# How do you size a fuse for a transformer?

## How do you size a fuse for a transformer?

In lieu of transformer inrush data, the rule of thumb is to select a fuse size rated at 300% of the primary full-load current and round up to the next larger standard size.

How do I know what size fuse I need?

The simple calculation is watts divided by volts equals amps. After you have calculated this, it is a simple case of adding around 10% to the value and choosing the nearest fuse to match. This is a more precise method then just guessing should you require a 3, 5, or even 13 amp fuse.

### Do transformers need fuses?

All transformers are connected to the common secondary circuit through secondary fuses. The purpose of these fuses is to disconnect a faulty transformer from the secondary circuit. The size of the secondary fuse must be such that it will blow on a primary fault between its transformer and the associated primary fuse.

What kind of fuses do pole mounted transformers need?

11kV/415V pole-mounted transformers shall be protected against fault currents by fuses on both the 11kV and the LV side of the transformer.

#### How to calculate circuit breaker / fuse for transformer?

AS per NEC 450.3, Max.Rating of C.B or Fuse is following % of its Current according to it’s Primary Voltage,% Impedance and Supervised/Unsupervised Condition. If Transformer is in supervised condition then Select Circuit Breaker near that size but if Transformer is in unsupervised condition then Select Circuit Breaker next higher size.

What are the fuse ratings for a transformer?

All fuse ratings 7, 10, and 15 A are type X or type KS fuse links. All fuse ratings 20 A and larger are type KS fuse links. Fuse link ratings recommended in these tables will carry the full load of the transformer continuously and will open in five minutes when the transformer load is approximately three times full load.

## Where does the fuse cutout go on a transformer?

Cutout (left) attached to a feeder line (connection at the right not shown). The lower wire goes to the transformer A set of pole top cutouts protecting a transformer on a 12.47 kV distribution line. One fuse is blown and the white tube is hanging down