Why split core current transformer are the best choice for power systemretrofits

May 26, 2026

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For adding energy monitoring to existing buildings, old distribution panels, or live production lines, the biggest pain point is power interruption. The split core current transformer (CT) was invented exactly to solve this problem. This article explains why split core CTs are the preferred choice for retrofit projects, covering structure, comparison, selection, applications, and installation.

1. What is a Split Core CT?

A split core current transformer is a current sensing device with a openable/clamp-on core. Unlike conventional solid core (closed) CTs, its magnetic circuit is split into two halves. You open the core using a spring or latch mechanism, clamp it around a cable, and then close it.

Core structure: separable iron core + coil + locking latch.
Primary advantage: Installation without power interruption. In hospitals, data centers, or factory lines, even a one‑minute outage can cause huge losses. Split core CTs can be clamped directly onto existing cables without affecting the load.

✅ The #1 reason buyers choose split core CTs: no power interruption – clamp and go.

2. Split Core vs. Solid Core CTs – Which One Should You Choose?

To help you decide quickly, here is a structured comparison table.

Feature	Split Core CT	Solid Core CT
Installation method	Open core, clamp around cable	Must thread cable through (requires cable disconnection or pre‑installation)
Power interruption needed	No ✅	Yes ❌
Typical applications	Retrofits, energy audits, temporary monitoring	New equipment production, labs, OEM
Accuracy class	Typically Class 0.5 / 1.0 / 3.0	Can reach Class 0.2 or even 0.1
Cost	Moderate (mechanical complexity)	Lower (simple structure, automated manufacturing)
Common use cases	Sub‑metering in buildings, PV monitoring	Panel meters, protection relays

Verdict:

For retrofit projects, existing buildings, energy audits – split core CTs are often the only viable choice, because you cannot shut down an entire building just to thread cables.
For new equipment design or high‑volume OEM – solid core CTs offer higher accuracy and lower cost, making them the first choice.

3. Key Factors for Selection – How to Choose the Right Split Core CT

As a supplier, we recommend selecting based on the following 4 dimensions:

① Hole Size (Window Diameter)

Must match the cable diameter. Common sizes: 10mm, 16mm, 24mm, 36mm, 45mm, 60mm, and larger.
Rule of thumb: hole size should be 2–5mm larger than the cable outer diameter to allow easy insertion without damaging insulation.
Note: For multi‑core cables or busbars, a larger window or rectangular opening is required.

② Rated Current

Common ranges: 5A, 50A, 100A, 200A, 400A, 600A, 1000A, 2000A, 5000A.
Selection principle: keep the measured current between 20% and 120% of the CT rating for best linearity.
Example: a 100kW device draws roughly 150A; choose a 200A or 300A CT.

③ Output Signal

Different outputs suit different back‑end devices – this is where engineers often make mistakes.

Output Type	Characteristics	Compatible Devices	Safety
5A / 1A	Traditional electromagnetic, secondary current proportional to primary	Mechanical meters, legacy analog input modules	⚠️ Open‑circuit secondary creates dangerous high voltage
333mV / 1V	Low‑power voltage output	Modern IoT data loggers, PLC analog inputs (voltage type)	✅ Safer – no open‑circuit risk
4‑20mA	Industrial standard analog, excellent noise immunity	DCS, long‑distance transmission (>50m)	✅ Safe, ideal for long runs

Supplier advice: For new IoT or smart meter retrofit projects, prioritise 333mV output split core CTs. They are compact, low‑power, and free from open‑circuit hazards.

④ Accuracy Class

Class 0.5 – ±0.5% error. Suitable for commercial sub‑metering and energy performance tracking.
Class 1.0 – ±1.0% error. Suitable for general industrial monitoring, equipment efficiency analysis.
Class 3.0 – ±3.0% error. For indicative monitoring only (e.g. on/off detection); not recommended for billing.

Pro tip: field conditions (cable position, adjacent conductors) affect real‑world accuracy. A Class 0.5 CT may perform closer to Class 1.0 in some retrofits. For billing or carbon accounting, always use high‑precision CTs and consider in‑situ calibration.

4. Top Applications in 2026

Split core CTs are seeing growing demand in the following fields:

① Smart Grids & Sub‑metering

Adding meters per department or circuit in commercial buildings and large factories.
Split core advantage: no power interruption, no disruption to tenants.

② Solar PV Monitoring

Monitoring PV inverter AC outputs, grid connection points, and load sides.
Most PV projects are rooftop retrofits where power cannot be cut. Split core CTs are standard.

③ Building Management Systems (BMS)

Monitoring HVAC, lighting, elevators, and other major loads.
Used with 4‑20mA or Modbus data loggers to optimise energy efficiency.

④ EV Charging Stations

Operators need energy metering per charging gun (especially for existing station retrofits).
For AC slow chargers, clamp onto input cable; for DC fast chargers, monitor AC input side.

5. Installation Best Practices

Correct steps:

1.Verify CT rating matches cable (hole size, current range).

2.Clean the cable surface – remove dust and grease.

3.Open the core latch, place the cable in the centre of the window.

4.Close the core until you hear a "click" confirming full lock.

5.Connect output leads – for 5A/1A types, always connect the load (meter or burden resistor) before closing the core to prevent open‑circuit.

6.Secure the CT using cable ties or mounting brackets to avoid vibration loosening.

Key technical details:

1.Polarity marking (P1/P2, K/L) – CTs are marked with P1 / P2 (primary direction) or K / L (secondary).

Current flow must go from P1 to P2; otherwise the secondary output polarity reverses, causing negative power readings.
If direction is uncertain, verify using forward/reverse active power on an energy meter after installation.

2.Preventing secondary open‑circuit (for 5A/1A types):

⚠️ A 5A/1A CT with open secondary can generate thousands of volts – a danger to people and equipment.
Safety measure: pre‑install a shorting block on the secondary terminals; remove it only after confirming all connections.

3.Safety with 333mV / 1V / 4‑20mA types:

These are low‑voltage outputs with no open‑circuit hazard, ideal for field installation.

Common mistakes:

❌ Cable not centred in the window (off‑centre increases error).

❌ Core not fully closed (air gap drastically reduces accuracy).

❌ Multiple cables passing through one CT (flux cancellation, invalid measurement).

6. FAQ Section

Q1: Can I install a split core CT while the power is on?

A: Yes. That is the main value of split core CTs. Provided you follow safe work practices (insulated gloves, no exposed cable damage) and for 5A/1A types ensure the secondary is connected or shorted, live installation is allowed.

Q2: What is the difference between 333mV and 5A output?

A:5A output: traditional method, secondary current proportional to primary. It works with old meters but has open‑circuit danger and cannot directly drive low‑voltage electronics.

333mV output: modern low‑power voltage output. Safer, connects directly to PLC or data logger analog inputs, and consumes very little power. For retrofit + IoT scenarios, 333mV is strongly recommended.

Q3: How do I choose the right hole size for my cable?

A: Measure the cable outer diameter (including insulation) with calipers, then select a CT with a hole size 3–5mm larger. Examples:

Cable OD 12mm → choose 16mm hole CT

Cable OD 20mm → choose 24mm hole CT

For busbars, measure width and thickness; choose a rectangular‑window CT or a larger round hole CT.

Q4: What is the accuracy difference between Class 0.5 and Class 1.0?

A: For a 100A circuit:

Class 0.5: max error 0.5A (suitable for energy analysis, carbon accounting).

Class 1.0: max error 1.0A (suitable for general equipment monitoring).
Recommendation: use Class 0.5 for commercial billing or government energy audits; Class 1.0 is sufficient for internal equipment efficiency evaluation.

Q5: Can one split core CT be used for both current and power measurement?

A: Not alone. Power measurement requires both current (CT) and voltage (PT or direct voltage input) , with calculation performed by an energy meter or data logger. The split core CT provides only the current signal.

Supplier Summary

As a professional manufacturer of split core current transformers, we confirm: for any retrofit project that requires adding current monitoring without power interruption, split core CTs are the only reasonable and mature solution on the market today. By correctly matching hole size, current rating, output signal, and accuracy class, you can achieve full electrical monitoring with minimal engineering cost and zero downtime.

We offer:

Full range of split core CTs from 10mm to 120mm aperture
Outputs: 5A, 1A, 333mV, 1V, 4‑20mA
Accuracy classes: Class 0.5 / 1.0 / 3.0
OEM customisation for lead length and terminal types

For more information, please contact us at sales@xfullstar.com