UFC 3 -520-01
June 10, 2002
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
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
The above examples do not include the effect of any losses; however, the
calculations provide approximate values that are usually adequate for use.
TRANSFORMER IMPEDANCE EFFECTS .
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
Isc -10% =
600 amperes = 10 600 amperes = 6,000 amperes
The 2 percent impedance transformer has the following expected short circuit
Isc -2% =
600 amperes = 50 600 amperes = 30,000 amperes