To choose a current transformer (CT), match its ratio to the primary current and the measuring device, select an accuracy class suited to the application, and confirm that the connected secondary circuit stays within the CT’s rated burden. These specifications work together: an unsuitable ratio can limit useful measurement, while excessive burden can affect accuracy and, in protection applications, performance during fault conditions. I recommend confirming the system details and applicable standard before comparing product prices or requesting a quotation.
This guide explains how to read CT ratios, distinguish metering and protection accuracy classes, calculate burden, and prepare a practical supplier inquiry. At ANNEZEE, we use these details to help buyers define a suitable specification rather than relying on a single headline value.
A CT ratio expresses the relationship between its rated primary current and rated secondary current. For example, a 100/5 A CT is designed to produce 5 A secondary when 100 A flows in the primary under its specified operating conditions. The ratio is nominal; actual performance also depends on the CT’s accuracy class, burden, frequency, and other design conditions.
Choose a primary rating that covers the expected operating current and any relevant system requirements. A rating that is much higher than normal operating current may provide less useful measurement resolution for some applications, while a rating that is too low may not suit expected current levels. Confirm the required secondary current with the meter, relay, or monitoring equipment: 1 A and 5 A are common options, but they are not interchangeable unless the connected equipment supports the selected input.
Where an application has variable loads, share the expected normal and maximum current with the supplier rather than choosing from the maximum value alone. For example, a proposed 200/5 A ratio may be appropriate for one installation and unsuitable for another, depending on actual load conditions and instrument requirements. This is a specification example, not a universal recommendation.
The accuracy class indicates the CT’s defined measurement or protection performance under conditions specified by the relevant standard. Metering CT classes are intended for measurement applications; protection CT designations describe performance relevant to protection duties. A class number should not be interpreted in isolation because the applicable standard, rated burden, current range, and test conditions determine what the designation means.
For metering, classes such as 0.2, 0.5, and 1 may appear in specifications, with the exact meaning governed by the applicable standard. Buyers should verify the standard and required operating range instead of assuming that the number guarantees identical performance across manufacturers or test conditions. For protection, designations such as 5P10 are used in some IEC-based specifications; the meaning and applicable requirements should be confirmed against the selected standard and project documentation.
In practical terms, specify a metering class for energy measurement or a protection class where the CT is part of a relay protection scheme. Some installations need separate cores for metering and protection because the functions have different performance requirements. Ask the supplier to confirm the core arrangement, class, rated burden, and standard for each required function.
Rated burden is the load the CT secondary is designed to supply while meeting its specified performance requirements. It is commonly stated in volt-amperes (VA) at the rated secondary current. The connected burden includes the input circuits of instruments and relays, wiring, and other components in the secondary loop—not just the meter shown on the drawing.
For a resistive approximation, burden can be calculated as VA = I² × R, where I is the secondary current in amperes and R is the total secondary-circuit resistance in ohms. At 5 A, a 10 VA burden corresponds to approximately 0.4 Ω; at 1 A, 10 VA corresponds to approximately 10 Ω. These calculations illustrate the relationship and do not replace the manufacturer’s stated rating or a complete assessment of circuit impedance.
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Long or undersized secondary wiring can add burden. Include the outgoing and return conductors when estimating the loop resistance, then add the burden of connected devices. If the calculated total is close to or above the CT’s rated burden, review the wiring, device inputs, CT selection, and applicable project requirements with a qualified electrical professional.
Record the system’s nominal voltage, frequency, expected primary-current range, installation type, and available mounting space. Identify whether the CT is for measurement, protection, or both, and note any environmental or enclosure requirements. These details help prevent a technically correct ratio from being paired with an unsuitable construction or connection arrangement.
Check the expected normal and maximum current, then confirm the secondary input required by the connected equipment. State the ratio in a clear format, such as 100/5 A, and identify whether the project requires a single ratio or multiple ratios. For a project with changing load conditions, discuss the expected operating range with the supplier before finalizing the rating.
Specify the purpose of each CT core and the accuracy class required by the project or applicable regulations. For billing or energy measurement, verify the relevant metering requirements with the utility, meter provider, or project engineer. For protection, provide the relay and system requirements so that the CT can be evaluated for the intended protection duty; do not select a protection class solely by comparing class labels.
List every device connected to the secondary circuit and obtain its burden or input impedance from its documentation. Estimate the wiring contribution using the conductor length, conductor size, and complete circuit path, then compare the total with the CT’s rated burden. Ask the supplier to confirm the rating at the selected secondary current and accuracy class.
Specify whether the design should be window, bar, or wound type, based on conductor arrangement and installation needs. Also confirm terminal layout, number of cores, polarity markings, frequency, insulation requirements, and any required test or product documentation. Standards and regulatory requirements vary by project and market, so state the applicable standard rather than assuming one applies universally.
A complete request helps us assess the application and respond with fewer clarification steps. Include the required ratio, secondary current, accuracy class, rated burden, application, frequency, installation type, number of cores, applicable standard, and estimated quantity. Drawings or device data sheets can also help clarify mounting, connection, and burden requirements.
At ANNEZEE, we can review the specification details you provide and discuss suitable CT configurations, product options, and documentation requirements. Availability, customization, minimum order quantities, and lead times depend on the requested design and order conditions, so we confirm these points during quotation rather than making a general promise.
The right current transformer is not selected by ratio, accuracy class, or rated burden alone. Match the ratio to the expected current and instrument input, select a class for the measurement or protection function, and verify that the complete secondary circuit remains within the rated burden. Then confirm construction, standards, and installation requirements before placing an order.
As a practical next step, prepare your current range, device inputs, wiring information, required class, and applicable standard. Send these details to ANNEZEE for a specification review and quotation discussion, so your purchasing decision is based on the actual application rather than an incomplete comparison.
Contact us to discuss your requirements of Current Transformer Buying Guide: Ratios, Accuracy Classes and Rated Burden. Our experienced sales team can help you identify the options that best suit your needs.