Increasing the voltage on step-up transformers is necessary in order to reduce losses in the process of power transmission. Our energy is equal to the product of voltage and current and time. And the losses in the wire are equal to the product of the square of the current and the resistance of the wire and time. Thus, by increasing the voltage using a transformer, we reduce the current, and hence the losses in the wire.
Step-down transformers are used to step down the voltage and to allow the use of low-voltage load consumers.
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