Current Transformers Selection Guide

A current transformer is an instrument transformer that reduces the high primary current by a set ratio and passes it to meters and relays. Busbar, cable and split-core types.

What is a current transformer? A current transformer is an instrument transformer that reduces the high current flowing through the primary conductor by a set ratio and passes it from its secondary to a measuring instrument, meter, reactive power relay or protection relay. For example, a transformer with a 100/5 A ratio delivers the 100 A current in the primary as 5 A in the secondary. It is used in power factor correction, energy monitoring and metering panels.

This guide explains what to look at when choosing products in the Current Transformers category.

Types of current transformers

• Busbar type (window): The busbar is passed through the rectangular window of the transformer and forms a single-turn primary. The dimension in the product name, such as 30x10 or 60x30, gives the size of busbar that can pass through the window, in millimeters.
• Cable type: Its window is designed for a cable; the product name gives a diameter such as ø 20 mm or a window size.
• Split-core: The core is opened and closed around the conductor; preferred when adding measurement to an existing panel without removing the busbar or cable.
• Residual current toroid: The phase and neutral conductors of the circuit are passed through it together; in the secondary only the unbalanced part of these currents, that is, the residual current, is seen. It is used with residual current relays and is not for load current measurement; confirm from the manufacturer data which relay it pairs with.

How to read the label

On a transformer that reads "400/5 A, 5 VA, class 0.5", the values are read as follows:

• 400/5 A: This is the transformation ratio; while 400 A flows through the primary, 5 A flows from the secondary. The secondary current is standardized as 5 A or 1 A.
• 5 VA: This is the rated power; the secondary circuit burden that the transformer can supply while maintaining its declared accuracy.
• Class 0.5: This is the accuracy class; it indicates that the ratio error at rated current does not exceed 0.5 percent. Classes 0.1, 0.2, 0.2S, 0.5, 0.5S, 1 and 3 are used for instrument transformers; the product data in this category includes 0.2, 0.5 and 1.

Manufacturers give different VA values in different classes for the same transformer; compare two products on the value in the same class.

What to look at when choosing a current transformer

1. Primary current: The nearest higher value that covers the continuous operating current of the line is chosen. An oversized primary makes the transformer operate at a small fraction of its rated current and increases the measurement error at low load. For the line current, you can find a preliminary value with the current and fuse calculation from active power tool.
2. Secondary current: It is determined by the current input of the device to be connected. If the distance between the transformer and the device is long, a 1 A secondary reduces the voltage drop in the cable and the burden on the transformer. LVCT models with 1/3V in their names have a voltage output, not a current output; they are used only with devices that support this input.
3. Window size: The busbar or cable, together with its insulation, must pass through the window; the Size Type attribute gives busbar sizes such as 30x10 and 60x30. If the busbar cross-section is not yet known, see the busbar current carrying capacity calculation tool and the busbars and distribution blocks category.
4. Rated power (VA): The current circuit consumption of the device and the burden of the secondary cable are added together; the transformer must cover this total. The most common cause of overburden is not the device but the long cable between the transformer and the device. On the other hand, in instrument classes such as 0.2, 0.5 and 1, the error limits are defined for a burden between 25 and 100 percent of the rated power; choosing a VA value far larger than necessary does not increase accuracy.
5. Accuracy class and seal: Indicative measurement and billing-grade measurement do not require the same class. The class required in the metering circuit and the sealing requirement are determined by the regulations in force and the distribution company; the Seal Type attribute separates sealed and unsealed models.

Points to watch in wiring

• Direction: Primary terminals are marked P1 and P2 and secondary terminals S1 and S2; the older marking uses K-L and k-l. If the transformer is installed backwards or the secondary terminals are swapped, the analyzer reads power in the opposite direction and the reactive power control relay makes the wrong step decision.
• Phase matching: The secondary of each transformer is connected to the current input of the same phase on the device.
• Open secondary: If the secondary circuit is disconnected from its burden while current flows through the primary, a voltage that can reach several kilovolts may appear across the terminals. Work on the circuit is carried out according to the manufacturer's instructions and occupational safety rules.
• Reactive relay transformer: The transformer feeding the relay is installed at the point where it sees the total current of the loads and capacitor steps, and on the phase shown in the relay's wiring diagram.

Brands in this category

• Schneider Electric: transformers with the METSECT5 code; split-core G types for busbar (GA, GD, GG, GJ), split-core H types for cable (such as HA, HD, HG), CC and M types with a single dimension in the name (such as 21 mm, 27 mm), and D types with two dimensions (such as 32x65, 54x102). VigiPact closed toroids are also under this brand.
• Entes: ENT.A series busbar-type transformers; the window size is written in the product name (such as 20x10, 30x10, 60x30, 80x30).
• ABB: transformers with the CT1M, CTA and TRFM codes.
• Siemens: transformers with the 4NC5 code.
• Tense: AT and ATM series low-voltage transformers; models with a ø 20 mm round window and with busbar windows such as 30 x 10 mm and 60 x 10 mm.
• Sigma: ST series toroidal current transformers.

Order notes

Products are sold per piece; power network analyzers that measure the three phases separately need one transformer per phase. Because the category also lists items that are not current transformers, such as energy meters, check the product type before adding to the cart. Some of the products are listed as made-to-order items; for these products, the delivery time is confirmed before the order. Product names carry the ratio, the VA rating and the window size or the manufacturer code (such as ENT.A30 400/5, METSECT5GD040, 4NC5232-2FE21); you can reach the product directly by typing the code into the search.

Current Transformer Types and Technical Comparison Table

Frequently asked questions

What does a 400/5 current transformer mean?

The first number is the primary rated current and the second is the secondary rated current. In a 400/5 transformer, when 400 A flows through the primary, 5 A flows from the secondary; the ratio is 80. This ratio is entered into the analyzer, meter or reactive relay, and the device calculates the actual line current by multiplying the secondary current it measures by the ratio. If the ratio entered into the device differs from the ratio on the label, all current, power and energy values are read wrong by the same factor.

Should a 5 A or a 1 A secondary be chosen for a current transformer?

First look at the current input of the device to be connected; the transformer is chosen with whichever secondary current the device was built for. If both inputs are supported, distance becomes the deciding factor. As the cable between the transformer and the device gets longer, its resistance and the burden it places on the transformer increase; a 1 A secondary causes less voltage drop on the same cable. For short connections inside the panel, a 5 A secondary is common.

How is the VA rating of a current transformer determined?

The current input consumptions of the devices connected to the secondary circuit and the burden of the outgoing and return cable are added together. The transformer's rated power must cover this total in the required accuracy class. The device consumption is given in VA in the user manual; it is small in digital analyzers, and the cable often makes up most of the total. Because the VA value of the same transformer changes with the class, the class and VA on the label must be read together.

When is a split-core current transformer preferred?

Because its core can be split into two parts, it is fitted around the conductor without removing the busbar or cable. It is preferred when adding an analyzer or energy monitoring to an existing panel afterwards, and where removing the busbar is difficult. In a newly built panel, busbar-type transformers with a solid body are usually used. Whether installation can be done while energized depends on the manufacturer's instructions and occupational safety rules.

Where is the current transformer of a reactive power control relay connected?

The transformer is installed at the point where it sees the total current of the facility's loads and capacitor steps, that is, on the mains side of the power factor correction panel connection. With a transformer that sees only the load current or only the capacitor current, the relay cannot measure the result when it switches in a step and does not work correctly. Which phase the transformer is installed on and the direction of its secondary terminals are shown in the relay's wiring diagram.

What happens if a current transformer is connected backwards?

In a transformer whose primary direction or secondary terminals are connected backwards, the phase of the current rotates by 180 degrees. The analyzer reads power on that phase as negative, that is, in the opposite direction; the energy and power factor values come out wrong. The reactive power control relay may see an inductive load as capacitive and manage the steps wrongly. Some devices report this condition with a reverse connection warning; the correction is made according to the device's manual.