tapping

A tap is a physical connection point soldered or welded to a transformer winding at some point between its two end terminals. Instead of using only the full coil length, a tap lets you access an intermediate voltage. If a transformer winding runs 480 volts end to end, a tap halfway along the wire might deliver 240 volts, or a tap at 80 percent might give 384 volts. The number and placement of taps determine what secondary voltages a single transformer can produce.

Taps serve load-matching and voltage regulation. A three-phase distribution transformer might have five taps spaced 2.5 percent apart, ranging from minus 10 percent to plus 10 percent of nominal voltage. Field workers adjust which tap is connected to the load to compensate for voltage drop over long feeder lines or to step voltage up or down within a narrow band. On some older equipment, changing taps requires de-energizing and manually moving a jumper or selector switch. Modern load-break switches and automatic tap changers under load (OLTC) allow tap adjustment while the transformer is live.

Tap configurations and constraints

A transformer winding is physically divided by tap leads; the wire is cut and each segment brought out as a separate terminal. Taps are most common on primary (input) windings because regulation of input voltage is easier than load-side voltage. Some designs use taps on both primary and secondary, but this multiplies complexity and cost. High-voltage power transformers may have 21 or more taps in 1.25 percent increments. Low-voltage control transformers might have just two or three. The tap lead must carry the full current of its winding segment, so wire gauge must account for the entire transformer load, not just the selected tap.

Corrosion and oxidation at tap connections cause chronic problems. If a tap terminal loosens, resistance rises, heating increases, and the connection can burn. Utilities inspect tap connections during maintenance and retorque terminals. Poor tap design with exposed copper and no sealant allows moisture ingress and green oxide formation. Industrial transformers in damp environments benefit from conformal coating or shrouded tap blocks.

The word tap comes from its mechanical origin: the switchman's hand-operated selector tap physically tapped from one connection point to another, like a phone switchboard. Today it persists as the name for any intermediate voltage point, whether switched manually, by servo drive, or by electronic control. Understanding available taps is essential when specifying a transformer, because you cannot add a tap to a finished unit without unwinding the coil.