Why do larger transformers, rated 100 kVA and above, typically have four external secondary bushings?

Larger transformers, particularly those rated at 100 kVA and above, typically have four external secondary bushings to accommodate larger internal leads and prevent contaminants from entering the transformer. The increased capacity necessitates much larger conductors to handle higher current loads safely and efficiently. These larger leads require additional space and secure attachment points, which the bushings provide.

Moreover, the bushings play a crucial role in protecting the internal components of the transformer from environmental factors. With four bushings, there is enhanced sealing and insulation, reducing the risk of moisture and other contaminants compromising the internal components. This design enhances the overall longevity and reliability of the transformer, which is particularly important in applications where consistent power supply is critical.

In contrast, the other choices do not accurately represent the primary reason for this design feature. While easier power distribution might be an outcome of having more bushings, it is not the main function. Aesthetic reasons are rarely a design priority for such functional equipment, and increasing voltage output does not relate to the number of bushings, as voltage is primarily determined by the winding ratios within the transformer itself.