What Is Current Transformer Ratio?

A current transformer ratio defines the relationship between the primary current flowing through the electrical system and the secondary current output provided to a measurement device.

The formula is:

CT Ratio = Primary Current ÷ Secondary Current

Example:

A 400A/5A CT means:

• Primary current: 400A
• Secondary output: 5A

The correct CT ratio should match the expected operating current range to ensure accurate measurement, prevent saturation, and maximize energy monitoring performance.

Quick Answers

How do I choose a CT ratio?

Select a CT ratio based on:

• Maximum expected current
• Normal operating current
• Meter input requirement
• Accuracy requirement
• Future load expansion

What does 400A/5A CT mean?

A 400A/5A CT converts a maximum primary current of 400A into a standard secondary output of 5A.

Should CT size match breaker rating?

Not always.

The CT should match the actual operating current, not only the breaker capacity.

What is the most common CT secondary output?

Common outputs:

• 5A
• 1A
• mA

Industrial energy meters commonly use 5A or mA inputs.

Introduction

Selecting the correct current transformer ratio is one of the most important decisions when designing an energy monitoring system.

A wrong CT ratio can cause:

• Incorrect energy readings
• Poor measurement accuracy
• CT saturation
• Reduced EMS data quality

Many engineering problems occur because users select CTs based only on the maximum circuit rating.

However, professional CT selection requires understanding:

• Actual load current
• Operating range
• Accuracy requirements
• Meter compatibility

This guide explains how to select the correct CT ratio for industrial and commercial applications.

Chapter 1 — Understanding CT Ratio

A current transformer ratio describes the transformation relationship between:

Primary Current

The actual current flowing through the electrical conductor.

Example:

• 100A
• 400A
• 1000A

Secondary Current

The reduced measurement signal output.

Common:

• 5A
• 1A
• mA

CT Ratio Formula

CT Ratio = Primary Current / Secondary Current

Examples

Example 1

CT:

100A/5A

Means:

Primary:

100A

Secondary:

5A

Ratio:

20:1

Example 2

CT:

500A/5A

Means:

Primary:

500A

Secondary:

5A

Ratio:

100:1

Chapter 2 — Why CT Ratio Selection Matters

The CT ratio determines how accurately the measurement system represents the real electrical load.

Correct Selection

Example:

Actual load:

300A

Selected CT:

400A/5A

Operating range:

Good

Result:

✔ Accurate measurement
✔ Stable operation
✔ Better energy analysis

Incorrect Selection Example

Actual load:

300A

Selected CT:

1000A/5A

Result:

The CT operates at only 30% capacity.

Problems:

• Lower accuracy
• Poor low-current performance
• Reduced measurement quality

Chapter 3 — Basic CT Ratio Selection Method

Follow these steps:

Step 1 — Determine Maximum Circuit Current

Identify:

• Transformer rating
• Breaker rating
• Cable capacity
• Load design

Example:

Main feeder:

400A

Step 2 — Determine Normal Operating Current

This is the most important factor.

Example:

Factory load:

250A–320A

Step 3 — Select CT Primary Rating

Recommended:

Choose a CT slightly above normal operating current.

Example:

Operating current:

300A

Recommended CT:

400A/5A

Step 4 — Match Meter Input

Confirm:

Energy meter supports:

• 5A input
• 1A input
• mA input

Chapter 4 — Common CT Ratios Explained

50A/5A CT

Ratio:

10:1

Applications:

• Small distribution circuits
• Lighting systems
• Small equipment

100A/5A CT

Applications:

• Commercial buildings
• Small industrial loads

200A/5A CT

Applications:

• Machine monitoring
• Medium feeders

400A/5A CT

One of the most common industrial CT ratios.

Applications:

• Factory feeders
• Distribution cabinets
• EMS systems

600A/5A CT

Applications:

• Large industrial systems
• High-current feeders

1000A/5A CT

Applications:

• Large power systems
• Main distribution equipment

Chapter 5 — How Much Oversizing Is Recommended?

Engineers should avoid selecting CTs exactly equal to maximum current.

Example:

Breaker:

400A

Do not automatically choose:

400A/5A

because actual load may be lower.

Recommended principle:

Normal operating current:

70–80% of CT rated current

Example:

Expected load:

300A

Choose:

400A/5A

instead of:

300A/5A

Benefits:

• Better measurement range
• Reduced overload risk
• Future expansion capability

Chapter 6 — CT Ratio and Accuracy Relationship

CT accuracy depends on operating current.

A CT performs best near its rated range.

Example:

400A/5A Class 0.5 CT

Best performance:

200A–400A range

Poor selection:

1000A/5A CT measuring 100A load

may reduce:

• Accuracy
• Resolution
• Energy calculation quality

Chapter 7 — CT Ratio Selection for Different Applications

Industrial Factory

Typical:

100A–2000A feeders

Recommended:

200A/5A

400A/5A

600A/5A

Commercial Building

Typical:

50A–400A circuits

Recommended:

100A/5A

200A/5A

400A/5A

Solar PV System

Selection depends on inverter output.

Example:

100kW inverter:

AC current:

≈150A

Recommended:

200A/5A CT

EV Charging Station

Depends on:

• Charger power
• Number of chargers
• Distribution capacity

Example:

300A charging feeder:

Recommended:

400A/5A CT

Data Center

Applications:

• UPS output
• PDU monitoring

Recommended:

High accuracy CT:

Class 0.5

Ratio:

Based on actual feeder current

Chapter 8 — CT Ratio Selection Example

Project:

Industrial factory energy monitoring

Electrical Information

Voltage:

400V Three Phase

Main feeder:

630A breaker

Normal load:

450A

Selection

CT Ratio

Choose:

600A/5A

Reason:

Operating load:

450A

≈75% CT capacity

Accuracy

Choose:

Class 0.5

Installation

Existing factory:

Choose:

Split Core CT

Final Solution

630A Feeder

↓

600A/5A Split Core CT

↓

Smart Energy Meter

↓

RS485 Modbus RTU

↓

EMS Platform

Chapter 9 — Common CT Ratio Selection Mistakes

Mistake 1

Choosing CT according to breaker rating only.

Problem:

Breaker ≠ actual load.

Mistake 2

Selecting oversized CT.

Example:

100A load

using:

1000A/5A CT

Problem:

Poor low-current accuracy.

Mistake 3

Ignoring future expansion.

Problem:

System upgrade requires CT replacement.

Mistake 4

Ignoring meter compatibility.

Example:

Meter requires mA input.

Selected CT:

5A output

Cannot connect directly.

Chapter 10 — YADA CT Ratio Solutions

YADA provides customized current transformer solutions supporting different application requirements.

SCT24L Split Core CT

Suitable for:

• Building monitoring
• Smart meters
• Retrofit projects

Available:

• Multiple current ratios
• Compact installation

SCT40L Split Core CT

Suitable for:

• Industrial feeders
• Higher current applications

Applications:

• Factory EMS
• Distribution monitoring

Customized CT Solutions

YADA supports:

• Custom primary current
• Secondary output customization
• Mechanical customization
• OEM requirements

Chapter 11 — CT Ratio + Energy Meter Integration

Complete system:

Electrical Load

↓

Current Transformer

↓

Energy Meter

↓

RS485 Modbus

↓

EMS / BMS

↓

Energy Analytics

Correct CT ratio ensures:

✔ Accurate energy data
✔ Reliable monitoring
✔ Better management decisions

Chapter 12 — FAQ

1. What is CT ratio?

CT ratio defines the relationship between primary current and secondary output current.

2. What does 400A/5A CT mean?

It converts 400A primary current into 5A secondary output.

3. Can I use a larger CT ratio?

Yes, but excessive oversizing may reduce measurement accuracy.

4. Should CT ratio equal breaker rating?

No.

Selection should consider actual operating current.

5. What CT ratio is commonly used?

Common industrial ratios:

• 100A/5A
• 200A/5A
• 400A/5A
• 600A/5A

6. How do I calculate CT ratio?

Divide primary current by secondary current.

Technical Glossary

CT Ratio

Relationship between primary and secondary current.

Primary Current

Current flowing in the monitored circuit.

Secondary Current

Reduced output current supplied to measurement devices.

Burden

Load connected to the CT secondary circuit.

Saturation

Condition where CT cannot maintain proportional output.

Key Takeaways

• CT ratio determines how electrical current is converted for measurement.
• Correct selection improves energy monitoring accuracy.
• Choose CT size based on normal operating current, not only breaker rating.
• Common industrial CT ratios include:
  • 100A/5A
  • 200A/5A
  • 400A/5A
  • 600A/5A
• Oversized CTs may reduce low-load accuracy.
• CT ratio must match:
  • Energy meter input
  • Accuracy requirements
  • Application conditions

Need Help Selecting the Right Current Transformer Ratio?

YADA provides professional CT solutions for:

✔ Industrial power monitoring
✔ Energy meters
✔ EMS systems
✔ Solar PV monitoring
✔ EV charging infrastructure

Solutions include:

• Split Core CT
• Precision Current Transformer
• Customized OEM CT

Contact YADA engineering team for:

• CT ratio recommendation
• Meter compatibility evaluation
• Customized current transformer solutions