Voltage Regulation

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 $100 \times 20/460 \approx 4.35\%$. 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.