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

To select a current transformer (CT), match its primary-to-secondary ratio to the expected line current, choose an accuracy class suited to metering or protection, and confirm that the connected wiring and instruments stay within the CT’s rated burden. For example, a 100:5 CT is designed to produce 5 A on its secondary when 100 A flows in its primary under its specified operating conditions. A suitable choice depends not only on these three ratings, but also on system voltage, installation space, frequency, and applicable standards. At ANNEZEE, we recommend sharing the full application details with your supplier before confirming a specification.

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Key takeaways

• Ratio: Choose a primary rating that reflects the normal operating current while allowing for the expected maximum current.
• Accuracy class: Select a metering or protection class according to what the CT must measure or help protect.
• Rated burden: Check the total secondary-circuit load, including instruments, wiring, and connection resistance, against the CT’s rating.
• Safety: Never leave an energized CT secondary open-circuited; follow the manufacturer’s instructions and applicable site procedures.

What a current transformer does

A current transformer reduces a high primary current to a smaller, proportional secondary current that can be measured by instruments or used by protective devices. It also provides electrical separation between the primary circuit and the secondary measuring circuit, subject to the CT’s design and insulation ratings. Common applications include switchgear, distribution panels, energy monitoring, industrial equipment, and power-system protection. A CT is selected as part of a measurement or protection circuit, not as a standalone rating decision.

How to select the CT ratio

Match the ratio to operating current

The ratio expresses the rated primary current in relation to the rated secondary current. Common secondary ratings include 5 A and 1 A, but the appropriate choice depends on the connected equipment and system design. For instance, a 200:5 CT has a nominal ratio of 40:1, while a 200:1 CT has a nominal ratio of 200:1. Confirm the instrument or relay input before specifying the secondary rating.

Start with the normal and maximum expected primary currents, rather than choosing a ratio based only on the circuit’s nameplate or breaker rating. An oversized ratio can reduce measurement resolution at lower operating currents, while a ratio that is too small may not suit the expected current range. The correct balance depends on operating profile, overload conditions, and the requirements of the connected meter or protection device. Ask your electrical designer or supplier to review the expected current range where these factors are uncertain.

Check the secondary circuit

The CT secondary rating must be compatible with the device receiving its output. A meter designed for a 1 A input should not be assumed to work with a 5 A CT secondary, or vice versa. Also confirm the number of instruments and the secondary wiring arrangement because these affect total burden. Make sure the CT’s polarity markings and terminal connections align with the intended circuit design.

Choosing an accuracy class

Accuracy class describes how closely a CT’s output follows its specified performance limits under defined conditions. The class alone does not guarantee the same measurement performance at every current level or burden; the applicable standard and the CT’s rating details matter. Metering CTs are generally selected to support measurement, billing, or monitoring requirements, while protection CTs are specified for their behavior during fault conditions. Do not substitute one type for the other without checking the application and system requirements.

Metering and protection are different needs

For measurement applications, buyers may encounter classes such as IEC 0.2, 0.5, or 1, and ANSI classes such as 0.3. These designations belong to different standards and should not be treated as directly interchangeable. A tighter class may be appropriate where measurement accuracy is important, but the required class depends on the applicable specification, operating range, and intended use. Confirm which standard governs the project before comparing offers.

Protection CTs are selected to provide suitable performance for a relay or protective function, including under elevated current conditions. Specifications may include designations such as 5P10 or 10P10 under IEC terminology, but the required class and limits depend on the protection scheme. Buyers should provide the relay details, fault-current information, and any required protection criteria to the CT supplier or engineering team. A metering accuracy label by itself is not enough to define protection performance.

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Understand rated burden

Rated burden is the load a CT secondary is designed to drive while meeting its stated performance requirements. It is commonly expressed in volt-amperes (VA) at a specified secondary current and power factor, as defined by the applicable standard or product specification. The total connected burden can include meter or relay inputs, terminal blocks, and the resistance of the secondary wiring. If this total exceeds the CT’s rated burden, accuracy or functional performance may be affected.

A useful estimate for a resistive secondary circuit is burden in VA = secondary current squared × total circuit resistance in ohms. For example, a 5 A secondary circuit with a total resistance of 0.2 Ω represents 5 VA of resistive burden (5 A × 5 A × 0.2 Ω). This is a calculation example, not a recommendation for a particular CT. Use the actual device input data, conductor length and size, connection details, and the supplier’s calculation method when checking a real installation.

Why the 1 A or 5 A choice affects burden

For a given circuit resistance, a 5 A secondary produces more burden in VA than a 1 A secondary because current is squared in the calculation. This can make the wiring run an important consideration, particularly when the CT is located far from its meter or relay. However, changing secondary current affects compatibility and must be planned as part of the complete measurement circuit. Do not select 1 A solely on the assumption that it will solve a burden issue.

Buyer selection checklist

1. Define the application: State whether the CT is for metering, monitoring, protection, or another specified function.
2. Provide current data: Share normal current, expected maximum current, and any relevant fault-current or operating information.
3. Confirm the ratio and secondary: Match the primary rating and 1 A or 5 A secondary to the connected equipment.
4. Specify accuracy requirements: Identify the standard, accuracy class, operating conditions, and any project-specific requirements.
5. Calculate burden: Include instrument inputs, wiring resistance, terminals, and other loads in the secondary circuit.
6. Check construction and fit: Confirm mounting style, dimensions, conductor or busbar arrangement, insulation requirements, and environment.
7. Review documentation: Request the applicable datasheet, drawings, and test or compliance documents required for your project.

Common selection mistakes to avoid

One frequent mistake is choosing a ratio from the maximum circuit rating without considering the normal operating current. Another is treating accuracy classes from different standards as equivalent without checking their definitions and test conditions. Buyers can also overlook lead resistance, especially where long secondary wiring connects the CT to a remote panel. Finally, an energized CT secondary must not be left open-circuited, because this can create a hazardous voltage; follow the product instructions and site safety procedures whenever working on CT circuits.

Working with a CT supplier

A complete inquiry helps a supplier assess whether the proposed CT fits the electrical and mechanical requirements. We recommend sending the single-line diagram or relevant circuit details, the required ratio and secondary current, accuracy or protection class, burden estimate, installation dimensions, and applicable standards. If some values are not yet confirmed, identify them as pending instead of assuming a standard configuration will suit the project. This makes it easier to clarify options before quotation or production planning.

At ANNEZEE, we can review the information you provide and discuss CT configurations against your stated application requirements. Availability, customization options, documentation, minimum order quantities, and lead times depend on the requested specification and order details, so confirm them during the inquiry process. We do not recommend finalizing a CT based on ratio alone. Share your operating current, connected equipment, burden details, and installation requirements to request a specification review and quotation.

Conclusion

The right current transformer combines a suitable ratio, an application-appropriate accuracy class, and enough rated burden for the complete secondary circuit. Begin by defining what the CT must measure or protect, then verify current range, instrument compatibility, wiring burden, physical fit, and the governing standard. Before placing an order, compare the supplier’s proposed specification with your project documents and resolve any missing electrical or mechanical details. If you are sourcing CTs for a B2B project, contact ANNEZEE with those requirements so we can discuss a suitable configuration and next steps.