How to Calculate Fuse Size for Transformers

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 use in small electronics to wide-scale local and national power distribution, all operate under the same principles. Electrical fuses are an over-current protection device designed to prevent an overload between a power source and a circuit such as a transformer and the branch circuit for which it is providing voltage.

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A transformer on a utility pole.

Calculating Fuse Size for the Primary Side of a Transformer

Step 1

The primary side of a transformer refers to high-voltage side of the transformer. Determine the volt-ampere or VA rating for the primary side of the transformer by multiplying the voltage and amperes and dividing the product by 1000. The result is the transformer's input amperage.

Step 2

Divide the VA rating by the voltage of the transformer. The quotient determines the percentage of amperage a fuse needs to be smaller than the VA rating of the transformer.

Step 3

Use a fuse size based upon the results using the National Electric Code standards as a guide.

Calculating Fuse Size for the Secondary Side of the Transfomer

Step 4

The secondary side of a transformer refers to low voltage side of the transformer. Determine the volt-ampere or VA rating for the primary side of the transformer by multiplying the voltage and amperes and dividing the product by 1000. The result is the transformer's output amperage.

Step 5

Divide the VA rating by the voltage of the transformer. The quotient determines the percentage of amperage a fuse needs to be larger than the VA rating of the transformer.

Step 6

Use a fuse size based upon the results using the National Electric Code standards as a guide.