How to Calculate Transformer CT Ratio 

Current transformers (CTs) are critical components in power systems. They reduce high current to a manageable level for monitoring and protection. An incorrect CT ratio may cause measurement errors, malfunctioning protections, and even safety hazards. This guide walks you through understanding and applying CT ratios correctly.

What is CT Ratio and Why It Matters

The CT ratio (Current Transformer Ratio) represents the relationship between the primary and secondary current. For example, a 1000:5 CT means 1000A primary current is reduced to 5A on the secondary side.

Importance of Selecting the Right CT Ratio:

• Ensures accurate metering (avoids billing errors)
• Ensures protection relays operate reliably
• Prevents CT overload and secondary overcurrent risks

Key Parameters to Understand Before Calculating CT Ratio

• Primary Current (Ip): Maximum current expected in operation
• Secondary Current (Is): Usually 1A or 5A depending on standard
• CT Ratio: Ip divided by Is

If the expected load is 800A and the secondary is 5A:

CT Ratio = 800 / 5 = 160:1

Common CT Ratios and Their Applications

CT Ratio	Application
100:5	Small commercial or residential panels
200:5	Small factories or commercial buildings
400:5	Medium industrial distribution
600:5	Mid-size buildings or factory systems
1000:5	Large industrial zones or switch rooms
2000:5	Main transformer protection in substations

How to Choose the Right CT Ratio: Tips and Pitfalls

• Match peak load current: Ensure the primary side handles expected max current
• Match your meters: Make sure meter input matches CT output (1A or 5A)
• Plan for future load increase: Leave some margin for growth
• Avoid underestimating current: CT may saturate and fail to measure properly
• Ensure compatibility with meters: Avoid incorrect readings or protection failures
• Check CT burden and accuracy class: For stable and accurate performance

Conclusion: The Right CT Ratio Ensures Safety and Efficiency

Choosing the right CT ratio is more than a technical step — it ensures accurate data and reliable protection across your power system. With a good understanding of key parameters and smart selection, you can avoid costly mistakes and maintain operational efficiency.

If you're sourcing CTs, transformer aluminium strips, copper-clad aluminum, or magnet wires, feel free to contact us for products that match your system perfectly.

Frequently Asked Questions 

1. What is the standard secondary current of a CT?

The standard secondary current is typically 1A or 5A, depending on the measurement system and application requirements.

2. Can I use a CT with a higher ratio than needed?

Using a CT with a higher ratio than required may lead to lower accuracy and underutilization of the CT's range, but it’s safer than using one that’s underrated.

3. What happens if I choose the wrong CT ratio?

Incorrect CT ratios can cause protection relays to misoperate, inaccurate meter readings, or CT saturation, which may lead to equipment damage or safety issues.

4. How do I know if my CT is compatible with the meter?

Check the meter’s input current rating (1A or 5A) and ensure your CT’s secondary matches it. Also, confirm the CT burden does not exceed the rated burden of the meter.

5. Can CT ratios be adjusted?

Standard CTs have fixed ratios. However, multi-ratio CTs are available for applications needing flexible configurations. These allow selection via tap changers or terminals.