Notice that the 2 percent impedance transformer has 5 times the short circuit

current of the 10 percent impedance transformer. The 2 percent impedance

transformer might require a complete redesign of downstream electrical

equipment to withstand the higher short circuit cur rents.

B-4.2

Impedance affects transformer regulation. As the impedance increases, the

voltage regulation tends to increase. Voltage regulation is defined as the voltage

change from no load to full load conditions:

100%

B-5

process for a simple transformer installation. Primary and secondary conductor sizes

are also determined.

EXAMPLE: A feeder supplies three-phase power to a 480 volt transformer. The

transformer steps down to 208Y/120 volts to a lighting panel with a continuous

load of 30 amperes on each phase. What is the required transformer kVA

capacity, and required amperage on the primary and secondary?

480 V

208Y/120 V

3-Phase

Transformer

Transformer Size

The transformer required kVA capacity is given by:

Transformers are provided in standard sizes. The next larger standard size

above 10.8 kVA is 15 kVA. So, choose a 15 kVA transformer for this load. If

additional load growth is anticipated, a larger transformer might have been

selected instead.

Primary Ampacity

Assume that the transformer will eventually be fully loaded. The required primary

amperage is:

15 *kVA * 1000

= 18 *amperes*

3 480 *V*

B-6

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