Choosing a current transformer means matching the circuit, the sensor and the meter, not just choosing an amperage. The wrong output can be incompatible with the meter, while the wrong size or measurement range can compromise an otherwise sound installation.
This guide covers the key factors that engineers and installers should consider when specifying current transformers for energy monitoring.
Start with the meter's permitted sensor inputs and the exact sensor datasheet. Record the minimum, normal and maximum expected current, conductor dimensions, cable route and required measurement accuracy. This is a selection guide, not an electrical installation procedure; installation requires qualified personnel and the applicable equipment instructions.
CT Types
Solid-Core CTs
Solid-core CTs have a continuous ring of magnetic material (typically silicon steel or ferrite) through which the conductor must be threaded. They are generally the most accurate and least expensive type, but installation requires disconnecting the conductor to pass it through the CT aperture. This means the circuit must be de-energised during installation, which may not be acceptable in critical facilities.
Solid-core CTs are best suited for new installations where they can be fitted during the initial electrical build, or for retrofit projects where planned shutdowns are available.
Split-Core CTs
Split-core CTs open around an existing conductor, avoiding the need to thread the conductor through a closed ring. That can simplify a retrofit, but it does not make live installation safe or remove isolation requirements. Have qualified personnel plan the work using the manufacturer's instructions and the site's electrical safety procedures. See Schneider Electric's installation guidance.
Core construction is one factor in performance. Compare the published error limits of the candidate sensors, and follow the specified closure and mounting instructions rather than assuming that every split-core design has the same accuracy.
Rogowski Coils
Rogowski coils use a non-magnetic winding, often flexible, around the conductor. They need an integrator, which may be separate or built into the measuring equipment. Compare the specified performance of the whole coil-and-meter pairing rather than assuming either technology is always more accurate. See our guide to Rogowski coils.
Current Rating
The rated primary current is a reference value, not a substitute for the specified measuring and overload limits. A current-output CT marked 100/5 has a nominal ratio of 100 A primary to 5 A secondary. Low-power voltage-output sensors use different output specifications; do not treat these as interchangeable inputs.
Sizing for the Load, Not the Breaker
A breaker rating alone does not describe the load profile. For example, a circuit protected at 400 A might normally carry 80–120 A, while still having higher peaks. Record both the ordinary operating range and the maximum conditions the sensor must withstand.
Compare candidate sensors' published error limits across that range, including low-load operation. There is no universal 25–100% selection rule: do not choose a smaller CT merely to improve the nominal ratio without checking its measuring, continuous-current and short-time ratings.
Future Load Growth
Consider anticipated load growth when sizing CTs. If a circuit is expected to increase from 100A to 180A within the next few years, selecting a 200/5 CT now avoids the cost and disruption of replacing the CT later. However, do not oversize excessively, as accuracy at low percentages of rated current will suffer.
Accuracy Class
An accuracy class describes error limits under prescribed conditions, not one percentage guaranteed at every load. Read the declared standard, current range, burden and frequency conditions in the sensor documentation. Beckhoff's terminology guide distinguishes these ratings.
For power and energy measurement, assess the meter and sensor together, including phase error and installation effects. A CT class alone does not establish the accuracy of the complete system or its suitability for billing. Confirm the requirements of the intended use before specifying equipment.
Burden
The burden of a CT is the impedance of the secondary circuit, including the monitoring device input, connecting cables, and any other devices in the secondary loop. It is specified in volt-amperes (VA) at rated secondary current.
For a current-output CT, check the permitted burden range, including the meter input and the complete secondary cable loop. Do not assume that the maximum VA rating is the only condition for the stated accuracy. Low-power voltage-output sensors have different loading requirements; use their specified input and cable limits.
For long cable runs between the CT and the monitoring device, calculate the cable resistance and confirm it does not push the total burden beyond the CT's rating. Using larger cross-section cables reduces resistance and burden.
Physical Size and Mounting
CTs must physically fit around the conductor and within the available space in the electrical panel. Key physical considerations include:
- Aperture size: The CT's internal opening must be large enough to accommodate the conductor (cable or busbar). Measure the conductor dimensions before specifying the CT.
- External dimensions: The CT must fit within the available space in the panel. In densely packed panels, clearances between phases and to the panel enclosure may be tight.
- Mounting orientation: Some CTs are designed for specific orientations (e.g., horizontal or vertical mounting). Check the manufacturer's installation guidelines.
- Cable length: Ensure the CT's secondary cable is long enough to reach the monitoring device. Extension of CT secondary cables should be done carefully to maintain burden and safety requirements.
Safety Considerations
Protect current-output secondaries
An open secondary on a conventional current-output CT can develop dangerous voltages while primary current flows. Qualified personnel must follow the manufacturer's isolation and approved shorting-device procedures before changing secondary wiring. Do not disconnect an energised CT secondary.
Practical safeguards include:
- Identify the sensor's output type before planning any wiring work
- Use the manufacturer's approved shorting arrangement for current-output CTs where required
- Use CTs with built-in shorting mechanisms where available
- Label CT secondary circuits clearly to prevent accidental disconnection
- Ensure all personnel working in panels with CTs are aware of this hazard
Internally burdened low-power sensors, such as specified 333 mV CTs, do not have the same secondary arrangement. Do not transfer wiring or shorting instructions between sensor types. Beckhoff explains the distinction; the exact product instructions remain authoritative.
Voltage Rating
Verify the sensor's insulation rating and permitted installation environment against the actual circuit. Do not assume that a device is suitable simply because the site is described as low voltage. Check the documented ratings and installation restrictions for the exact sensor.
Installation by Qualified Personnel
CT installation involves working inside live or potentially live electrical panels. This work must only be performed by qualified electricians following appropriate safety procedures, including risk assessment, isolation where required, and use of appropriate PPE.
Output Type
CTs are available with different secondary output types:
- Current output (1A or 5A): Traditional CTs produce a proportional current output. 5A secondary is the most common worldwide; 1A secondary is used for longer cable runs (lower burden).
- Voltage output (0-333mV or 0-1V): Some CTs include an internal burden resistor and produce a voltage output directly. These are safer (lower open-circuit voltage) and simpler to install, but the output type must be compatible with the monitoring device's input.
Ensure that the CT output type matches the monitoring device input. Connecting a 5A CT to a voltage input, or vice versa, will produce incorrect readings and may damage the equipment.
CT Selection Checklist
- Determine the actual operating current range of each circuit to be monitored
- Check the measuring range, low-load error limits and thermal ratings against that current profile
- Define the required system accuracy, then verify the sensor and meter pairing
- Select the CT type: solid-core (new installation), split-core (retrofit), or Rogowski coil (high current, tight spaces)
- Verify the physical dimensions: aperture size, external dimensions, and cable length
- Confirm the output type matches the monitoring device input
- Calculate the secondary circuit burden (cable resistance + meter input impedance) and verify it does not exceed the CT's rated burden
- Verify the voltage rating is appropriate for the system voltage
- Review the installation environment for any special requirements (temperature range, humidity, vibration)
Specifying CTs for EpiSensor
Start with the ZEM electricity monitor family, then use the product datasheets and installation guides to confirm the supported sensor pairing. Primary range, output type, phase count, conductor size, installation category, accuracy requirement and calibration all have to match.
Do not assume that an arbitrary current-output or voltage-output CT is interchangeable. Record the manufacturer and part number in the design, verify polarity and orientation during commissioning, and compare the installed reading with a known reference before relying on the data.
If the pairing is unclear, contact us with the circuit's current profile, conductor dimensions and the intended monitor. Resolve those details before ordering rather than selecting a sensor by its headline current rating alone.
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