Calculator | Transformers

Transformer sizing calculator.

Calculate transformer kVA from volts and amps, or calculate transformer current from kVA and volts. Works for single phase and three phase, with the formula shown.

This calculator handles the arithmetic step. NEC Article 450 and the exact exam question still control overcurrent protection, conductor sizing, tap rules, and grounding.

Direct answer: transformer sizing starts with kVA and current. For single phase, kVA equals volts x amps divided by 1,000. For three phase, kVA equals volts x amps x 1.732 divided by 1,000. To find current, rearrange the same formula.

Worked example

75 kVA transformer, 480 volt, three phase

Inputs

Transformer size: 75 kVA
Voltage on this side: 480 V
Phase: three phase

Result

Current equals 75 x 1,000 divided by 480 x 1.732. That comes out to about 90.21 amps on the 480 volt side.

Here is the trap: primary current and secondary current are not the same unless the voltages are the same. Read which side the question asks for before you touch the calculator.

Common misses

Transformer mistakes that cost easy points

Forgetting 1.732 on three phase. The same volts and amps give a different kVA answer once the circuit is three phase.
Using the wrong voltage side. Primary current uses primary voltage. Secondary current uses secondary voltage.
Calling current the breaker size. Current is only the arithmetic result. The breaker answer comes from the code rule the question asks you to apply.
Mixing kVA and VA. kVA is thousands of VA. Multiply by 1,000 when solving for amps.

Related practice

After the transformer math, drill the code step

Transformers on the examHow transformer questions are usually written and where candidates slip.NEC calculations practiceTransformer current, motors, dwelling load, voltage drop, and fill problems.Wire size calculatorRun conductor sizing by voltage-drop target after you know the current.Take the free diagnosticSee whether transformer math is actually slowing you down.

FAQ

Transformer sizing calculator questions

How do you calculate transformer kVA?

For single phase, multiply volts by amps and divide by 1,000. For three phase, multiply volts by amps by 1.732 and divide by 1,000.

How do you calculate transformer current from kVA?

For single phase, multiply kVA by 1,000 and divide by volts. For three phase, multiply kVA by 1,000 and divide by volts times 1.732.

Which voltage should I enter for a transformer question?

Enter the voltage on the side the question asks about. Primary current uses primary voltage. Secondary current uses secondary voltage.

Does this size the transformer breaker?

No. This calculator gives kVA or current. Overcurrent protection, conductor sizing, tap rules, and grounding depend on the code rule in the question.

Do electrician exams test transformer sizing?

Yes. Transformer questions commonly test kVA, primary current, secondary current, and the difference between single-phase and three-phase formulas.

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