Why is CT Secondary is 1A or 5A?1A vs 5A Current Transformer Guide

Dec 24, 2025

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Introduction

If you have worked with electrical measurement systems, protection relays, or power monitoring equipment, you may have noticed that most current transformers (CTs) have a secondary output rated at 1A or 5A.

Many engineers ask:

Why is CT secondary current always 1A or 5A? Why not 2A, 3A, or another value?

The answer is related to decades of electrical engineering standardization, measurement accuracy, safety requirements, and system compatibility.

The secondary current rating of a current transformer is not randomly selected. The values of 1 ampere (1A) and 5 amperes (5A) have become international standards because they provide the best balance between:

Measurement accuracy
Cable transmission loss
Equipment compatibility
Protection performance
Installation cost

According to the IEC 61869-2 standard for current transformers, the standard rated secondary currents are 1A and 5A.

This article explains:

What CT secondary current means
Why CTs use 1A or 5A output
The difference between 1A CT and 5A CT
Advantages and disadvantages of each option
How to choose the correct CT secondary current for your application

What Is CT Secondary Current?

A current transformer (CT) is an electrical device used to reduce high primary current into a smaller standardized secondary current that can be safely measured by meters, relays, and monitoring systems.

A typical CT ratio is written as:

Primary Current / Secondary Current

Examples:

1000/5A CT Primary current: 1000A Secondary current: 5A or: 1000/1A CT Primary current: 1000A Secondary current: 1A

The primary side is connected to the high-current power circuit, while the secondary side connects to:

Energy meters
Protection relays
Power monitoring devices
Electrical control systems

The secondary current represents the actual primary current proportionally.

For example:

A 1000/5A CT means:

When the primary current reaches 1000A
The secondary current becomes 5A

A measuring device can then safely process this smaller current signal.

Technical current transformer di — Current Transformer Working Principle Diagram

Why Are CT Secondary Currents Standardized at 1A and 5A?

The reason CT manufacturers mainly produce 1A and 5A secondary current transformers is because these values have become industry standards.

There are several important reasons behind this standardization.

1. Historical Compatibility With Electrical Equipment

The 5A CT secondary current has been widely used for many decades.

When early electrical systems were developed, most measuring instruments and electromechanical protection devices were designed around a 5A input.

Traditional equipment such as:

Analog ammeters
Electromechanical relays
Protection devices
Panel meters

were commonly manufactured with 5A current inputs.

Because millions of electrical installations were already using 5A systems, maintaining this standard ensured compatibility between:

Current transformers
Measuring instruments
Protection equipment
Switchgear systems

Changing the standard would require replacing a huge amount of existing electrical infrastructure.

Therefore, 5A remains widely used today, especially in existing substations and industrial electrical systems.

2. Standardization Reduces Engineering and Manufacturing Costs

One major advantage of having only two common secondary ratings is standardization.

Electrical manufacturers can design equipment around:

1A input
5A input

instead of creating many different versions.

This reduces:

Manufacturing complexity
Spare part requirements
Engineering design costs
Maintenance difficulties

For system designers, choosing between 1A and 5A becomes much easier because most electrical equipment supports one of these two standard inputs.

3. CT Secondary Current Affects Cable Loss

One of the most important reasons engineers choose between 1A and 5A CTs is cable power loss.

The power loss in CT secondary wiring follows the formula:

P=I2RP=I^2R

Where:

P = power loss
I = secondary current
R = resistance of secondary cable

Because the current is squared, even a small increase in current causes a large increase in losses.

Compare:

1A CT: 12=11^2=1

5A CT: 52=255^2=25

A 5A CT secondary circuit can have approximately 25 times higher cable loss compared with a 1A CT circuit using the same cable resistance.

This is why 1A CTs are preferred for applications with:

Long cable distances
Remote protection panels
Large substations
Distributed monitoring systems

For short cable distances, the difference may not be significant, which is why 5A CTs are still common.

4. CT Burden Considerations

Another important factor is CT burden.

The burden of a current transformer refers to the maximum load that can be connected to the CT secondary while maintaining its specified accuracy.

Burden includes:

Relay input impedance
Meter input impedance
Cable resistance
Terminal resistance

It is usually expressed in VA.

For example:

A CT may be rated:

1000/5A 15VA Class 0.5 or: 1000/1A 10VA Class 0.5

The CT must provide enough voltage to drive the connected burden.

The relationship is:

VA=I×VVA = I \times V

A lower secondary current means less voltage drop across cables and lower transmission losses.

Therefore, 1A CTs are often advantageous when the secondary circuit has higher impedance or longer wiring distances.

1A CT vs 5A CT: What Is the Difference?

Although both CT types perform the same basic function, their applications are different.

Feature	1A CT	5A CT
Secondary current	1 ampere	5 amperes
Cable loss	Very low	Higher
Long-distance wiring	Excellent	Less suitable
Traditional systems	Less common	Very common
Cable size requirement	Smaller	Larger
Installation cost	Lower cable cost	Lower equipment replacement cost
Modern digital systems	Preferred	Still available
Existing substations	Sometimes requires upgrades	Widely compatible

Why Is 1A CT Better for Long-Distance Applications?

One of the biggest advantages of a 1A current transformer is its ability to reduce secondary circuit losses over long distances.

In modern electrical systems, CTs are often installed far away from measuring devices or protection relays.

Examples include:

Large power substations
Renewable energy plants
Industrial distribution systems
High-voltage switchgear installations

In these applications, the distance between the CT and the relay panel can reach tens or even hundreds of meters.

Because CT secondary cables have resistance, current flowing through the cable creates power loss.

The loss is calculated using:

P=I2RP=I^2R

Since current has a squared relationship with loss:

1A current produces relatively low losses
5A current produces significantly higher losses

For example:

Assume the secondary cable resistance is: R=1ΩR=1\Omega

For a 1A CT: P=12×1=1WP=1^2 \times 1=1W

For a 5A CT: P=52×1=25WP=5^2 \times 1=25W

The 5A CT produces 25 times more cable loss.

Therefore, when the CT secondary wiring is long, using a 1A CT can improve:

Measurement accuracy
Energy efficiency
Protection reliability

This is why many modern substations and digital protection systems prefer 1A secondary current transformers.

Why Is 5A CT Still Widely Used?

Although 1A CTs provide advantages for long-distance applications, 5A CTs are still extremely common worldwide.

There are several reasons.

1. Existing Electrical Infrastructure

A large number of industrial plants, substations, and commercial buildings already use 5A CT systems.

Replacing all existing equipment with 1A systems would require:

New protection relays
New meters
New wiring configurations
Additional engineering costs

Therefore, maintaining 5A compatibility is often the most practical solution.

2. Short Distance Measurement Applications

For installations where the CT and measuring device are close together, cable loss is usually very small.

Examples:

Electrical panels
Motor control centers
Low-voltage distribution cabinets
Compact switchboards

In these cases, a 5A CT works effectively because:

Cable length is short
Voltage drop is minimal
Existing equipment compatibility is important

3. Traditional Protection Systems

Many conventional protection systems were designed around 5A inputs.

Examples include:

Overcurrent protection relays
Transformer protection systems
Generator protection equipment

Although modern digital relays increasingly support 1A inputs, 5A remains common in many legacy installations.

1A vs 5A Current Transformer: Which One Should You Choose?

Choosing between a 1A CT and a 5A CT depends mainly on the electrical system design.

There is no universal answer.

The correct choice depends on:

Cable length
Relay or meter requirements
System voltage level
Installation environment
Existing equipment

Application Comparison: 1A CT vs 5A CT

Application Condition	Recommended CT Secondary
Short cable distance	5A CT
Existing 5A protection system	5A CT
Long-distance secondary wiring	1A CT
Large substations	1A CT
Digital protection systems	1A CT
Renewable energy projects	1A CT
Industrial control panels	5A CT
New electrical installations	Usually 1A CT

When Should You Choose a 1A Current Transformer?

A 1A CT is usually recommended when:

1. The Secondary Cable Is Long

If the CT is far away from the measuring equipment, cable resistance becomes an important factor.

A 1A output reduces:

Voltage drop
Cable heating
Measurement errors

2. The System Uses Modern Digital Relays

Many modern protection devices are designed for low-current CT inputs.

Advantages include:

Better accuracy
Lower wiring losses
Easier integration with digital monitoring systems

3. The Project Requires Energy Efficiency

Large electrical installations often operate continuously for decades.

Reducing secondary circuit losses can improve:

Long-term reliability
Energy efficiency
System performance

When Should You Choose a 5A Current Transformer?

A 5A CT remains a good choice when:

1. Existing Equipment Uses 5A Input

If the relay, meter, or monitoring device requires a 5A input, selecting a 5A CT avoids unnecessary replacement costs.

2. Cable Distance Is Short

For short secondary connections:

Cable losses are low
Accuracy remains stable
Installation is simple

3. Replacement Projects Require Compatibility

Many maintenance projects involve replacing old CTs while keeping existing equipment.

In these cases, a 5A CT is often the easiest solution.

Does 1A CT Have Better Accuracy Than 5A CT?

A common misunderstanding is that:

"1A CT is always more accurate than 5A CT."

This is not necessarily true.

The accuracy of a current transformer depends on:

Accuracy class
Core material
Burden rating
Manufacturing quality
Operating conditions

For example: A: 1000/1A Class 0.5 CT, and: 1000/5A Class 0.5 CT

can have the same accuracy class.

The difference is mainly related to:

Secondary current level
Cable losses
Application suitability

A high-quality 5A CT can be more accurate than a poorly designed 1A CT.

Can a 1A CT Replace a 5A CT?

In most cases:

No, a 1A CT cannot directly replace a 5A CT.

The reason is that connected equipment must match the CT secondary rating.

For example:

A protection relay designed for 5A input may not operate correctly with a 1A CT.

Before replacement, engineers must check:

Relay input rating
Meter specifications
CT ratio
Burden requirements
Accuracy class

If a system needs conversion between 1A and 5A, special CT conversion devices may be required.

Can a 5A CT Be Used Instead of a 1A CT?

The same principle applies.

A 5A CT cannot simply replace a 1A CT.

The entire secondary circuit must be designed for the selected current rating.

Important parameters include:

Rated secondary current

Rated burden

Accuracy class

Protection requirements

How to Calculate CT Secondary Burden?

CT burden calculation helps engineers select the correct CT.

The total burden includes:

Total Burden=Meter Burden+Relay Burden+Cable BurdenTotal\ Burden = Meter\ Burden + Relay\ Burden + Cable\ Burden

Cable burden is:

VA=I2RVA = I^2R

Where:

I = secondary current
R = cable resistance

Because the current is squared, 5A CT systems experience much higher cable burden.

Example:

Assume:

Cable resistance: R=0.5ΩR=0.5\Omega

For 5A CT:

VA=52×0.5VA=5^2 \times0.5 VA=12.5VAVA=12.5VA

For 1A CT:

VA=12×0.5VA=1^2\times0.5 VA=0.5VAVA=0.5VA

This demonstrates why 1A CTs are preferred for long secondary circuits.

How to Select the Right CT Secondary Current: 1A or 5A?

Choosing between a 1A and 5A current transformer depends on the application requirements, system design, and connected equipment.

There is no universal answer. The correct CT secondary current should be selected based on:

Cable distance
Relay or meter input
Accuracy requirements
CT burden
Existing system compatibility

1. Check Equipment Compatibility

The first step is confirming the input rating of the connected devices.

Before selecting a CT, check:

Protection relay input current
Energy meter specification
Monitoring system requirements

A 1A CT must connect to a 1A input device, and a 5A CT must connect to a 5A input device.

They cannot normally be exchanged directly without system modification.

2. Consider Secondary Cable Distance

Cable length is one of the most important factors.

Choose 1A CT when:

CT is installed far from the relay panel
Secondary cable runs are long
Low transmission loss is required

Because cable loss follows:

P=I2RP=I^2R

a 5A CT produces significantly higher losses than a 1A CT under the same cable resistance.

Choose 5A CT when:

Cable distance is short
CT and measuring devices are installed nearby
Existing equipment already uses 5A input

3. Consider CT Burden and Accuracy

The CT burden includes:

Meter burden
Relay burden
Cable resistance
Terminal resistance

The total burden must be within the CT rated burden.

A proper CT selection should consider:

Secondary current
Accuracy class
Rated burden
Protection or metering requirements

Incorrect burden selection may reduce accuracy or affect protection performance.

1A CT vs 5A CT: Quick Selection Guide

Application	Recommended CT
Long cable distance	1A CT
Digital protection system	1A CT
Large substations	1A CT
Existing 5A equipment	5A CT
Short-distance measurement	5A CT
Traditional industrial systems	5A CT

Common Questions About CT Secondary Current

Why is CT secondary current standardized at 1A and 5A?

Because these values provide a good balance between:

Equipment compatibility
Measurement accuracy
Cable loss control
Manufacturing standardization

Is 1A CT better than 5A CT?

Not always.

A 1A CT is usually better for long-distance applications because it reduces cable losses.

A 5A CT is often better for existing systems where equipment already uses 5A inputs.

The best choice depends on the project requirements.

Can a 1A CT replace a 5A CT?

No. The CT secondary rating must match the connected equipment.

Before replacement, engineers should verify:

Relay input rating
Meter compatibility
Burden requirements
Accuracy class

Why don't manufacturers use other CT secondary currents?

The electrical industry has standardized around 1A and 5A because common ratings simplify:

Equipment design
Installation
Maintenance
Spare parts management

Future Trend of CT Secondary Current

With the growth of:

Smart grids
Digital substations
Renewable energy systems
Intelligent protection equipment

the demand for 1A current transformers is increasing because of their lower cable losses and better performance over long distances.

However, 5A current transformers will continue to be widely used because of existing infrastructure and compatibility requirements.

Need A 1A or 5A Current Transformer Solution?

Selecting the correct CT secondary current is essential for accurate measurement and reliable protection.

Xfullstar Technologies provides:

1A current transformers
5A current transformers
Metering CTs
Protection CTs
Split core CTs
Customized current transformer solutions

Our engineers can help you choose the correct CT ratio, accuracy class, and burden rating according to your application.

Conclusion

CT secondary current is standardized at 1A and 5A because these ratings provide the best balance between performance, compatibility, and cost.

In general:

1A CT → Better for long-distance wiring and modern digital systems.
5A CT → Better for traditional systems and short-distance applications.

Understanding the difference between 1A vs 5A current transformers helps engineers design safer, more efficient, and more reliable electrical systems.

For more information, please contact us at sales@xfullstar.com