1. What is a Control Transformer Sizing Calculator?

Definition: This calculator determines the proper size (in volt-amperes) for a control transformer based on the total load and inrush requirements.

Purpose: It helps electrical engineers and technicians select appropriately sized control transformers for industrial and commercial applications.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( VA \) — Required transformer size (volt-amperes)
• \( \sum(Load\ VA) \) — Sum of all connected loads (volt-amperes)
• \( Inrush \) — Additional capacity for inrush currents (volt-amperes)

Explanation: The total load VA represents the continuous power requirement, while the inrush accounts for temporary startup currents of devices like contactors and relays.

3. Importance of Proper Transformer Sizing

Details: Correct sizing ensures reliable operation, prevents overheating, and maintains proper voltage regulation for control circuits.

4. Using the Calculator

Tips: Enter the total load VA (sum of all connected devices) and any additional inrush requirements. All values must be ≥ 0.

5. Frequently Asked Questions (FAQ)

Q1: What is inrush current?
A: The momentary surge current that occurs when electromagnetic devices (like contactors) are first energized.

Q2: How do I determine the load VA?
A: Sum the VA ratings of all connected devices or multiply their current ratings by the operating voltage.

Q3: What's a typical inrush value?
A: For control circuits, inrush is typically 1.5-2 times the steady-state load, but check manufacturer specifications.

Q4: Should I add a safety factor?
A: It's recommended to add 10-20% to the calculated VA for safety margin and future expansion.

Q5: What if I have multiple loads with different inrush?
A: Use the largest single inrush value, not the sum of all inrush currents.