Transformer kVA: what it means for the Texas exam

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://bondingjumper.com/charts/bondingjumper/transformer-kva.svg" alt="A transformer drawn as two coils around a core, primary on the left marked 480 volts and 54 amperes, secondary on the right marked 208Y/120 volts and 125 amperes, with the nameplate 45 kVA in the middle and the note same kVA, different current" width="100%"><p><a href="https://bondingjumper.com/guides/transformer-kva?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=transformer-kva.svg">Same kVA in and out. Lower voltage means higher current. by Bonding Jumper</a></p>

Why kVA and not kW

The windings heat according to the current through them, and the core according to the voltage across it. Neither cares whether the current is in step with the voltage. A transformer feeding motors at a power factor of 0.8 heats just as much at its rated kVA as one feeding heaters at 1.0, even though the motors get fewer kilowatts. See apparent vs true power.

Current on each side

Ignoring losses, the kVA is the same on both sides, so the current changes inversely with voltage.

• Single-phase: amperes = kVA × 1,000 ÷ volts.
• Three-phase: amperes = kVA × 1,000 ÷ (1.732 × line volts).

A worked example with invented numbers: a 45 kVA three-phase transformer, 480 volts delta primary, 208Y/120 volts secondary.

• Primary: 45,000 ÷ (1.732 × 480) = 45,000 ÷ 831.4 = about 54.1 amperes.
• Secondary: 45,000 ÷ (1.732 × 208) = 45,000 ÷ 360.3 = about 124.9 amperes.

The winding voltages are 480 on the delta side and 120 on the wye side, so the turns ratio of each winding pair is 4 to 1, even though the line voltages are 480 and 208. How the windings are connected is covered in wye vs delta.

Sizing a transformer

Add the load in volt-amperes, allow for any continuous loads and future growth, and pick the next standard kVA size at or above it. Running a transformer above its kVA rating for long periods shortens its insulation life. Overcurrent protection for transformers has its own rules in Article 450, and a transformer feeding a building system is a separately derived system with its own bonding and grounding.

Common mix-ups

• Using kW for the rating. Use kVA, and convert only if the question gives power factor and asks for kW.
• Using phase voltage in the three-phase formula. Use line-to-line voltage with 1.732.
• Expecting the same current on both sides. The lower-voltage side carries more current.
• Using 1.732 on a single-phase transformer.

On the Texas journeyman exam

Transformers sit in the services, service equipment and separately derived systems area, 10 scored items across both portions under the PSI bulletin. Study to find full-load current on either side and then size the conductors or protection from it. Our blog walks through transformer calculations from kVA to amps.

Practice it

The transformer calculator gives the current on each side and the turns ratio. Every term in this set is in the electrical terms glossary.

Know the terms? Take the free 10-question check

Take the free 10-question check. It needs no signup and explains every answer, so you see whether theory, grounding or wiring methods costs you the most points. When you want the full method worked topic by topic, the Bonding Jumper course comes second.

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.