The primary (or input) voltage of a transformer must be matched closely to its supply voltage in order for it to operate correctly. If the transformer is to be used with a range of different supply voltages then it is possible to provide it with either a muliple-tapped primary or a number of independent primaries which may be connected in series or parallel to match the supply.

Some common arrangements are 0-115 V, 0-115 V or 0-220-240 V. The former provides two primaries which may be connected either in parallel to operate from a 115 V supply or in series for a 230 V supply. The latter provides for 220 V or 240 V operation.

An alternative arrangement offering greater flexibility is to provide one or more taps for fine adjustment to the other side of the zero tap.

E.g. 10-0-200-220-240 provides no fewer than six possible values for the primary voltage. The values 200, 220 and 240 V are available when the neutral is connected to the 0 V tap. Connecting the neutral to the 10 V tap makes 210, 230 and 250 V connections available.