The 4-20 mA loop versus RS485 decision is made at the receiving end of the cable, not inside the transformer. If the device that reads the current has an analog input, a loop-powered 4-20 mA current transmitter is the shortest path to a working system. If that device sits on a digital bus and the project needs a metro-grade smart CT, an RS485 split-core current transformer is the shortest path. The measuring core can be almost identical; the output architecture is what changes wiring, commissioning effort, cost structure and the compliance evidence a buyer should request.

This comparison places two HEYI products on either side of that line. The KCT-L is a True RMS current transformer/transmitter: a loop-powered, split-core current transmitter with a 4-20 mA DC output, designed for analog loop integration. The KCD is a smart split-core current transformer with RS485 communication - a metro-grade smart CT used in Beijing Metro Lines and Hangzhou Metro Lines, and suited to elevator monitoring and green building energy monitoring.

Instead of repeating datasheets, this article works as a decision framework: what each output architecture does well, where each one creates hidden cost, how to compare them step by step, and what evidence to collect before a project commits to either signal type.

Problem Definition: The Output Signal Is Usually Specified Last

A current transformer that measures accurately can still fail a project if its output does not match the device that has to read it. Ratio, accuracy class, burden and window size are normally decided early; the output line is often added at the end of the bill of materials, by which point the receiving cabinet or the monitoring platform has already been fixed.

HEYI treats these as controlled risks. Measurement deviation, insulation failure, overheating, wrong wiring, unstable output signal, installation mismatch and product damage during transportation are managed through accuracy calibration, insulation resistance testing, withstand voltage testing, polarity testing, ratio testing, temperature rise control, a flame-retardant housing design and a secure terminal structure. Every unit passes routine inspection before shipment. Output selection sits directly on top of two of those risk categories - wrong wiring and installation mismatch - because a 4-20 mA analog loop and an RS485 bus are wired differently and must be labelled differently.

Retrofit projects raise the stakes. Split-core CTs can reduce installation time by 50%-70% in retrofit projects compared with traditional closed-core CTs, and because the primary cable does not have to be disconnected, shutdown cost and labour cost fall as well. That saving only materialises if the output side can be commissioned in the same window, without waiting for an extra transducer, a gateway or a firmware change.

The output question therefore appears in two practical forms: can the cabinet that already exists read this CT, and can the monitoring platform collect from it? Answering those two questions with the receiving equipment in front of you resolves most of the 4-20 mA versus RS485 debate in a single meeting.

Industry Background: Analog Loops and Digital Buses Are Expanding Together

Both output architectures are growing, because they serve different parts of the same market. 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). Asia Pacific dominated the market with a 40.15% revenue share in 2025 (Straits Research), and split core current transformers are identified as the fastest-growing segment, driven by ease of installation in retrofitting and smart grid applications (Fact.MR).

The demand pull comes from applications where current data is collected continuously. 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). At the same time, IEC 61869-2 - which replaced IEC 60044-1 - remains the core international standard for inductive current transformers and defines accuracy classes such as 0.5 and 0.5S, while revenue grade current transformers used for utility billing are required to meet ANSI C12.20 or IEC 61869-2 accuracy classes, typically 0.2 or 0.15 (Eaton / ANSI).

Market concentration also matters to the buying decision. The top five global players - ABB, Siemens, GE, Schneider Electric and Arteche - collectively held approximately 40% of the current transformer market share in 2024 (Global Market Insights). The rest of the demand is served largely by specialist manufacturers, which is why the practical question in most projects is not which brand is largest, but which output architecture and which level of customization the receiving system actually requires.

The Two Output Architectures: KCT-L and KCD

KCT-L: True RMS current transmitter with a 4-20 mA DC output

The KCT-L is a True RMS current transformer/transmitter, supplied as a loop-powered, split-core current transmitter with a 4-20 mA DC output. The signal it produces is the classic process interface: one measurement, one analog channel, carried on the same two wires that also power the transmitter.

For the installer this is the least ambiguous interface to commission, because the receiving device only has to provide an analog input and the transmitter performs the conversion. The engineering consequence is simple to plan: the number of analog input channels required equals the number of current points being measured. Where a cabinet already has spare analog inputs - a PLC card, a generator panel, a power monitoring meter with an analog channel - the KCT-L slots into an existing architecture instead of demanding a new one.

The limitation is equally clear. An analog loop carries one value per channel, so scaling to dozens of measurement points means dozens of analog inputs and dozens of cable pairs pulled back to the panel. That is a cost and layout decision, not a measurement quality decision.

KCD: smart split-core CT with RS485 communication

The KCD is a smart split-core current transformer with RS485 communication. It is a metro-grade smart CT used in Beijing Metro Lines and Hangzhou Metro Lines, and it is suited to elevator monitoring and green building energy monitoring - environments where current data has to reach a digital monitoring layer rather than a single analog input.

RS485 is a networked interface, and that changes the wiring logic of a project. Several devices can share one communication path and be addressed individually, so the number of cable runs back to a monitoring panel does not grow one-for-one with the number of measurement points. That makes the KCD attractive in buildings, metro systems and multi-point installations where current values are consolidated into one platform.

The trade-off is configuration work. The receiving system must be able to accept RS485 devices, the bus has to be wired consistently, and the commissioning team has to map each device into the monitoring platform. Projects that already own that capability will find the digital path cheaper; projects that do not will find the analog loop faster to close.

Comparison Table: 4-20 mA Loop vs RS485 in Project Terms

The table below compares the two HEYI products on the criteria that usually decide a specification. It contains product facts and integration criteria only.

Note: the table reflects the published product facts for the KCT-L and the KCD. No accuracy, current range or price figures are claimed for either model beyond the product and standard references listed here; additional figures should be confirmed against the specific order specification.

Step-by-Step Breakdown: How to Choose Between 4-20 mA and RS485

The sequence below follows the order in which a project actually commits money: receiver, point count, installation window, accuracy requirement, wiring ownership and sample validation.

Use Cases: Where Each Output Fits

The KCD's reference deployments make the digital case concrete: it is a metro-grade smart CT used in Beijing Metro Lines and Hangzhou Metro Lines, and it is suited to elevator monitoring and green building energy monitoring. Those are environments where many current points feed a central monitoring platform, and where the value of the measurement comes from the data flow rather than from a single meter reading.

• Metro and rail systems: KCD, a metro-grade smart CT with RS485 communication, referenced in Beijing Metro Lines and Hangzhou Metro Lines.
• Elevator monitoring: KCD, suited to elevator current monitoring that reports into a building management layer.
• Green building and commercial building monitoring: KCD for digital collection; HEYI current transformers are also specified for energy metering systems and commercial buildings.
• Energy meter projects: customizable ratios such as 100/5A, 500/5A, 1000/5A and 2000/5A with accuracy classes up to 0.5 / 0.2 depending on model.
• Retrofit and switchgear work: split-core CTs for low-voltage switchgear, power distribution cabinets, generator panels and ATS cabinets, with a 4-20 mA loop where the cabinet reads analog and RS485 where the data is consolidated digitally.
• Industrial control and protection: feeder monitoring and relay protection systems, where the output type must match the relay or PLC input that receives it.

Cost and Maintenance: What Actually Changes the Budget

Unit price is a poor basis for comparison, because the two architectures put cost in different places. HEYI's own comparison states that its current transformers may carry a slightly higher unit cost than low-cost generic CTs, but that this is offset by lower replacement cost, lower installation risk, smaller measurement deviation and fewer project mismatches. In retrofit projects, split-core CTs also lower shutdown cost and labour cost by avoiding cable disconnection.

On the analog side the cost driver is channel count: each 4-20 mA loop needs an input, terminals and a pair of wires. On the RS485 side the cost driver is configuration: bus wiring, addressing and data mapping into the monitoring platform. A project that already owns one of those two capabilities will usually find that architecture cheaper overall, even when the CT unit price is identical.

Maintenance behaves the same way. HEYI current transformers are described as offering stable output, a clear wiring structure, reliable insulation and better compatibility with meters, relays, PLCs and monitoring systems, which reduces maintenance frequency, wiring errors, replacement difficulty and long-term system operating risk. Current transformers do not directly consume or save energy; their contribution is measurement quality, which supports better energy metering, load monitoring, abnormal current detection and energy management in buildings, factories, switchgear and utility distribution systems.

Where HEYI ELECTRICAL Fits in This Decision

HEYI ELECTRICAL (Wenzhou Heyi Electrical Co., Ltd) is a current transformer and current sensor manufacturer founded in 2012 in Wenzhou, Zhejiang, China. The company operates a 10,000 square metre factory with 56 employees and a five-engineer R&D team, and exports about 95% of its output to Southeast Asia, South Korea, Europe, South America, Australia, Africa, the Middle East and North America. Its product range covers current transformers, current sensors, shunts, digital energy meters, current transmitters and split core current transformers - which is why both architectures discussed here, the 4-20 mA transmitter and the RS485 smart CT, can come from the same supplier and be evaluated against the same quality system.

Compared with generic current transformers, HEYI positions its range on wider product selection, more stable accuracy, reliable insulation performance, flexible installation structures and OEM/ODM customization support. Accuracy classes reach up to 0.5 / 0.2 depending on model, with customizable ratios such as 100/5A, 500/5A, 1000/5A and 2000/5A for low-voltage switchgear, power distribution cabinets, energy metering systems, commercial buildings and industrial control panels. According to its published certification information, HEYI holds ISO 9001, CE and RoHS certifications.

For buyers, the practical implication is that the output decision and the supplier decision do not have to be handled separately. The same engineering team can propose a KCT-L where the cabinet reads analog and a KCD where the platform reads RS485, and the sample stage can be used to confirm which one fits. Product information is available at www.heyiele.com.

FAQ: 4-20 mA Loop vs RS485 Current Transformer Decisions

Conclusion: Let the Receiver Choose the Output

The 4-20 mA loop and RS485 are not competing quality levels; they are two integration paths, and the receiver decides between them. The KCT-L True RMS current transformer/transmitter suits projects where a meter, PLC, controller or panel reads an analog input and where a loop-powered split-core transmitter can be commissioned without adding hardware. The KCD smart split-core CT with RS485 communication suits projects where current data must reach a digital platform - metro-grade deployments such as Beijing Metro Lines and Hangzhou Metro Lines, elevator monitoring and green building energy monitoring - and where several devices should share one communication path.

Whichever path is chosen, three checks protect the decision: confirm the receiving device's input type, confirm the accuracy class against the actual purpose using IEC 61869-2 classes (and ANSI C12.20 or IEC 61869-2 classes of typically 0.2 or 0.15 for revenue grade billing), and validate with a sample before scaling. Split-core construction then delivers its benefit: installation without disconnecting the primary cable, with installation time reduced by 50%-70% in retrofit projects.