Current Transformer Buying Guide: Ratios, Accuracy Classes and Rated Burden

To choose a current transformer (CT), first match its ratio to the primary current and the measuring device’s required secondary input. Then select an accuracy class appropriate to the application and confirm that the connected meters, relays, and secondary wiring stay within the CT’s rated burden. At ANNEZEE, we recommend evaluating these specifications together rather than choosing a CT by ratio alone.

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This guide explains how to compare CT ratios, accuracy classes, and rated burden, along with the practical details buyers should confirm before requesting a quotation. The examples are illustrative selection references, not a substitute for checking the applicable product specification and local requirements. If you provide your application details, we can help clarify which information to include in a product inquiry.

Key Takeaways

• Choose a ratio that suits the expected primary current and the connected equipment’s secondary input.
• Match the accuracy class to the intended use, such as measurement or protection, and confirm the standard used to define it.
• Check total secondary burden, including instruments and wiring, against the CT’s rated burden.
• Specify installation, insulation, frequency, and terminal requirements alongside the three headline specifications.
• Never leave the secondary circuit of an energized CT open; follow the manufacturer’s instructions and applicable safety procedures.

What a Current Transformer Does

A current transformer reduces a larger alternating current in a power circuit to a smaller, proportional secondary current that can be measured or used by protective equipment. It allows compatible instruments and relays to monitor a circuit without carrying its full primary current. CTs are used in applications such as switchboards, distribution equipment, energy metering, and electrical protection.

A CT is specified for a particular installation and intended use. Its ratio, accuracy class, rated burden, frequency, insulation level, mounting arrangement, and secondary connection all affect whether it is suitable. Buyers should review the complete specification rather than assuming that two CTs with the same ratio are interchangeable.

How to Select the CT Ratio

Match the primary rating to the circuit

The ratio expresses the relationship between the rated primary current and rated secondary current. For example, a CT marked 100/5 A is designed for a rated primary current of 100 A and a rated secondary current of 5 A. A 100/1 A CT has the same stated primary rating but a different secondary output, so it must be matched to equipment designed for a 1 A input.

Begin with the circuit’s nominal and expected operating current, then check the relevant design conditions, including possible overloads and the measurement range required. Do not select a ratio solely because its primary rating matches a nameplate value; the actual load profile and the connected instrument’s input rating matter too. Where current varies substantially, ask the supplier or design engineer to review whether the proposed ratio meets the measurement or protection objective.

Confirm secondary compatibility

Common secondary ratings include 5 A and 1 A, but neither is automatically the right choice for every project. The meter, relay, or monitoring device must accept the selected secondary current, and the wiring arrangement must be suitable for it. Verify device documentation before ordering, particularly when replacing an existing CT or adding a CT to an established panel.

Ratio selection also affects how the measurement system uses its range. A ratio that is poorly matched to normal operating current can make readings less useful for the intended task, while a ratio chosen without regard to the instrument can prevent correct connection. Specify both the primary and secondary ratings in the inquiry to avoid ambiguity.

Choose an Accuracy Class for the Application

Measurement and protection requirements differ

Accuracy class describes performance under defined conditions in the relevant product standard. Metering applications prioritize reliable current measurement over the specified operating range, while protection applications require CT performance suited to the connected protective function, including behavior under fault conditions. The required class and designation depend on the governing standard and the device or system design.

When comparing offers, identify whether the specification follows an IEC, IEEE, or other applicable standard. Class names and performance requirements are not necessarily interchangeable across standards, so a label alone may not establish equivalence. Ask suppliers to state the standard, the complete class designation, and any conditions or ratings associated with it.

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Consider the working range and purpose

For energy or panel metering, confirm that the CT’s specified accuracy is appropriate at the current levels the system is expected to measure. For protection, the engineer should select the class and related characteristics according to the relay and protection scheme. If one installation requires both measurement and protection functions, check whether a CT with separate cores or another specified arrangement is needed.

Accuracy should not be treated as a standalone promise that applies in every installation. Performance is evaluated under specified conditions, and the complete circuit—including burden—must be considered. For this reason, provide the intended use and connected device details when asking for a recommendation.

Understand Rated Burden

Rated burden is the load a CT secondary is designed to supply under specified conditions. It is commonly stated in volt-amperes (VA) at the rated secondary current, although the applicable standard and product documentation define the exact rating basis. The total connected burden includes instrument or relay inputs, secondary leads, and relevant connections.

For a resistive-equivalent load, burden can be estimated using the relationship VA = I² × Z, where I is secondary current in amperes and Z is the relevant impedance in ohms. As a simple illustration, a 5 A secondary circuit with an equivalent impedance of 0.2 Ω represents 5 VA. This calculation is a screening aid; use the manufacturer’s specified method and account for the actual circuit and applicable requirements.

Do not assume that a higher rated burden is always a better choice. The CT must still meet the application’s accuracy and performance requirements, and the connected circuit should remain within the specified burden range. Long secondary runs can make lead resistance a relevant part of the calculation, so include the wiring layout when the distance is significant.

Buyer’s CT Selection Checklist

Common Buying Mistakes to Avoid

• Specifying only a ratio: A ratio does not define the accuracy class, burden, installation style, or insulation requirements.
• Ignoring the secondary input: Confirm whether connected devices require a 1 A or 5 A input before selecting the CT.
• Leaving out wiring burden: Include the secondary cable route and connected equipment when assessing the total load.
• Comparing class labels without standards: Request the governing standard and complete designation before treating two offers as equivalent.
• Opening an energized secondary circuit: An energized CT secondary must not be left open. Follow approved safety procedures and manufacturer instructions, including appropriate shorting arrangements where required.

Working with a CT Supplier

A clear inquiry helps a supplier assess the request and identify missing information. Include the primary and secondary ratings, intended application, required accuracy class and standard, rated burden, frequency, installation dimensions, and any relevant insulation or environmental requirements. If you are replacing an existing unit, provide its nameplate details and mounting information where available.

At ANNEZEE, we can review the information you provide and discuss the CT specification needed for your project. Before placing an order, confirm the final technical data, drawings or dimensions, quantity, packaging expectations, and delivery requirements with the supplier. This step helps align the quotation with the intended installation rather than relying on an incomplete specification.

Conclusion: Make the Specification Complete

The right current transformer is selected by matching its ratio to the circuit and secondary device, choosing an accuracy class for the measurement or protection task, and keeping the connected load within the rated burden. Buyers should also confirm the governing standard, frequency, insulation needs, and physical installation details. No single specification can determine suitability on its own.

As a next step, collect the circuit current, device input rating, intended function, estimated wiring burden, and mounting requirements before requesting a quotation. Share those details with ANNEZEE for a focused discussion of your CT requirements. A complete specification makes product evaluation clearer and supports a more informed purchasing decision.

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