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How to transformer taps adjust voltage?

The ratio between the number of windings in the primary and secondary coil’s of a transformer determines the voltage ratio. If a transformer has 100 primary windings and 25 secondary windings, the ratio is 100:25 = 4:1. If the primary is fed with 480VAC, the ratio of primary to secondary voltage is 4:1 or 480:120. Taps are used to adjust for voltage differences at the transformer’s primary winding.A 5% adds or subtracts 5% of the windings.