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Voltage Regulation

Also known as percent regulation · transformer regulation · generator regulation · no-load to full-load droop

%VR=100 (Vnl−Vfl)Vfl\%VR = \frac{100 \, (V_{nl} - V_{fl})}{V_{fl}}

Worked example: 480 V no load, 460 V full load → 4.35% — press Try an example to run it live, then adjust anything.

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See also
Transformer Percent-Impedance Voltage Drop — Predicts the same sag before it is built, from the nameplate percent impedance.Percent Voltage Drop — The cable-side equivalent — volts lost in the run rather than inside the source.
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Voltage Regulation explained

VnlVfl%VR

Regulation is the answer to "how much does the voltage sag when I actually use it?" — the gap between no-load and full-load terminal voltage, as a percentage of the full-load figure. A transformer reading 480 V unloaded and 460 V at rated load regulates at 100×20/460≈4.35%100 \times 20/460 \approx 4.35\%. Small is good: a stiff source barely notices the load, a soft one wanders.

Note the denominator, because it is the standard mistake. Regulation is referenced to the full-load voltage, not the no-load one, since full load is the condition the equipment must work in. Dividing by 480 instead of 460 gives 4.17%, and while the difference looks trivial here, it grows fast on a poorly regulated source and it will not match anyone else's numbers.

Different machines carry wildly different figures for good reasons. A distribution transformer might regulate at 2–4%; a welding transformer is designed for terrible regulation on purpose, because a drooping characteristic is what keeps arc current stable as the operator's hand wobbles. Regulation can also go negative — a generator or transformer feeding a leading power factor can deliver more volts under load than without it, which is why the sign, not just the size, matters on circuits with capacitor banks or a lot of inverter-based generation.

Voltage Regulation formula

%VR=100 (Vnl−Vfl)Vfl\%VR = \frac{100 \, (V_{nl} - V_{fl})}{V_{fl}}
Where
  • %VR\%VR= Voltage regulation (%)
  • VnlV_{nl}= No-load voltage (V)
  • VflV_{fl}= Full-load voltage (V)

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