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A center tapped transformer is a transformer with a tap in the middle of the secondary winding, usually used as a grounded neutral connection, intended to provide an option for the secondary side to use the full available voltage output or just half of it according to need. This is often done in North American residential wiring.

o--+  +--o    \
   3||E   115  |
   3||E--o     > 230
   3||E   115  |
o--+  +--o    /

A tap on a transformer is an additional connection somewhere in the middle of the winding, used either in conjunction with or instead of the connections at the ends of the winding to provide different winding ratios. A center tap is a special case in which the tap is halfway between the ends of the secondary winding, such that it can be used to split the voltage output in half.

In the UK, a 400 VAC three-phase line is used for power distribution. This makes it convenient to supply power to UK homes — just run one leg into the house and the line-to-neutral voltage is
400 VAC ÷ √3 = 230 VAC.
That is, in a three-phase system, line-to-line voltage is the line-to-neutral voltage multiplied by √3. Homes in the UK use 230 VAC for all their electrical needs (except applications which provide their own transformers such as shaver sockets and low-voltage halogen lighting).

In North America, the distribution voltage is much higher. A transformer, often a pole pig, is required to bring the voltage down from three-phase (typically) 13.8kV to a safer level to be used in the house. Complicating the matter is that 115 VAC is used for lower power electrical needs while 230 VAC is used for higher power applications such as ovens and air conditioners.

In order to supply both of these voltage levels, the transformer is center tapped. The circuit breaker panel in the house will use only one side of the center tap for 115 VAC needs, completing the circuit with the neutral connection on the center tap itself. Both ends, with a double-pole circuit breaker, are used to provide 230 VAC power from the extreme ends of the transformer windings, bypassing the center tap. In order to balance the load on the transformer, alternating sides of the center tap are used for 115 VAC power. An unbalanced transformer will heat unevenly, shortening both its lifespan and maximum power output capability.

Not all 115 VAC power in the US is run off of center tapped transformers. In certain industrial and commercial settings a three-phase system similar to the UK system is used. The power is provided at 208 VAC line-to-line, giving 120 VAC line-to-neutral. However it is important to remember that three-phase systems like this share a single neutral connection for all three legs, so it may be necessary to plan for third harmonics depending on the application.

Center tapped three-phase installations are also available by center tapping one phase of a three-phase transformer. This offers efficient three-phase 230 VAC power and convenient single-phase 115 VAC power in the same package.

Dual-voltage power systems are generally referred to by a format such as 230/115 or 400/230. The number before the slash is the line-to-line (L-L) voltage and the number after the slash is the line-to-neutral (L-N) voltage. If the L-L voltage is twice the L-N voltage, then it is provided by a center tapped transformer. If it is slightly less, it is three-phase power.

Other uses for multi tap transformers include modular design, buck-boost applications, and voltage regulation. For example, a computer power supply intended to be sold in multiple countries will have a multi tap transformer to switch the primary side from accepting 115 VAC to 230 VAC and still provide the same output on the secondary side. A voltage regulating transformer has a voltage sensor which detects when the incoming voltage is too high or too low, and automatically switches the taps in order to keep the secondary side voltage close to the expected values.

Note: Voltage levels are approximate. In North America, wall voltage could be anywhere from 110 VAC to 120 VAC depending on many factors.

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