In this article, we will calculate the appropriate fuse for a transformer. To do this, we only need to calculate the primary amperage and allow a certain margin to the fuse capacity to prevent it from blowing during startup surges.
A fuse is designed to blow (open), which means it will disconnect the circuit when the amperage exceeds the fuse’s capacity for a brief period.
The goal is to protect the transformer, so the fuse blows, but the transformer remains undamaged in the event of a short circuit.
Summary: Choosing a Fuse for a Transformer
The fuse should be selected based on the primary amperage of the transformer and the load it supplies.
Minimum
We recommend selecting the next highest fuse value above the primary amperage of the transformer.
Maximum
The maximum should be three times the primary amperage, as any value higher than this is non-compliant and leaves the transformer overly exposed to potential damage.
Keep in mind the type of load, especially if startup conditions might cause amperage spikes that exceed the fuse’s capacity and lead to it blowing.
App for Calculating the Fuse
We recommend the following app, which displays the minimum and maximum fuse capacity, primary transformer amperage, appropriate cable gauge, circuit breaker size, grounding wire gauge, and more.
There is a free version:
And a PRO version as well.
Calculating Transformer Amperage
Primary Amperage of a Three-Phase Transformer
Primary Amperage of a Single-Phase Transformer
Considering the Type of Load
The fuse should be chosen based on the primary amperage of the transformer, but it is also essential to consider the type of load it will supply.
For instance, if the transformer supplies an electric motor that almost fully uses the transformer’s capacity, such as in an agricultural well, and the motor starts at full voltage (direct-on-line), this can result in a large surge current during startup. This surge may exceed the fuse’s capacity momentarily.
If the fuse does not have a sufficient margin, it may blow during motor startup.
In cases like this, it is crucial to provide an appropriate margin to avoid such issues.
However, if the motor is not large relative to the transformer’s capacity and has a soft start (e.g., with a reduced-voltage starter), the surge current during startup is much lower, and the fuse does not need as much margin.
In fact, using a soft starter is recommended to prevent motor damage and extend its lifespan.
Selecting a Fuse According to NOM 001 SEDE 2012
According to the 2012 Mexican standard, specific tolerances for fuses are suggested, based on two critical factors:
- The transformer’s impedance
- Whether the location is supervised or not
The maximum adjustment for the fuse should be the next highest value above the calculated primary amperage.
For a Supervised or Unsupervised Location, with Transformer Impedance Below 6%
Up to 300% of the primary amperage is allowed.
For a Supervised or Unsupervised Location with Transformer Impedance Between 6% and 10%
Up to 300% of the primary amperage is still allowed.
Example:
If the primary amperage is 2.6 A, then:
2.6 A x 3 = 7.8 A
The maximum permissible fuse size is 8 A.
Therefore, we can conclude that the maximum fuse capacity allowed by the standard is three times the primary amperage of the transformer. However, this is the upper limit; a fuse with a lower capacity can be selected based on the type of load and whether there will be periods where the amperage is significantly high for a not-so-brief duration.
Fuse Tables for Single-Phase and Three-Phase Transformers
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