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. Use the voltage for the side the question asks about.

What is the biggest transformer calculation trap?

Using the wrong side. Primary current uses primary voltage. Secondary current uses secondary voltage. A correct formula with the wrong voltage gives a clean wrong answer.

Is transformer current the same as breaker size?

No. Transformer current is the arithmetic result. Overcurrent protection is a separate Article 450 decision based on the table and protection arrangement in the question.

When do I use 1.732 in transformer math?

Use 1.732 for three-phase transformer current or kVA calculations. Do not use it for single-phase transformer math.

What NEC article covers transformers?

NEC Article 450 covers transformers and transformer overcurrent protection. The arithmetic may be basic electrical math, but the protection question sends you to Article 450.

NEC Article 450 | transformers

Transformer Calculations for Electrician Exams

Transformer questions are short. The miss usually happens before the math, when the candidate uses the wrong side, forgets kVA means thousand VA, or turns a current answer into a breaker answer.

Last reviewed July 2026

The two current formulas

Single-phase transformer current

I = (kVA x 1,000) / V

Use the same formula on either side. The voltage changes because the side changes.

Three-phase transformer current

I = (kVA x 1,000) / (V x 1.732)

The 1.732 belongs to three-phase math. If the question is single phase, leave it out.

If you want the arithmetic checked after the setup, use the free transformer sizing calculator.

Worked example: 45 kVA, 480 V to 208 V, three phase

A 45 kVA three-phase transformer has a 480 V primary and a 208 V secondary. Find primary and secondary current before any overcurrent-protection decision.

Convert kVA to VA: 45 kVA x 1,000 = 45,000 VA.
Primary current: 45,000 / (480 x 1.732) = 54.1 A.
Secondary current: 45,000 / (208 x 1.732) = 124.9 A.
The secondary current is higher because the secondary voltage is lower.
Neither answer is automatically the breaker size. A protection question sends you to Article 450.

Where Article 450 enters

Transformer current is basic math. Transformer protection is a code question. Article 450 tells you how to handle overcurrent protection based on the transformer, the current, and whether the question gives primary-only or primary-and-secondary protection.

On the exam, run the current first. Then read what protection arrangement the question gives you. If you skip that second step, you will pick an answer that looks like good math but answers the wrong question.

Wrong answers the exam wants you to pick

Forgetting kVA means thousand VA

Twenty five kVA is 25,000 VA. If you divide 25 by voltage, the answer will be too small by a factor of 1,000.

Using primary voltage for a secondary-current question

The formula is fine. The input is wrong. Read the side the question asks for, then use that side's voltage.

Treating current as the overcurrent device

A calculated current tells you the transformer load current. It does not finish an Article 450 protection question by itself.

How to drill this for the exam

Write three labels before you solve: side, phase, protection. Side picks the voltage. Phase decides whether 1.732 appears. Protection tells you whether the problem ends with math or moves into Article 450.

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