Current transformers (CTs) are the fundamental sensing element in most electrical energy monitoring systems. They measure the alternating current flowing through a conductor by electromagnetic induction, producing a proportional low-current output signal that can be safely measured by monitoring equipment. Selecting the right CT for each application is critical: an incorrect choice can lead to inaccurate measurements, safety hazards, or installation difficulties that undermine the entire monitoring deployment.
This guide covers the key factors that engineers and installers should consider when specifying current transformers for energy monitoring.
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 have a magnetic core that separates into two halves, allowing the CT to be clamped around an existing conductor without disconnecting it. This makes them the most popular choice for retrofit energy monitoring, as they can be installed on live circuits by a qualified electrician (following appropriate safety procedures).
Split-core CTs are slightly less accurate than equivalent solid-core CTs because the air gap at the split introduces a small error. High-quality split-core CTs minimise this with precision-machined mating surfaces, achieving accuracy comparable to solid-core designs.
Rogowski Coils
Rogowski coils use a flexible, non-magnetic winding that wraps around the conductor. They cannot saturate, have a very wide measurement range, and are lightweight and flexible. However, they require an external integrator circuit and are generally less accurate than iron-core CTs at lower currents. See our dedicated article on Rogowski coils for a detailed comparison.
Current Rating
The primary current rating of a CT defines the maximum current it is designed to measure accurately. CTs are specified by their ratio, for example 100/5 or 200/1, meaning a primary current of 100A or 200A produces a secondary current of 5A or 1A respectively.
Sizing for the Load, Not the Breaker
A common mistake is to select a CT based on the circuit breaker rating rather than the actual operating current. A circuit protected by a 400A breaker may typically carry only 80-120A. A 400/5 CT measuring 100A is operating at only 25% of its rated current, where accuracy is significantly degraded.
Best practice is to select a CT where the normal operating current falls between 25% and 100% of the CT's rated primary current. For the example above, a 150/5 or 200/5 CT would provide much better accuracy at the actual operating current.
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
CTs are rated by accuracy class according to standards such as IEC 61869-2 (international) or IEEE C57.13 (North America). Common accuracy classes for energy monitoring include:
- Class 0.2: Maximum ratio error of 0.2% at rated current. Used for revenue metering and billing applications.
- Class 0.5: Maximum ratio error of 0.5% at rated current. Suitable for most energy monitoring and sub-metering applications.
- Class 1: Maximum ratio error of 1% at rated current. Adequate for general monitoring and load profiling where high precision is not critical.
- Class 3: Maximum ratio error of 3%. Only suitable for indication purposes, not recommended for energy monitoring.
It is important to note that these accuracy specifications apply at rated current and rated burden. Accuracy degrades when operating current is significantly below rated current or when the connected burden exceeds the rated burden.
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.
Every CT has a rated burden (for example, 5 VA or 15 VA). The actual burden of the connected secondary circuit must not exceed this rated burden, or accuracy will be degraded. In practice, modern electronic energy meters have very low input impedance (often less than 0.1 VA), so the burden is usually dominated by the resistance of the connecting cables.
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
Never Open-Circuit a CT Under Load
This is the single most important safety rule when working with current transformers. If the secondary circuit of a current transformer is opened while current is flowing in the primary conductor, the CT will attempt to maintain the magnetic flux by driving the secondary voltage to dangerous levels, potentially thousands of volts. This creates a lethal shock hazard and can cause the CT to overheat, potentially leading to fire or explosion.
Practical safeguards include:
- Always short-circuit the CT secondary before disconnecting the monitoring device
- 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
Voltage Rating
CTs must be rated for the system voltage of the circuit they are installed on. The CT's insulation must withstand the full system voltage (including transients) between the primary conductor and the secondary winding. For low-voltage systems (up to 690V), most CTs are adequately rated. For medium-voltage systems (above 1 kV), specific medium-voltage CT designs are required.
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
- Select a CT ratio where normal operating current falls between 25-100% of rated primary current
- Choose the appropriate accuracy class for the application (Class 0.5 for most energy monitoring)
- 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)
CTs and EpiSensor
EpiSensor's wireless energy monitoring sensors are designed to work with a wide range of current transformers, including split-core CTs and Rogowski coils. The sensors accept both current and voltage CT outputs, providing flexibility in CT selection. EpiSensor can provide guidance on CT selection for your specific installation, and our documentation includes compatibility tables for commonly used CT models.