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Transformer Taps, Full Capacity and Why They Matter
Taps let the primary winding be adjusted so the secondary lands at the right voltage when the incoming supply runs high or low. The common arrangement is two steps above and two below nominal at 2.5 percent each. Full capacity means the unit still carries its full kVA on any tap, and that is the part a quote often leaves out.
What taps do
The incoming primary voltage is rarely exactly nominal. It sags at the end of a long feeder and runs high near a substation. Taps change the primary turns slightly so the secondary still lands where it should.
The standard arrangement
Two steps above and two below nominal, each 2.5 percent, giving a range of plus or minus 5 percent. On a 12470 volt primary each step is about 312 volts.
Full capacity, and why the phrase matters
Full capacity taps mean the transformer still delivers its full kVA on any tap position. Reduced capacity taps derate the unit off nominal. A quote that says only "with taps" has not told you which, and the difference is real capacity you either have or do not.
When a manufacturer offers a unit built at one voltage "with a tap" for another, ask two questions. Are the taps full capacity, and how many steps above and below nominal. Those two answers decide whether one unit genuinely covers both systems.
De-energised only
Taps on a distribution pad-mount are changed with the unit de-energised, using an external handle. They are not a load tap changer and cannot be moved under load.
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