Transformer Calculations: kVA to Amps for Electricians

Why transformers are on the exam

The services, service equipment and separately derived systems area carries 10 scored items across both portions, 4 of them on Calculations, and the PSI Candidate Information Bulletin (updated 9/15/2026) lists transformers within it. A transformer feeding a panel is a separately derived system, and the NEC's overcurrent protection rules for transformers are in Article 450. We cite by number only, so find the sections in your own 2026 book and read the conditions there.

The two formulas

A transformer's rating is in kilovolt-amperes (kVA), and one kVA is 1,000 volt-amperes. To find the current on either side, divide the volt-amperes by the voltage, and for three-phase also by 1.732, the square root of 3.

• Single-phase current = (kVA × 1,000) ÷ voltage
• Three-phase current = (kVA × 1,000) ÷ (1.732 × line voltage)

Use the line voltage for the side you are working on. The primary and secondary currents are different because the voltages are different, and the power is the same on both sides, ignoring losses.

Worked example: single-phase

A 25 kVA single-phase transformer has a 480 volt primary and a 240 volt secondary.

• Primary current: 25,000 ÷ 480 = about 52.1 A
• Secondary current: 25,000 ÷ 240 = about 104.2 A

The check is that the lower-voltage side carries the larger current, here about twice, because the voltage is half.

Worked example: three-phase

A 75 kVA three-phase transformer has a 480 volt primary and a 208 volt secondary.

• Primary current: 75,000 ÷ (1.732 × 480) = 75,000 ÷ 831.4 = about 90.2 A
• Secondary current: 75,000 ÷ (1.732 × 208) = 75,000 ÷ 360.3 = about 208.2 A

If your calculator has a square-root key, enter 3 and press it. Otherwise 1.732 is close enough for exam-style rounding, but look at the options: if two choices differ by less than the effect of rounding, carry more digits.

The three steps that follow

1. Convert to current on each side. As above.
2. Find the overcurrent protection rule. Article 450 has rules for the primary and secondary protection of transformers, with percentages that depend on the current and the arrangement. Read the condition in your book. The percentages are not for memory.
3. Round to a standard device rating. 240.6(A) lists standard ratings. Read the question to see whether it allows the next size up.

Then size conductors from the load or the device rating, with the ampacity steps in our conductor ampacity guide, and the grounding conductors from their tables in our grounding electrode conductor guide.

Mistakes to avoid

• Using phase voltage in the three-phase formula. Use the line voltage the question gives.
• Using 1.732 on a single-phase unit. It belongs only to the three-phase formula.
• Mixing primary and secondary. Label each current as you write it.
• Forgetting the 1,000. A kVA rating is thousands of volt-amperes. A figure off by 1,000 is the commonest wrong option.
• Applying exceptions. The bulletin tells you not to consider exceptions or optional methods unless a question directs you to them.

Drill it

Make ten transformers with different kVA and voltage pairs. For each, write both currents and check that power on the two sides is equal. Check each pair with the transformer calculator, which shows every step. For three-phase power with a power factor, check your arithmetic with the Ohm's law calculator. Then take a mixed set from our calculations portion guide and time the full chain, from kVA to device size.

Find your weak spot first

Transformers are one of several types inside the services area. Take the free 10-question check to see where your score sits across all ten areas. No signup, and every answer comes with an explanation.

FAQ

How do I convert kVA to amps?

Multiply kVA by 1,000 and divide by the voltage. For three-phase, also divide by 1.732.

Why is secondary current larger than primary on a step-down transformer?

Power is nearly equal on both sides, so the lower-voltage side carries more current.

Where are transformer overcurrent rules?

In Article 450. Read the specific section and conditions in your 2026 NEC.