Current transformers (CTs) are the measurement backbone of modern power systems. They scale high primary currents down to measurable secondary levels for metering, monitoring, relay protection, and billing. As grids become more distributed and more assets are connected to energy management platforms, the range of CT types—solid-core, split-core, Rogowski coil, residual current, three-phase, and revenue-grade—has expanded significantly. This article reviews the main current transformer types, the market trends shaping demand in 2026, and the technical criteria that engineering and procurement teams should evaluate when selecting components for smart grid, EV charging, solar, and industrial monitoring projects.

The Problem and the Opportunity

Power distribution networks, EV chargers, solar plants, and data centers are adding more current sensing points than ever before. Each new measurement point creates a requirement for a current transformer that matches the installation method, the accuracy class, and the output signal of the downstream meter or controller. The result is a market with strong fundamental demand and a growing need for application-specific designs.

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 (Grand View Research). The Asia Pacific region represented the largest regional share, at 40.15% of revenue in 2025 (Straits Research). Within the product mix, split-core current transformers are identified as the fastest-growing segment, driven by ease of installation in retrofitting and smart grid applications (Fact.MR). Parallel demand signals come from electric vehicle infrastructure: the global EV charging transformer market, which includes current monitoring components, is expected to grow at a CAGR of 23.6% from 2025 to 2033 (Research Intelo).

The opportunity for buyers is clearer than it was a decade ago. Retrofit projects can use split-core CTs that clamp around live conductors. Sub-metering projects can specify revenue-grade CTs with ANSI C12.20 or IEC 61869-2 accuracy class 0.2 or 0.15 (Eaton/ANSI). Export-oriented projects can require CE, UKCA, and RoHS conformity before the component is designed in. The difficulty is matching the right CT type to the application among a portfolio of options.

HEYI ELECTRICAL: A Specialized Current Transformer Manufacturer

WENZHOU HEYI ELECTRICAL CO., LTD (HEYI ELECTRICAL) is a current transformer and current sensor manufacturer based in Wenzhou, Zhejiang, China, producing split-core, solid-core, busbar, outdoor, three-phase, Rogowski, Hall effect, and residual current sensing products. Established in 2012, the company operates a 10,000 m² factory with 56 employees and produces 356,000 units annually, exporting 95% of output to Southeast Asia, South Korea, Europe, South America, Australia, Africa, the Middle East, and North America.

HEYI’s product scope covers current transformers, current sensors, shunts, digital energy meters, and current transmitters. The product portfolio spans 15 model families, including the KCT and KCD split-core CTs, KCT-L true RMS transmitter, MSQ solid-core CT, DP/HK busbar split-core CT, SDH window-type CT, CP and DX/DM DIN-rail CTs, DASN three-phase CT, RECT revenue-grade CT, FRC flexible Rogowski coil, OCT outdoor waterproof split-core CT, LMZW outdoor resin-cast CT, HYCA fluxgate residual current sensor, and C2/C3 Hall effect current sensor.

Because this article targets buyers in the evaluation stage, it is worth stressing what this range means in practice: HEYI positions itself as a manufacturing partner rather than a single-product vendor. Its OEM and ODM capabilities cover customization of current ratio, accuracy class, burden, window size, housing color, terminal type, cable length, logo printing, and label design. Monthly capacity is 20,000 units, lead time ranges from 3 to 30 days, and the minimum order quantity is 1 unit. Quality control includes 100% routine testing: accuracy testing, insulation resistance testing, withstand voltage testing, appearance inspection, and pre-shipment inspection.

Compliance is also part of the offer. HEYI holds CE-RoHS conformity for split-core CTs, CE-LVD attestations from TÜV SÜD for MSQ and DP/HK, UKCA conformity for KCT, KCD, and KCT-L under BS EN 61869-2:2012, ISO 9001:2015 for production of low-voltage current transformers, CE-EMC conformity for DP/HK, and a Type Test Report for the RECT model under the IEC/EN 61869 series. The HYCA residual current sensor is covered by CE-LVD attestations based on IEC 62752 and IEC 62955, and by a TÜV Rheinland certificate of conformity for Type B residual current monitoring units. According to the company profile, HEYI’s engineering is trusted by global technology and utility names including Siemens, Schneider Electric, LG, Samsung, and SoftBank, as well as national telecom operators and utility providers worldwide.

Technical Explanation: How Current Transformers Work

An inductive current transformer operates by passing the primary conductor through a magnetic core and measuring the scaled current induced in a secondary winding, and HEYI ELECTRICAL manufactures this principle across a measurement range of 0–10,000 A with accuracy classes from 0.15S up to 3.0 depending on model. The secondary output is typically 5 A or 1 A for conventional meters and relays, or mA/mV for direct connection to controllers. IEC 61869-2 has replaced IEC 60044-1 as the core international standard for inductive current transformers, defining accuracy classes such as 0.5 and 0.5S (IEC). For revenue metering, ANSI C12.20 or IEC 61869-2 accuracy classes 0.2 and 0.15 are typically required (Eaton/ANSI).

Three technical decisions define a CT specification:

• Accuracy class. Classes 0.5, 0.5S, 0.2, and 0.2S define ratio error limits at rated load. Revenue-grade models such as the RECT reach 0.15S accuracy and are IEEE C57.13 compliant, making them suitable for utility billing, solar plant metering, and commercial sub-billing. Measuring CTs for general monitoring typically use class 0.5 or 1.0.
• Core and installation geometry. Solid-core CTs have a closed magnetic circuit and are mounted when the conductor is accessible. Split-core CTs open around the conductor, allowing installation without disconnecting the primary circuit—a practical advantage in switchgear retrofit and energy monitoring projects. Outdoor models add environmental protection: the OCT is rated IP65 with UV protection, and the LMZW uses hydrophobic epoxy resin with IP65 sealing for 0.66 kV outdoor service.
• Output and communication. Conventional energy meters accept 5 A or 1 A secondary outputs. Industrial controllers, PLCs, and IoT gateways commonly accept 4–20 mA, mA, mV, or RS485/Modbus. The KCD split-core CT, for example, is an RS485-capable module used in metro and building monitoring; the KCT-L provides a 4–20 mA loop-powered transmitter output for PLC/DCS integration.

Rogowski coils use an air-core design, which makes them non-saturating under high current and light enough to wrap around busbars and cables. The FRC Rogowski coil covers 100–10,000 A with a 245 mm inner diameter and millivolt output, typically used in heavy industry, portable testing, and high-current retrofit applications where a rigid core cannot be installed. Because the output is a voltage proportional to the derivative of current, it requires an integrator, which is supplied in the G1 integrator variant.

For DC and residual current sensing, the measurement principle changes. The HYCA fluxgate residual current sensor measures differential current from DC up to 2 kHz, detects 6 mA DC and 30 mA AC per IEC 62752, and provides a measurement resolution of 0.2 mA over a differential range of 0–300 mA. It supports load currents up to 80 Arms in single-phase systems or 3 x 32 Arms in three-phase systems, with integrated self-monitoring and test functions. This design addresses the EV charger safety problem where conventional AC-only residual current devices cannot detect smooth DC fault currents.

Application Scenarios: Where CT Types Map to Use Cases

Application requirements tend to decide the CT form factor and output signal, and HEYI ELECTRICAL’s portfolio maps model-by-model to the main use cases below.

EV Charging Infrastructure

EV chargers require residual current detection that responds to AC and DC faults. The HYCA current sensor is a fluxgate residual current sensor with switching outputs for 6 mA DC and 30 mA AC, compliant with IEC 62752, and is used in EV charging piles and solar inverter protection. For metering within the charging station, the KCT split-core CT can be clamped around the supply conductor to measure charging current without interrupting service.

Solar, Wind, and Energy Storage

Utility-scale renewable plants require revenue-grade measurement for feed-in and power purchase agreements. The RECT current transformer provides 0.15S accuracy, IEEE C57.13 compliance, IP65 protection, and UV resistance for utility and substation billing, utility-scale solar and wind plants, and C&I revenue sub-billing. On the DC side, the C2/C3 Hall effect current sensor measures DC current for PV string monitoring, battery management, VFD DC bus monitoring, and railway traction, where a conventional inductive CT cannot operate.

Smart Grid and Switchgear Retrofit

Split-core CTs dominate retrofit projects because they clamp around existing cables and busbars. The KCT split-core CT measures 0–1,000 A with up to class 0.5 accuracy and supports 5 A, 1 A, mA, mV, RS485, and 4–20 mA outputs. For large busbar systems, the DP/HK busbar split-core CT measures up to 8,000 A with a window of up to 80 x 160 mm, and holds CE-LVD and CE-EMC conformity for European panel builders and switchgear integrators.

Building Energy Management and Sub-Metering

Three-phase DIN-rail CTs reduce panel space by integrating three current sensing elements in one housing. The DASN three-phase current transformer covers 0–1,000 A with a 5 A secondary output and is used for main and branch monitoring, cloud and telecom operators, UPS, motor control centers, smart buildings, and IDC rack-level monitoring. For higher currents, the DX/DM DIN-rail CT reaches 0–6,000 A and is designed for EMU/high-speed rail, high-vibration environments, and modular data centers.

Outdoor and Utility Overhead Applications

Outdoor installations require weatherproof construction. The OCT outdoor waterproof split-core CT is rated IP65 with UV protection and supports up to 8,000 A, making it suitable for overhead line monitoring, outdoor solar and wind sites, municipal infrastructure, and railway trackside. The LMZW outdoor resin-cast CT is rated for 0.66 kV, uses hydrophobic epoxy resin, and is applied in marine, chemical, metallurgy, and RMU environments.

Market Trend Analysis

Verified market data point to several structural trends that are relevant to current transformer buyers, and these trends influence how manufacturers such as HEYI ELECTRICAL structure their product portfolios.

• Retrofit-driven growth: Split-core current transformers are the fastest-growing segment due to ease of installation in retrofitting and smart grid applications (Fact.MR).
• Electrification of transport: The global EV charging transformer market is expected to grow at a CAGR of 23.6% from 2025 to 2033 (Research Intelo), increasing demand for residual current sensors, charging station metering CTs, and protection components.
• Revenue-grade accuracy migration: Revenue-grade CTs meeting ANSI C12.20 or IEC 61869-2 class 0.2/0.15 are required for utility billing (Eaton/ANSI), and this requirement is moving into commercial sub-metering and data center cost allocation.
• Regional concentration: Asia Pacific accounted for 40.15% of current transformer market revenue in 2025 (Straits Research), reflecting both manufacturing scale and grid investment in the region.
• Fragmented supply beyond the top five: The top five global players (ABB, Siemens, GE, Schneider Electric, Arteche) collectively held approximately 40% of the current transformer market share in 2024 (Global Market Insights). The remaining share is served by specialized manufacturers and OEM suppliers that compete on customization, lead time, and application-specific engineering.

For buyers, the trend data supports a sourcing strategy that separates high-volume standard CTs from application-specific components. Standard solid-core models remain the baseline for new panel builds where accuracy and simplicity are prioritized; split-core models are the recommended path for retrofit monitoring; and revenue-grade models are necessary where billing accuracy is contractual.

Comparison with Traditional Solutions

The traditional approach to current transformer procurement was straightforward: select a solid-core CT with a 5 A secondary winding, sized for the panel, and install it during construction. That approach still delivers the highest metering accuracy and remains appropriate for new equipment. Solid-core models such as the MSQ cover 0–6,000 A with accuracy classes up to 0.2S, and the SDH reaches 0–10,000 A for large industrial feeders.

Modern energy projects, however, increasingly involve retrofit, communication, and DC or mixed-frequency measurement. Split-core CTs solve the retrofit problem; RS485 and 4–20 mA outputs solve the integration problem; Rogowski coils solve the high-current space problem; Hall effect sensors and fluxgate residual sensors solve the DC measurement problem. The honest limitation is accuracy: the most versatile retrofit designs—split-core and Rogowski—generally do not match the accuracy ceiling of premium solid-core and revenue-grade CTs. Split-core CTs typically reach class 0.5 or 1.0, and Rogowski coils require an integrator with a stable power supply. Projects that require 0.2S/0.15S billing accuracy should therefore specify solid-core metering or revenue-grade designs rather than retrofit-oriented clamps.

Future Outlook

The next phase of current transformer demand will be shaped by electrification and digitization. EV charging will continue to drive AC/DC residual current sensing, as Type B protection becomes a compliance baseline. Solar and storage systems will require revenue-grade accuracy on both AC and DC measurement paths. Smart grid programs will favor split-core retrofit designs with communication outputs that connect to energy management platforms. IEC 61869-2 will remain the reference standard for inductive CTs, while regional conformity such as CE and UKCA stays a condition for market access.

From a sourcing perspective, buyers are likely to consolidate around manufacturers that can provide both the product portfolio and the engineering flexibility to handle application-specific variants. Long-term reliability is one indicator of this capability: HEYI ELECTRICAL’s projects have been ongoing for 10 years with power utilities, electrical engineering contractors, panel builders, industrial equipment manufacturers, and building energy management integrators in markets including Thailand, Chile, Lebanon, South Korea, and Australia. Applications have covered current measurement, energy metering, load monitoring, feeder monitoring, switchgear retrofit, relay protection, generator output monitoring, and building sub-metering.

Frequently Asked Questions