Voltage Regulation Calculator

Enter a source's no-load and full-load voltage to calculate its voltage regulation (VR%), the voltage drop under load, and a qualitative regulation rating.

Quick Facts

Formula
VR% = (V_NL − V_FL) / V_FL × 100
V_NL is the no-load (open-circuit) voltage; V_FL is the terminal voltage under rated load.
Ideal value
0%
The output voltage would be unaffected by load — perfect regulation.
Typical transformers
1% – 6%
Larger power transformers trend lower due to lower internal impedance.
Negative regulation
Possible
A leading power-factor load can make full-load voltage exceed no-load voltage.

Your Results

Calculated
Voltage Regulation (% of full load)
-
VR = (V_NL − V_FL) / V_FL × 100
Voltage Regulation (% of no load)
-
Alternate basis: (V_NL − V_FL) / V_NL × 100
Voltage Drop
-
V_NL − V_FL, in your chosen unit
Regulation Quality
-
Qualitative rating based on |VR%|

Ready

Enter no-load and full-load voltage, then press Calculate.

How to Calculate Voltage Regulation

Voltage regulation (VR) describes how well a transformer, generator, or alternator holds its output voltage steady as electrical load is applied. Ideally, a source's terminal voltage would stay exactly the same whether it is supplying no current or its full rated current — but internal winding resistance and leakage reactance cause the terminal voltage to sag (or occasionally rise) as current flows. Voltage regulation quantifies that change as a percentage, letting you compare sources of different sizes on equal footing.

The voltage regulation formula, explained

The standard (IEEE/US) definition is:

VR% = (VNL − VFL) / VFL × 100

where VNL is the no-load voltage — the terminal voltage measured with the load disconnected — and VFL is the full-load voltage measured with the rated load connected. A smaller VR% means the source's output voltage is more stable across the full range of loading. This calculator also reports the alternate, no-load-basis version some references use, VR% = (VNL − VFL) / VNL × 100, which gives a slightly smaller number for the same two voltages.

What causes voltage drop under load

As current flows through a transformer's or generator's internal impedance (winding resistance plus leakage reactance), it produces an internal voltage drop, I × Z, that subtracts from the ideal output voltage. The size of that drop — and therefore VR% — depends on the load current magnitude and on the load's power factor. Lagging power-factor loads (most motors and inductive loads) increase VR, while leading power-factor loads (lightly loaded or capacitive lines) can reduce it or even flip its sign, producing a small voltage rise instead of a drop.

Reading your result: typical values and practical notes

  • Excellent (under 2%): typical of large power transformers with low internal impedance.
  • Good to fair (2–10%): common for small-to-medium distribution transformers and generators under normal loading.
  • Poor (over 10%): signals high internal impedance relative to the load, an undersized source, or a heavily loaded, long feeder — worth investigating before committing equipment to that circuit.
  • Always measure VNL and VFL at the same tap setting and frequency; comparing voltages taken under different conditions produces a meaningless VR%.

Frequently Asked Questions

What is voltage regulation in electrical engineering?
Voltage regulation (VR) measures how much a power source's output voltage changes between no load and full load. It is calculated as VR% = (V_NL − V_FL) / V_FL × 100, where V_NL is the open-circuit (no-load) voltage and V_FL is the terminal voltage under rated load. A lower VR% means the source holds its voltage steadier as load changes.
What is a good voltage regulation percentage?
For power transformers, 1-6% is typical, with small distribution transformers often near 2-5% and large power transformers closer to 1-3% due to lower internal impedance. Under 2% is considered excellent; above 10% usually signals high internal impedance or an undersized source for the load.
Can voltage regulation be negative?
Yes. If the load has a leading power factor — a lightly loaded or capacitive line, for example — the full-load voltage can exceed the no-load voltage, producing a negative VR%. This voltage rise under load is more common on lightly loaded, capacitive transmission lines.
Why do some references divide by no-load voltage instead of full-load voltage?
The full-load basis, VR% = (V_NL − V_FL) / V_FL × 100, is the standard convention used on most transformer nameplates and in most textbooks. Some references instead divide by V_NL, which gives a slightly different percentage for the same two voltages — always confirm which base a spec sheet uses before comparing numbers.

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