All Transformer windings have an internal resistance due to the length of the copper wire making up that winding.

From "Ohms Law" we know that when a current is passed through a resistance, there will be a potential difference or "Voltage Drop" across that resistance.

This is calculated as V = I.R. , where V=Voltage Drop , I=Current , R=Resistance.

Applying this logic to the secondary winding of a transformer, we can see that when no current is flowing in the winding, there will be no voltage drop across the winding. In other words, if we measure the No-Load output voltage of a secondary winding, we will be measuring the actual induced voltage (induced EMF).
However, once we connect a load across this winding, current will flow, and there will be a voltage drop across the winding. Hence, the voltage measured "On-Load" will be smaller than the voltage measure at "No-Load".
This voltage difference is known as "Regulation" and is normally expressed as a percentage of the full-load voltage.

Secondary winding designed to provide 10 Volts when delivering its rated current.
If we assume a no-load voltage of 11 Volts, we can then calculate :

Regulation = (11 - 10) / 10 = 1 / 10 = 0.1 = 10%

*NB* Some transformer manufacturers will divide by the higher figure (No-Load voltage) instead of dividing by the smaller figure. This gives a false lower regulation value.

Typical regulation figures for transformers from 15VA to 5000VA are shown below.

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