1. What is a Current Transformer Sizing Calculator?

Definition: This calculator determines the current transformer (CT) ratio based on primary and secondary current values.

Purpose: It helps electrical engineers and technicians properly size current transformers for metering and protection applications.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• CT Ratio — Current transformer turns ratio (dimensionless)
• Primary Current — Current in the primary winding (Amperes)
• Secondary Current — Current in the secondary winding (Amperes, typically 5A)

Explanation: The ratio represents how much the primary current is reduced in the secondary circuit.

3. Importance of Proper CT Sizing

Details: Correct CT sizing ensures accurate current measurement, proper relay operation, and system protection.

4. Using the Calculator

Tips: Enter the primary current in Amperes and secondary current (default 5A). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical secondary current value?
A: Most CTs have 5A secondary current, though 1A is also common in modern systems.

Q2: How do I select primary current?
A: Primary current should be the maximum expected load current in the circuit.

Q3: What if my ratio isn't a standard value?
A: Choose the next higher standard CT ratio (e.g., for 287:5 calculation, use 300:5 CT).

Q4: Why is secondary current typically 5A?
A: 5A is a safe, standardized value that matches most meters and relays while providing sufficient signal strength.

Q5: What about CT accuracy class?
A: Accuracy class (0.2, 0.5, 1.0 etc.) is separate from ratio and depends on application requirements.