How-To Guide

How to Choose a Control Transformer

Choose a control transformer by the actual control-circuit burden, not just the steady-state load. In most industrial panels that means adding the continuous burden, checking the largest expected inrush burden, and then rounding to a standard VA size that the manufacturer supports.

Difficulty: IntermediatePosted: 2026-03-15

Quick answer

Choose a control transformer by the actual control-circuit burden, not just the steady-state load.

Table of contents

  1. Start with the real control burden
  2. Round to a real standard size
  3. What to verify before buying
  4. Common mistakes
  5. FAQ

When this matters

This matters when a starter bucket, control panel, or retrofit needs a control transformer for coils, relays, indicators, or similar control loads and the team wants a solid first-pass selection before checking the final catalog table.

Start with the real control burden

A control transformer is selected from the burden it has to support. That means the continuous control load plus the largest inrush event the circuit expects, not just the quiet steady-state condition after everything has already pulled in.

This is why control-power sizing references look at coil burden and inrush burden together when they walk through a transformer choice.

Round to a real standard size

Once the estimated burden is understood, the next step is to move up to a standard VA size the manufacturer actually offers. That keeps the early estimate aligned with real catalog options instead of an exact number that cannot be purchased.

  • List the steady-state burden of relays, contactors, indicators, and similar control loads.
  • Identify the largest inrush burden expected at one time.
  • Apply a practical design margin and round up to a standard VA size.
  • Review the protection and fuse recommendations separately.

What to verify before buying

  • Primary and secondary voltage requirements
  • Largest expected inrush burden
  • Continuous control burden
  • Standard VA size availability
  • Primary and secondary protection strategy

Common mistakes

  • Sizing from holding VA only and ignoring the largest inrush load.
  • Picking a transformer size without checking common standard VA steps.
  • Treating transformer sizing and fuse review as the same decision.
  • Assuming every coil or inrush event happens the same way in every circuit.

Important note

Use this page as a sizing path, not a final release. Always verify the complete control burden, transformer rating table, protection, and wiring against the actual manufacturer's data and the finished control circuit.

FAQ

Should I size a control transformer from sealed VA only?

Usually no. The largest expected inrush burden also needs to be part of the estimate.

Does the VA estimate finish the job by itself?

No. The final transformer choice still needs a catalog-table check plus protection and wiring review.

Technical Information Notice

The information in this article is provided for general educational and reference purposes. Industrial equipment selection, installation, and operation should always be verified against manufacturer documentation, applicable electrical codes, and the requirements of the specific application.

Strike Industrial does not design electrical systems and cannot evaluate every operating condition. Before installing or modifying industrial equipment, consult qualified personnel such as a licensed electrician, controls engineer, or equipment manufacturer when appropriate.