guidelines for transformers
guidelines for transformers

The selection of transformer fuse depends on ampere rating and speed. Transformers manufacturers in India consider few factors for the same, such as-

  • Anticipated normal transformer loading schedule, including repetitive peak loads and emergency peak loads
  • Inrush currents and long term over current that occurs on re-energizing the transformer following cold-load pickup.
  • The protection level provided to the transformer against overcurrents
  • Coordination with secondary side and primary side overcurrent protective devices
  • Safeguarding downstream conductors from damaging overcurrents

Earlier, it was a complex thing for manufacturers to select a transformer primary fuse ampere rating and speed because the process involved hard-to-understand graphical solutions. They used time-current characteristic curves mentioned for several overcurrent protective devices.

What is the loading capability?

Loading capability is a term used for the amount of load can be carried out by the transformer primary fuse even after the extended loss of source voltage, without operating and carry on without break. There are two load ability values for each fuse ampere rating –

  • The minimum of the repetitive peak load capability and
  • The hot load and cold load pickup capability

These three capabilities are explained further-

  1. Continuous peak load capability – This type of capability can be explained as the ability of the transformer primary fuse to carry on a repetitive peal load regardless of time or duration.
  2. Hot load pickup capability – It is the ability of the transformer primary fuse (on load) to withstand both magnetizing and load-in rush currents related to the re-energizing of the transformer.
  3. Cold load pickup capability – It is the ability of the transformer primary fuse to withstand both the magnetizing and load-inrush currents linked to the re-energizing of the transformer.

Manufacturers selling the units to industries for commercial, industrial, and institutional usage are making transformers as per the size to accommodate maximum peak-load conditions skipping overloading. This is why the combined load-inrush and magnetizing current is no more severe than the inrush current flow under hot-load pickup condition. In this situation, the primary fuse is loaded to the peak-load capability.

How to size fuse for the transformer?

When you ask a manufacturer, he simply explains the transformer first. You will get to know that transformers are devices used for transferring voltage between circuits from one level to another using electrical induction technique. Transformers are initially used to turn high voltage to low voltage for power distribution.

A transformer can be intended in different sizes for use in a small electric circuit to national power distribution; the operating principle for each size is the same. Electrical fuses are an over-current protection device that helps prevent overload between a circuit and a power source, such as the transformer and the branch circuit for which it is supplying voltage power.

Manufacturers calculate the fuse size for the primary side of a transformer in the following ways-

  1. They determine the volt-ampere or VA rating for the transformer’s primary side using formula –

Voltage X Amperes



  1.     VA


Transformer voltage

The quotient helps determine the amperage percentage required by a fuse to be smaller when compared to the VA rating of the transformer.

  1. Use a fuse size based on the results as per NEC standards

Manufacturers calculate fuse size for the secondary side of the transformer with these steps-

  1. They determine the VA by using formula-

Voltage X Amperes           = Output Amperage



  1.   VA rating


Transformer voltage

The quotient determines the percentage of fuse amperage needs to be greater than the VA rating of the transformer

  1. Use a fuse size as per the results.

This guide is issued by transformers manufacturers India. If you have any doubt or query related to the transformer or its parts then get in touch with the manufacturers directly.