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The primary and secondary have an adjustable number of turns which can be selected without reconnecting the transformer.

An idéal transformer would have no losses, and would therefore be 100% efficient. In practice, energy is dissipated due both to the resistance of the windings known as copper loss or I2 R loss, and to magnetic effects primarily attributable to the core (known as iron loss méasured in watts per pound). Transformers are, in general, highly efficient. Large power transformers (over 50 MVA) may attain an efficiency as high as 99.75%. Small transformers, such as a plug-in "power brick" used to power small consumer electronics, may be less than 85% efficient.