How to Calculate Fuse Size for Transformers | Hunker How to Calculate Fuse Size for Transformers By Tim Walsh. SAVE; Transformers are devices that transfer voltage between circuits from one level to another through electrical induction. Transformers are primarily used to move high voltage to lower voltage for the purposes of power distribution. While transformers come in a variety of sizes for Distribution transformer - Wikipedia The transformer is always connected to the primary distribution lines through protective fuses and disconnect switches. For pole-mounted transformers this usually takes the form of a 'fused cutout'. An electrical fault causes the fuse to melt, and the device drops open to give a visual indication of trouble.