Some transformers compensate for line drop and percent voltage regulation by having multiple taps by which the volts/turn ratio can be adjusted.
In the image below, we have a transformer whose secondary is open. For all intents and purposes, there is very little current flowing in the primary (in this case we will say it is negligible). The .25Ω resistance in the primary is the resistance of the winding. In this example, the supply voltage and the terminal voltage will be the same value. There is a potential secondary voltage of 120 volts but there is no current flowing in the secondary.
When current flows in the secondary circuit, things start to change. Total line drop in the primary circuit is 150 V due to the volt drop across the resistance of the primary circuit (75 V+ 75 V). This means that the terminal voltage applied to the primary of the transformer is only 1050 V. With the 10:1 turns ratio on the transformer, the secondary voltage is 105 V. This is 15 V lower than is desired.
To compensate for the line drop, it would be lovely if we could reduce the turns ratio of the transformer to 9.5:1. Some transformers provide taps to do that.
 Any increase in the number of turns in the primary will reduce the volts per turn ratio, thus lowering the secondary voltage.
 Any decrease in the number of turns in the primary will increase the volts per turn ratio, thus increasing the secondary voltage.
 The tap percentages indicate the percentage of the full turns that are effective when the tap is used.
 When the taps are in the primary winding, the new turns ratio can be calculated by multiplying this percentage by the full turns ratio:
Example:
 Consider a 360 kVA 1200V – 120V transformer.
 It has a turns ratio of 10:1 on the 100% tap.
Tap percentage

Equation

New ratio

New secondary voltage (at 1200V)

95%

10 x 0.95 =

9.5:1

126.3 V

97.5%

10 x 0.975 =

9.75:1

123 V

100%

10 x 1 =

10:1

120 V

102.5%

10 x 1.025 =

10.25:1

117 V

Video Alert!
This video shows how to use a multitap transformer to boost secondary voltage.
Attributions
Multitap transformer video by The Electric Academy is under a Creative Commons Attribution License.