How to Select the Right Current Transformer for Your Application

Important Points for Selecting a CT

• Current Transformer Output
• Conductor Size
• Load Size or Amperage Range
• Accuracy Rating
• Form Factor
• Regulatory Requirements
• Using Rogowski Coils

Help me choose the right CT

If you have a power measurement project coming up, chances are you have narrowed down your search for a power meter to a few choices. Whether it’s a multi-circuit application or a high-precision metering in an industrial setting, the next step in project preparation is selecting the right current transformer to maximize the performance of your power meter. When going through the selection process, it can be helpful to answer a few, basic application questions to reach a decision and consider several parameters including current transformer output, conductor size, amperage range, and accuracy. If you need help deciding, reach out to your power meter manufacturer so they can help guide you to the CT that best meets your project’s measurement objectives and budget.

CT OUTPUT:

What current transformer output is your power meter compatible with?

Current transformers are available with several output options, some of the most popular of which include 333mV, 5A, or 80mA. A critical question in the current transformer selection process, it is important to note which output your metering equipment is compatible with. While it is possible that the meter may work with multiple output options, it may not be possible to make in-field adjustments to this setting or it may need to be configured by the factory.

Unlike typical split-core or solid-core current transformers, Rogowski coils have a unique output that is generally rated at a low AC voltage (e.g., 150mV or less) per 1000A. In addition, there is an inherent 90-degree phase shift. Many meters and other measurement devices require a higher signal than what a Rogowski can provide on its own and are not configured to compensate for the phase shift, so it is important to work with your meter manufacturer to determine whether this specialized CT is directly compatible with your device.

CONDUCTOR SIZE:

Are you measuring around large busbars/conductors or small branch circuits?

The dimensions of the conductor are a critical consideration and can be one of the leading deciding factors in CT selection. Any CT that is used needs to be able to physically fit around the conductor you plan to measure. At the same time, oversizing a CT to accommodate a small conductor may not make sense in terms of both cost and the space required in the electrical panel, which may not have enough room to accommodate a large, rigid current transformer. In this situation, a flexible Rogowski coil can make it easier to measure in crowded electrical panels or switchgear because they can easily slip around oversized bus bars in tight spaces, making them an ideal compromise between large window size and flexible functionality.

LOAD SIZE:

How many amps will you be measuring?

Like the physical dimensions, the size of the load under measurement is a key consideration. All current transformers have a current input range, or amperage range, specification which indicates the size of the load they can effectively measure. If the load fluctuates throughout the day—for example, when occupancy is low during evening hours—it can be helpful to choose a current transformer with a broad current sensing range, such as a flexible Rogowski coil. It is also important to note that, if a load goes outside the sensor’s range, the meter may not be able to measure the load accurately, so it is important to always choose a sensor with a range that matches what you intend to measure.

ACCURACY RATING:

Does the project involve billing tenants for their consumption?

When it comes to tenant billing, selecting equipment with the highest accuracy is of the utmost importance. In fact, in any application where “money changes hands,” power monitoring equipment must meet certain accuracy requirements and is often labeled “revenue grade” to indicate its conformance to accuracy standards. What does revenue grade accuracy mean? It is generally understood to be better than 1% accuracy and, more often, in the range of 0.5% accuracy or better. Before selecting a revenue grade sensor, be sure to check which industry accuracy standards they meet to ensure the accuracy class matches your project requirements. A common revenue grade accuracy standard is IEC 60044-1 0.5 Class.

On the other hand, if you’re simply collecting overall consumption trend data for a facility, a 1% accuracy sensor may be sufficient, and you may not need to upgrade to a revenue grade model.

FORM FACTOR:

Will the project be new construction or a retrofit application?

This question may also be framed as, “Will a split-core or solid-core current transformer be a better for my application?” Although either sensor type may be used for any job, it is almost always easier to use a split-core, or a Rogowski coil, current transformer for a retrofit application because it can easily open to fit around a conductor and does not require wire disconnection as part of the installation process. Alternatively, while a facility is still under construction, installing a solid-core CT does not require much additional work since facility shutdowns or wire disconnection are not yet disruptive. Another consideration is cost: Although the up-front price of a solid core CT is lower, the initial savings is negligible when compared to the largely uncalculated installation cost which must include shutdowns and disconnections, adding time and labor to the overall project.

REGULATORY REQUIREMENTS:

Does your application require a sensor that meets UL or other regulatory certifications?

A UL Listed current transformer has undergone rigorous testing to ensure that it complies with nationally recognized safety standards. Unlike a current sensor that is a UL Recognized Component, which means it is intended to be a component within a complete system or product, a UL Listed sensor can be sold as an end-user product and is designed to minimize installation hazards such as shock or fire. It may be that your application mandates a UL Listed current sensor to meet safety code requirements. If this is the case, be sure to look for CTs with a UL Listed marking that indicates they comply with XOBA UL2808 and CSA C22.2 61010-1.

Another key regulatory requirement is a CE mark. This mark is required for products used in the European Economic Area (EEA) which includes countries like Germany, France, Spain, Italy, and others. Unlike other quality marks, such as UL, the CE mark on a product means that it conforms with European safety, health, and environmental standards. The CE mark should be visible on product labeling and documentation.

A third regulatory requirement you may encounter concerns Measurement Canada approval. Tenant billing applications in Canada may require both a Measurement Canada approved meter and current transformers, each of which must meet rating, design, accuracy, testing, and other requirements. For example, a few characteristics of Measurement Canada approved CTs include that they must be solid core, meet an accuracy class of 0.6% or better, and be either 5A, 80mA, or 100mA output devices. The nature, scope, and location of your project will dictate whether Measurement Canada approval is required. Check the product labeling and documentation to determine whether a sensor meets the regulatory requirements.

USING ROGOWSKI COILS:

My power meter does not work directly with Rogowski coils. Is there a way that I can still use a rope CT?

Nearly any project can benefit from Rogowski coil current transformers which offer many advantages including a large window size, broad amperage range, lightweight flexibility, and no saturation point. However, if your power meter only accepts 333mV, 5A, 1A, or another standard output, it will not directly work with a Rogowski coil. Fortunately, there is a simple solution to this challenge and that is to use an integrator. An integrator is an electronic device that makes it possible to change the output of a Rogowski coil to a commonly accepted output, such as 333mV or 5A, so that it can work with a host a power meters, protection relays, or other devices. By adjusting the input ranges to match nearly any system, an integrator is a simple solution to solve a common compatibility dilemma and bridges the divide between Rogowski coils and industrial metering equipment.