What are Primary Voltage Taps?
In some cases, the actual supply voltage to the primary of the transformer is either slightly higher or lower than the nameplate rating. Taps are provided on most transformers on the primary winding to correct this condition and maintain full rated output voltage and capacity. Standard taps are usually in 2 1/2% or 5% increments.
Example: The transformer has a 480V primary rating and the incoming voltage is at 504V. The primary connection should be made at the +5% tap in order to maintain the nominal secondary voltage.