Current Transformer Basics: Types, Accuracy Classes, and Selection Criteria for Buyers

A current transformer (CT) is an instrument transformer that converts a high primary current into a proportional low secondary current for measurement, monitoring, and protection. This article explains the main CT families, accuracy concepts, and the selection logic buyers need when sourcing from a Chinese current transformer manufacturer.

What Is a Current Transformer?

A current transformer is a passive measuring device used in low-voltage and medium-voltage AC systems. Its primary winding is connected in series with the conductor carrying the load current; its secondary winding provides a reduced current — typically 5 A, 1 A, mA, mV, or a standardized process signal such as 4-20 mA — to meters, relays, controllers, and energy-management systems. Because the secondary output is isolated from the primary circuit, the CT also provides galvanic isolation and enables safe instrumentation.

CTs are widely used in smart grid and switchgear panels, power distribution systems, renewable energy plants, EV charging infrastructure, data centers, and industrial facilities. The global current transformer market was estimated at USD 2.63 billion in 2024 and is projected to reach USD 3.90 billion by 2030, according to Grand View Research. Asia Pacific accounted for the largest share at 40.15% in 2025, according to Straits Research. These figures indicate that buyers and engineers are increasingly relying on CTs for energy efficiency, sub-metering, and grid reliability.

Core CT Categories Buyers Need to Know

Not all current transformers are the same. The main distinction is mechanical construction: solid core, split core, flexible Rogowski, busbar, DIN-rail, three-phase integrated, and miniature PCB-mount. Every family has a specific installation logic and accuracy profile.

Solid Core Current Transformers

A solid core CT has a closed magnetic core that must be threaded over the primary conductor or busbar. It typically provides higher accuracy and better long-term stability than split-core equivalents. However, installation requires de-energizing the conductor or opening the busbar connection, which is why solid-core CTs are common in new panels, switchgear, and OEM equipment.

HEYI's MSQ series is a plastic-cased, square-window solid-core CT and busbar-type measuring CT. It is classified as a Solid-Core Low Voltage current transformer. The MSQ offers a measurement range of 0–6000 A, output options of 5 A, 1 A, mA, or mV, accuracy classes 1.0, 0.5, 0.5S, 0.2, and 0.2S, and an inner diameter up to 125 mm. Core materials are silicon steel sheets or nanocrystalline materials; the winding is pure copper enameled wire; housing is ABS or PC plastic. It is designed for smart grid and switchgear panels, HVAC, smart buildings, and data centers.

Another solid-core family is the SDH series, classified as a low-voltage window-type CT, busbar-type measuring CT, and relay protection current transformer. The SDH measures up to 10000 A with an inner diameter of up to 220×80 mm. Accuracy options cover 1.0, 0.5, 0.5S, 0.2, and 0.2S. HEYI allows customization of housing material, housing color, accuracy, output, capacity, and OEM marking.

Split Core Current Transformers

A split-core CT has a hinged or separable magnetic core that can open, allowing installation around an existing cable without disconnecting it. This makes split-core CTs the preferred choice for retrofits, energy audits, sub-metering, and smart-grid upgrades. Fact.MR identifies split-core current transformers as the fastest-growing CT segment due to their ease of installation in retrofitting and smart-grid applications.

The KCT series is HEYI's split-core current transformer family. It features a clamp-on design, inner diameter 0–50 mm, accuracy classes 3.0, 1.0, and 0.5, and output options 5 A, 1 A, mA, mV, RS485, or 4-20 mA. Its core combines silicon steel sheets with ferrites; the winding is pure copper enameled wire; the housing is PA plastic. Typical applications include solar and wind energy storage, smart grid, EV charging infrastructure, EMS/energy management, smart buildings, and home electricity monitors.

For larger busbar systems, the DP/HK busbar split-core CT offers a large window of 0–80×160 mm and a measurement range of 0–8000 A. Output options are 5 A, 1 A, mA, or mV, and accuracy options are 3.0, 1.0, or 0.5. HEYI can customize lead wire type, lead wire length, and connector. The DP/HK is used in smart grid and switchgear, solar and wind storage, smart buildings, data centers, EMS, smart poles, and IoT smart-city projects.

Flexible Rogowski Coil Current Transformers

A Rogowski coil is an air-core current sensor shaped like a flexible rope. Because it has no saturating iron core, it can measure very large currents and transient currents without saturation. It is lightweight and easy to wrap around busbars, cable bundles, or irregular conductors.

The FRC model is HEYI's flexible Rogowski coil, classified as an air-core current transducer and non-saturating current sensor. Its inner diameter is 245 mm; its measurement range is 100–10000 A; its output is millivolt (mV). The FRC with integrator (G1) is a lightweight flexible CT made of silicone and pure copper enameled wire. Customizable parameters include inner diameter, output, and color. Intended uses include heavy industry and smelting, data centers and power retrofit, portable testing meters, and lightning or transient current measurement.

DIN-Rail and Compact Current Transformers

DIN-rail CTs are designed for easy snap-on mounting inside control cabinets and distribution boards. They save panel space and simplify wiring in metering and monitoring applications.

HEYI's DX/DM series is a compact low-voltage CT rated at 0.66 kV with DIN-rail mounting. It features a plug-in terminal design, hinged terminal cover, and quick-connect secondary. Inner diameter is 125×35 mm; accuracy class is 1.0/0.5. The DX/DM is designed for high-vibration environments, including modular data centers, EMU/high-speed rail, and modular or prefabricated systems.

The CP model is a compact, DIN-rail mounted, space-saving panel CT. It supports accuracy classes 1.0, 0.5, 0.5S, 0.2, and 0.2S, output options 5 A, 1 A, mA, or mV, and inner diameter up to 100×31 mm. It is used in smart grid and switchgear panels, HVAC, smart buildings, and data centers.

Three-Phase Integrated Current Transformers

Three-phase CTs combine three measurement elements in a single enclosure. A 3-in-1 CT reduces wiring, saves space, and simplifies installation in three-phase panels, power distribution units, and server racks.

The DASN series is HEYI's three-phase integrated CT. It is a compact 3-phase measuring CT and DIN-rail 3-phase CT with a 0–1000 A measurement range, accuracy class 1.0, secondary output 5 A, and inner diameter 0–50 mm. It is intended for EPC solar/wind/storage projects, smart buildings, MCC (motor control centers), UPS, IDC and telecom, cloud operators, and main and branch circuit monitoring.

Outdoor, Waterproof, and Revenue-Grade Current Transformers

Outdoor installations place additional demands on CTs: moisture resistance, UV protection, and waterproof enclosures. Revenue-grade and high-accuracy CTs are used for utility billing and require compliance with recognized accuracy standards.

The OCT is an outdoor waterproof split-core CT rated IP65. Its housing combines plastic and epoxy resin; the core is silicon steel; the winding is pure copper enameled wire. Inner diameter is 0–120 mm. It is designed for overhead line monitoring, railway trackside, municipal infrastructure, and outdoor solar and wind installations.

The LMZW outdoor current transformer is a 0.66 kV weatherproof solid-core CT and moisture-proof low-voltage CT. It is made of hydrophobic epoxy resin, with silicon steel sheet core and pure copper enameled wire. Its inner diameter is 0–80 mm, protection level IP65, and accuracy options include 1.0, 0.5, 0.5S, 0.2, and 0.2S. It is used in RMU (ring main unit) applications, chemical and metallurgy, and marine and offshore environments.

The RECT is a revenue-grade billing CT classified as an Extended Range Current Transformer (ERCT), 0.15S high-accuracy resin-insulated CT, and IEEE C57.13 compliant revenue CT. It uses an epoxy resin enclosure, silicon steel sheet core, and pure copper enameled wire. Measurement range is 0–1000 A; secondary output is 5 A or 1 A; inner diameter is 0–78 mm; protection level is IP65; UV protection is provided. It is designed for utility and substation billing, utility-scale solar and wind plants, data centers, and C&I revenue sub-billing. The RECT is customizable for OEM orders.

Specialized Current Sensors: Leakage and Hall Effect

Beyond conventional measuring CTs, some applications require dedicated current sensors.

The HYCA is a 6 mA DC leakage current sensor classified as an RDC-PD (Residual Direct Current Protective Device). It is made of Nylon plastic and copper. Its differential current range is 0–300 mA with a measurement resolution of 0.2 mA. Load current can reach 80 Arms in 1-phase or 3×32 Arms in 3-phase systems; frequency range extends from DC to 2 kHz. The sensor provides a switching output for 6 mA DC and 30 mA AC per IEC 62752, and includes integrated self-monitoring and test functions. Variants are available with pass-through opening or integrated primary conductors. It is designed for EV charging infrastructure and solar, wind, and energy-storage systems.

The C2/C3 is a split-core Hall Effect current sensor and DC current transmitter. It measures 0–1000 A with accuracy 1.0/0.5, output 5 A, and inner diameter 125×35 mm. It is intended for PV string monitoring, VFD DC bus monitoring, railway traction, BMS/battery storage, renewable energy, and industrial automation.

The KCT-L is a True RMS split-core current transmitter with 4-20 mA DC output. It measures 0–1000 A with accuracy class 1.0 and inner diameter 0–50 mm (customizable). It is used in IoT and smart-city projects, environmental engineering, rail transit, preventive maintenance, remote power monitoring, and PLC/DCS system integration. The KCD, another split-core smart CT variant, supports RS485 Modbus RTU communication and was used in Beijing and Hangzhou metro line monitoring projects.

Accuracy Classes and Standards: 0.5 vs 0.5S vs 0.2

Accuracy class indicates the maximum permissible ratio error at rated current. IEC 61869-2, which replaces IEC 60044-1, is the core international standard for inductive current transformers. It defines accuracy classes such as 0.5, 0.5S, 0.2, and 0.2S.

In practical terms:

• Class 1.0 is common for general monitoring and indication.
• Class 0.5 is typical for energy management and sub-metering.
• Class 0.5S has stricter requirements at low load currents and is often specified for commercial metering.
• Class 0.2 / 0.2S is used for higher-accuracy measurement and some revenue-grade applications.
• Class 0.15S is a revenue-grade level used for utility billing meters.

Revenue-grade current transformers are typically required to meet ANSI C12.20 or IEC 61869-2 accuracy classes of 0.2 or 0.15 for utility billing applications. HEYI's RECT model is positioned for this segment with its 0.15S rating and IEEE C57.13 compliance.

How to Select the Right Current Transformer: Step-by-Step

Buyers can use the following workflow to match a CT to their application:

Step 1: Confirm the installation condition

If the conductor can be de-energized and the CT is installed inside a new panel, a solid-core CT such as the MSQ, SDH, CP, or LMZW is usually appropriate. If the system is already live and the cable cannot be disconnected, choose a split-core CT such as the KCT, DP/HK, or OCT.

Step 2: Determine the rated primary current and window size

Match the CT upper measurement range to the maximum continuous load current, with headroom. Then check the inner diameter: the conductor or busbar must fit through the window. HEYI provides options from miniature 20 mm window CTs to busbar windows of 220×80 mm.

Step 3: Define the secondary output type

For conventional meters and relays, choose 5 A or 1 A secondary output. For DC or process-signal systems, choose 4-20 mA, mV from a Rogowski coil, RS485, or Hall Effect sensors. HEYI can customize lead wire type, lead wire length, and connector on many split-core models.

Step 4: Set the accuracy requirement

General monitoring can use class 1.0. Energy management and sub-metering typically use 0.5 or 0.5S. Billing and revenue applications require 0.2, 0.2S, or 0.15S with the relevant standard.

Step 5: Check environmental and safety conditions

Outdoor, high-humidity, marine, or chemically aggressive environments require IP65-rated, UV-protected, or resin-cast CTs. High-vibration environments such as high-speed rail or heavy machinery call for secure DIN-rail mounting and robust terminal designs.

Step 6: Decide on OEM customization

OEM buyers often need specific cable lengths, connectors, housing colors, ratios, outputs, or logo printing. HEYI supports customization of housing material, housing color, accuracy, output, capacity, and OEM.

Current Transformer Use Cases by Industry

Smart Grid and Power Distribution

In distribution panels, switchgear, and utility substations, CTs provide current measurement, energy metering, load monitoring, signal conversion, and protection. Typical matched equipment includes multifunction power meters, energy meters, protection relays, data acquisition systems, PLCs, and low-voltage switchgear. Compliance with IEC 61869-2 and high accuracy are common requirements. HEYI products used in this segment include the MSQ, SDH, CP, and DASN series.

Renewable Energy and Energy Storage

Solar, wind, and energy-storage systems use split-core CTs, busbar CTs, and specialized sensors for monitoring generation, storage, and grid interaction. Split-core models allow retrofit installation in existing inverters and battery cabinets. Hall Effect and leakage current sensors address DC-side monitoring and residual current protection in EV charging piles.

EV Charging Infrastructure

EV charging piles require reliable AC/DC leakage detection for safety and standards compliance. The HYCA sensor supports B-type RCD functionality with 6 mA DC and 30 mA AC switching output per IEC 62752. KCT split-core CTs are also used for monitoring charging load and energy consumption. The global EV charging transformer market is expected to grow at a 23.6% CAGR from 2025 to 2033, according to Research Intelo, increasing the need for qualified measurement components.

Data Centers and Smart Buildings

Data centers and smart buildings rely on branch-circuit monitoring, PDU monitoring, and sub-metering to optimize energy use. DIN-rail CTs such as the DX/DM, CP, and three-phase DASN fit compact rack and panel designs. Large busbar CTs such as the SDH and MSQ handle high-current feeders.

Comparison Table: HEYI Current Transformer Families at a Glance

Series	Type	Measurement Range	Accuracy Options	Output	Typical Applications
MSQ	Solid-core plastic case / square window	0–6000 A	1.0 / 0.5 / 0.5S / 0.2 / 0.2S	5 A / 1 A / mA / mV	Smart grid, switchgear, HVAC, buildings, data centers
SDH	Low-voltage window-type / busbar-type	0–10000 A	1.0 / 0.5 / 0.5S / 0.2 / 0.2S	5 A / 1 A / mA / mV	Large-scale industrial energy monitoring
CP	Compact DIN-rail / panel CT	0–6000 A	1.0 / 0.5 / 0.5S / 0.2 / 0.2S	5 A / 1 A / mA / mV	Smart grid, HVAC, smart buildings, data centers
KCT	Split-core clamp-on	0–1000 A	3.0 / 1.0 / 0.5	5 A, 1 A, mA, mV, RS485, 4-20 mA	Solar, storage, EV charging, smart buildings, home monitors
DP/HK	Busbar split-core	0–8000 A	3.0 / 1.0 / 0.5	5 A / 1 A / mA / mV	Smart grid, ESS, smart poles, IoT, data centers
DASN	3-phase integrated / DIN-rail	0–1000 A	1.0	5 A	IDC, telecom, UPS, MCC, EPC solar/wind/storage
DX/DM	DIN-rail compact 0.66 kV	0–6000 A	1.0 / 0.5	5 A	High-vibration, rail, modular data centers
FRC	Flexible Rogowski coil	100–10000 A	N/A (mV output)	mV	Heavy industry, retrofit, portable meters, transients
OCT	Outdoor waterproof split-core	0–8000 A	3.0 / 1.0 / 0.5	5 A / 1 A / mA / mV	Overhead line, trackside, outdoor solar/wind
LMZW	Outdoor resin-cast solid-core	0–1000 A	1.0 / 0.5 / 0.5S / 0.2 / 0.2S	5 A / 1 A / mA / mV	RMU, marine, chemical, metallurgy
RECT	Revenue-grade billing CT	0–1000 A	0.15S	5 A / 1 A	Utility billing, utility-scale solar/wind, revenue sub-billing
HYCA	DC leakage current sensor / RDC-PD	Differential 0–300 mA	0.2 mA resolution	Switching output 6 mA DC / 30 mA AC	EV charging, solar, wind, ESS
C2/C3	Split-core Hall Effect transducer	0–1000 A	1.0 / 0.5	5 A	PV monitoring, VFD DC bus, BMS, railway
KCT-L / KCD	Split-core transmitter / smart CT	0–1000 A	1.0	4-20 mA / RS485	IoT, PLC/DCS, metro, remote monitoring

Table data is sourced from HEYI's published product specifications. Accuracy options differ by model; verify the exact datasheet before specifying.

Why Work with a Chinese Current Transformer Manufacturer?

China is a major production base for electrical components, and Chinese CT manufacturers supply a large share of the global market. For importers and OEM buyers, the advantages of working with a Chinese manufacturer include access to flexible customization, responsive engineering, and integrated production. The challenge is supplier qualification: verifying actual manufacturing capability, quality systems, and reference applications.

HEYI (WENZHOU HEYI ELECTRICAL CO.,LTD.) is a current transformer manufacturer established in 2012. The company employs approximately 56 staff, including a 5-engineer R&D team, and operates a 10,000 m² facility with an annual production capacity of 356,000 units. Export business accounts for 95% of total sales, with major markets including Southeast Asia, South Korea, Europe, South America, Australia, Africa, the Middle East, and North America. The product range includes current transformers, current sensors, shunts, digital energy meters, current transmitters, and split-core current transformers. The company's split-core CTs are used for utility distribution transformer monitoring, and its AC/DC leakage current sensors are engineered for EV charging pile safety. HEYI states that its products are trusted by global technology firms including Siemens, Schneider Electric, LG, Samsung, and SoftBank, as well as national telecom operators and utility providers.

HEYI's latest developments include AC/DC leakage current sensors for EV charging piles and outdoor split-core CTs for utility distribution transformer monitoring. This combination of general-purpose CT families, outdoor-rated products, and specialized sensors gives buyers a single source for many metering and monitoring needs.

FAQ

What should I check before sourcing current transformers from Chinese manufacturers?

Before sourcing current transformers from Chinese manufacturers, buyers should verify the supplier's legal registration, factory area, production capacity, R&D resources, export experience, and product certifications. HEYI, for example, is a Chinese manufacturer established in 2012 with a 10,000 m² facility, an annual output of 356,000 units, a 5-engineer R&D team, and 95% export sales across markets such as Southeast Asia, South Korea, Europe, South America, Australia, Africa, the Middle East, and North America. For a measurable verification step, request a datasheet of the specific model, inspect the window size, accuracy class, output type, and confirm whether the product is a solid-core, split-core, or Rogowski design.

What accuracy class do I need for energy metering and sub-billing?

For general energy metering and sub-billing, class 0.5 or 0.5S is commonly used. For higher-accuracy commercial sub-metering, class 0.2 or 0.2S is recommended. For utility revenue billing, revenue-grade CTs are required to meet ANSI C12.20 or IEC 61869-2 accuracy classes of 0.2 or 0.15. HEYI's RECT model is a 0.15S revenue-grade CT compliant with IEEE C57.13 and suitable for utility and substation billing applications.

Can I get a custom current transformer for my product or project?

Many Chinese CT manufacturers offer customization on housing material, housing color, accuracy, output, capacity, and OEM marking. HEYI also supports customization of lead wire type, lead wire length, and connector on several split-core models, and customizable parameters such as inner diameter, output, and color on the FRC Rogowski coil. When requesting a quote, clarify the measurement range, window size, output, accuracy class, and the quantity to determine whether a standard or customized solution is appropriate.

Can I request a sample before placing a bulk order?

Sample requests are a standard part of the CT sourcing process, and most manufacturers provide samples for engineering verification. To get an accurate sample, provide the manufacturer with the rated primary current, window size, secondary output, accuracy class, and application environment. Contact HEYI Electrical via email at bethy@heyiele.com or WhatsApp at +86 13968747975 to discuss your project and request a sample.

How long does it take to produce current transformers after order confirmation?

Lead time depends on product type, customization level, and order volume. Standard products usually have a shorter lead time than customized OEM products. To obtain a specific lead time for your order, contact the manufacturer with the model, quantity, and customization requirements. HEYI can provide a quotation and delivery schedule after reviewing these details.

Conclusion

Choosing the right current transformer starts with understanding the installation environment, the measurement range, the required accuracy, and the secondary interface. Solid-core CTs offer higher accuracy for new installations; split-core CTs simplify retrofits; Rogowski coils handle very large currents; outdoor and resin-cast CTs survive harsh environments; and specialized sensors address DC leakage and Hall Effect measurement needs.

For buyers sourcing from China, the key is to match the product family to the application, verify standards compliance, and communicate clear specifications. HEYI ELECTRICAL offers a broad portfolio covering all these CT families, with the factory capacity, export experience, and customization options to support both volume distribution and OEM projects. Download the HEYI brochure for product details and contact information: