EXAMPLE: What is the rated secondary current of a 30-kVA single -phase

transformer with a rated secondary voltage of 240 volts?

30 *kVA * 1000

= 125 *amperes*

240 *V*

EXAMPLE: What is the rated secondary current of a 100-kVA three-phase

transformer with a rated secondary voltage of 480 volts?

100 *kVA * 1000

= 120 *amperes*

3 480 *V*

B-3.2

The above examples do not include the effect of any losses; however, the

calculations provide approximate values that are usually adequate for use.

B-4

B-4.1

For a given kVA rating, a transformer will provide a higher short circuit current

as its impedance is lowered. Transformer impedance is usua lly expressed as a

percent. A transformer rated at 10 percent impedance can supply 100%/10% = 10

times its rated secondary current into a short circuit. A transformer rated at 4 percent

impedance can supply 100%/4% = 25 times its rated secondary current into a short

circuit. Notice that two transformers of equal kVA capacity can have significantly

different short circuit currents. This feature must be evaluated as part of the transformer

sizing and selection process.

EXAMPLE: Compare the secondary sho rt circuit current of a 500-kVA, 480 volt

secondary, three -phase transformer with a 10 percent impedance to an equal

capacity transformer with a 2 percent impedance.

First, calculate the rated secondary current:

500 *kVA * 1000

= 600 *amperes*

3 480 *V*

The 10 percent impedance transformer has the following expected short circuit

current:

100%

600 *amperes *= 10 600 *amperes *= 6,000 *amperes*

10%

The 2 percent impedance transformer has the following expected short circuit

current:

100 %

600 *amperes *= 50 600 *amperes *= 30,000 *amperes*

2%

B-5

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