The *percent impedance* (%Z) is the percent of the rated load impedance possessed by a transformer. The percent impedance is important in that it allows us to:

- Calculate available fault currents (both individual and bank).
- Determine whether two transformers are suitable for paralleling.

In the last chapter, we learned how to determine the short-circuit voltage of a transformer. We now can use that voltage to determine the percent impedance:

**E _{rated}/E_{short circuit} = %Z**

**Video Alert!**

**Fault-Current Calculations**

To calculate the fault current available from a transformer if a *dead short* occurs across the secondary terminals, use the formula:

**I _{rated (Secondary)}/%Z = Fault current**

Remember to use the percentage as a decimal not the full number. For example, 2.5% is actually .025.

**Video Alert!**

How to calculate fault current using %Z:

**Example #1**

What is the available fault current of a step-down transformer rated at 50 kVA, 1200 V – 120 V and 2.75% percent impedance?

I(full load) =

I(full load) = 417 Amps

I(short circuit) =

I(short circuit) = 15,151 Amps

**Exampe #2**

A transformer is rated 20KVA, 4800 V – 240 V. With the secondary short-circuited, it takes 96 volts to reach the rated primary current (4.2 amps). Determine the %Z and the available fault current.

%Z =

**%Z = 2**

I(rated secondary) =

I(rated secondary) = 83 Amps

I(Short circuit) =

**I(Short circuit) = 4150 Amps**

# Attributions

How to determine the percent impedance of a transformer video by The Electric Academy is under a Creative Commons Attribution License.

How to calculate fault current using percent impedance video by The Electric Academy is under a Creative Commons Attribution License.