 V207 to 253 V (±10%)
USB 5 V supply5 V4.75 to 5.25 V (±5%)

Ranges are typical and vary by device and region. Always check the applicable standard or datasheet.

FAQ

What does a low voltage regulation mean?

A low VR means the output voltage stays close to its no-load value as load rises, so the supply is "stiff". A good regulated supply has under 1%, while an unregulated transformer-rectifier supply may be 5 to 15%.

Which voltage is the base, VFL or VNL?

The usual power-engineering definition divides by the full-load voltage, as used here. Some texts divide by VNL, giving a slightly smaller number. The calculator shows both, so you can match your source.

Can voltage regulation be negative?

Yes. With a leading (capacitive) load, the voltage at the load can be higher than the no-load voltage, so VFL > VNL and VR is negative. This is related to the Ferranti effect on lightly loaded lines.

How do I improve regulation?

Reduce source and cable resistance (larger conductors, shorter runs), use a stiffer transformer or generator, add feedback regulation (a linear or switching regulator, an AVR on a generator), or use tap changers to track the load.

Educational tool only. The model uses a single resistive source drop at one operating point. Real regulation also depends on load power factor, reactance, temperature and frequency. Follow the applicable electrical code and standards for any real installation.

Variables

Symbol Quantity Unit
VR Voltage regulation (percentage) %
V_NL No-load voltage (output voltage with zero load current) V
V_FL Full-load voltage (output voltage at rated load current) V

What it means

Voltage regulation (VR) is a measure of the change in output voltage of a power supply or generator from no‑load to full‑load, expressed as a percentage of the full‑load voltage. The formula is VR = (V_no‑load − V_full‑load) / V_full‑load * 100%. A lower VR indicates a more stable supply, which is important for sensitive equipment. Voltage regulation is a key specification for voltage regulators, transformers, and generators. It accounts for the internal resistance of the source and the drop across leads. In power systems, good regulation ensures that loads receive voltages within acceptable limits. Example: A supply has no‑load voltage 120V and full‑load voltage 114V. VR = (120−114)/114 * 100 = 6/114*100 ≈ 5.26%. This indicates a 5.26% drop under load.

Worked example

Voltage Regulation – Practical Example

Real‑World
Scenario: A small generator produces 120 V at no load and 115 V at full load. Calculate the voltage regulation.
ParameterValue
VNL120 V
VFL115 V
FormulaVR = (VNL − VFL) / VFL × 100%
1Difference: 120 – 115 = 5 V
2Divide by full‑load: 5 / 115 = 0.04348
3Convert to %: 0.04348 × 100 = 4.35%
Final Design VR = 4.35% ✓ Acceptable regulation
Why: Lower regulation means better voltage stability – 4.35% is typical for good generators.

Common mistakes

  • No‑load voltage: The output voltage when no load is connected (or minimum load).
  • Full‑load voltage: The output voltage at rated load.
  • Regulation: A lower percentage indicates better voltage stability.
  • Sign: Typically V_NL > V_FL (positive regulation).
  • Power factor: Voltage regulation depends on load power factor.

Applications

Voltage regulation (VR) measures the change in output voltage from no‑load to full‑load as a percentage of the full‑load voltage. It indicates how well a power supply or transformer maintains its output voltage under varying load. A lower VR means a more stable supply. Engineers use this parameter to design regulated power supplies, to specify transformers, and to assess the quality of distribution networks. By calculating VR, they can ensure that the voltage delivered to equipment remains within acceptable limits. This formula is essential for power quality and reliability. Understanding voltage regulation helps in troubleshooting voltage fluctuations.

  • Transformer and generator voltage stability analysis
  • Design of voltage‑regulated power supplies
  • Power quality assessment in distribution systems
  • Sizing of automatic voltage regulators (AVR)
  • Educational understanding of load regulation

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  • Published July 10, 2026
  • Last updated August 26, 2026