# Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 kVA single-phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.

Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 kVA single-phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.

### Right Answer is: 520 Amp

#### SOLUTION

$\begin{array}{l} \begin{array}{*{20}{l}} {{\text{Normal Full Load current}} = \dfrac{{{\text{Nameplate volt Amp}}}}{{{\rm{Line Volt}}}}}\\ {}\\ {\dfrac{{10,000}}{{480}} = 20.8A} \end{array}\\ \\ {\text{Maximum Short Circuit Amps}} = \dfrac{{{\text{Full – Load Amp}}}}{{{\rm{4\% }}}}\\ \\ = \dfrac{{20.8}}{{4\% }} = 520A \end{array}$

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.

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