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Transformer Sizing Practice for the California Exam

Transformer current comes straight from the kVA and the voltage. Single phase divides by voltage, three phase divides by voltage times 1.732. On the California exam, calculations are part of the Determination of electrical system requirements domain. Here is the pattern, one worked example, and a question to try.

Last reviewed June 2026

One worked example

A 45 kVA, 208-volt, three-phase transformer. What's the secondary current?

  1. 1
    Name the problem

    Three-phase transformer current from kVA and voltage.

  2. 2
    Pick the formula

    Three-phase current is the VA divided by the voltage times 1.732.

    I = VA / (E × 1.732)

  3. 3
    Pull the numbers

    45000 VA, 208 volts.

  4. 4
    Run the arithmetic

    Voltage times 1.732 first, then divide.

    I = 45000 / (208 × 1.732)

    I = 45000 / 360.3 = 124.9 A

  5. 5
    Check it

    About 125 amps on the secondary. Sprinkle the 1.732 anywhere voltage shows up on three phase.

Now try one

Your turn. A 15 kVA single-phase transformer with a 120-volt secondary.

What's the secondary current, in amps?

  1. 1
    Run the arithmetic

    Divide the VA by the volts.

    I = 15000 / 120 = 125 A

  2. 2
    Check it

    125 amps. Single phase is just VA over volts.

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